User:Serendipodous/indigo/page 15

Kuiper belt

Jupiter family comets vs ecliptic comets

Tisserand parameter of 2 separates the Jupiters from the Halleys

There is almost no overlap in orbital period; Jupiters with a period of more than 20 years are almost nonexistent, as are Halleys with periods of less than 20

(KBOs) reaching a Neptune-crossing orbit can be slingshot, by encounters with different planets, to very low perihelion distances (q < 2.5 au)

their orbits were assumed to start with low inclinations (i < 5)

inclination distribution of ECs becomes wider over time due to scattering encounters with Jupiter.

The escape of bodies from the classical Kuiper belt at 30-50 au is driven by slow chaotic processes in various orbital resonances with Neptune.

Because these processes affect only part of the belt, with most orbits in the belt being stable, questions arise about the overall efficiency of comet delivery from the classical KB.

suggested that the scattered disk should be a more prolific source of ECs than the classical KB because SDOs can approach Neptune during their perihelion

nclination distribution of SDOs (median i ' 25 is much broader than that of ECs (median i ' 13◦.

This raises a question whether the scattered disk can produce the narrow inclination distribution of ECs

The Halley-type comets (HTCs) have longer orbital periods and larger inclinations than do most ECs.

Levison et al. (2006) considered the scattered disk as the main source of HTCs and showed that some SDOs can evolve into the Oort cloud and back, thus providing an anisotropic source of HTCs

presumed source populations of trans-Neptunian objects with cometary sizes (∼1-10 km) are not well characterized from observations.

kozai mechanism can gradually diffuse objects to the oort cloud

the centaur region has a mean life time of 9 my

Brasser & Morbidelli (2013) found that the scattered disk needs to contain ∼ 2 × 109 bodies with diameter D > 2.3 km to provide an adequate source of ECs, and the Oort cloud needs to have ∼ 4 × 1010 to ∼ 1011 bodies with D > 2.3 km to explain the flux of new Oort cloud comets

Levison et al. (2006), on the other hand, required that there are ∼ 3 × 109 SDOs with D > 10 km to produce the observed population of HTCs, assuming physical evolution (more SDOs would be needed if they accounted for physical disruption/fading of HTCs).

The number of comets produced in the model at t = 4.5 Gyr can be inferred from the number of comets in the original transplanetary disk, which in turn can be calibrated from the number of Jupiter Trojans.\\ This is because the Trojan implantation efficiency from the original disk is well-determined and because the size distribution of Trojans is well characterized from observations

SPCs are defined as bodies showing cometary activity and having short orbital periods (P < 200 yr)

The period range is arbitrary, because there is nothing special about the boundary at the 200-yr period, and the orbital period distribution of known comets appears to continue smoothly across this boundary. With P < 200 yr, SPCs are guaranteed to have at least one perihelion passage in modern history,

The orbital distribution of SPCs shows clear evidence for two populations, which are historically known as JFCs and HTCs.

periods 5 . P . 20 yr, which is similar to the orbital period of Jupiter (PJ = 11.9 yr).

Tisserand parameter for Jupiter

ECs with 2 < TJ < 3, and (2) nearly isotropic comets (NICs) with TJ < 2.

HTCs (20 < P < 200 yr) are NICs (TJ < 2). The NIC category is broad, however, and includes LPCs as well

Orbits with TJ > 3 are generally not Jupiter crossing, and are therefore typically not classified as cometary

several known comets with P < 20 yr have large orbital inclinations, or even retrograde orbits

Np(2.5) > 1000 is required to obtain a steady-state population of large active HTCs that is consistent with observations (number of perihelia, au)

To fit the ratio of the returning-to-new OCCs, model implies that Np(2.5) . 10, possibly because the detected long-period comets are smaller and much easier to disrupt than observed HTCs.

The escape of bodies from the classical Kuiper belt at 30-50 au (hereafter classical KB) is driven by slow chaotic processes in various orbital resonances with Neptune. Because these processes affect only part of the belt, with most orbits in the belt being stable, questions arise about the overall efficiency of comet delivery from the classical KB.

Duncan and Levison suggested that the scattered disk should be a more prolific source of ECs than the classical KB (see Gladman et al. (2008) for a formal definition of these dynamical classes). This is because SDOs can approach Neptune during their perihelion passages and be scattered by Neptune to orbits with shorter orbital periods.

the inclination distribution of SDOs (median i ' 25◦; Nesvorn´y et al. 2016) is much broader than that of ECs (median i ' 13◦). This raises a question whether the scattered disk can produce the narrow inclination distribution of ECs

The Halley-type comets (HTCs) have longer orbital periods and larger inclinations than do most ECs. The HTCs population is not well characterized from the existing observations, because the observational biases for long orbital periods are more severe than for ECs

some SDOs can evolve into the Oort cloud and back, thus providing an anisotropic source of HTCs.

the presumed source populations of trans-Neptunian objects with cometary sizes (∼1-10 km) are not well characterized from observations.

Rickman et al. (2017) suggested thereare ∼ 109 SDOs with D > 2 km from modeling of ECs. Levison et al. (2006), on the other hand, required that there are ∼ 3 × 109 SDOs with D > 10 km

While some of these estimates may appear to be high, it is not obvious whether they are implausibly high, because we just do not know from observations how many small objects there are in the distant regions.

The number of comets produced in the model at t = 4.5 Gyr can be inferred from the number of comets in the original transplanetary disk, which in turn can be calibrated from the number of Jupiter Trojans (Nesvorn´y & Vokrouhlick´y 2016, hereafter NV16). This is because the Trojan implantation efficiency from the original disk is well-determined (Nesvorn´y et al. 2013; see also Morbidelli et al. 2005) and because the size distribution of Trojans is well characterized from observations

SPCs are defined as bodies showing cometary activity and having short orbital periods (P < 200 yr). The period range is arbitrary, because there is nothing special about the boundary at the 200-yr period, and the orbital period distribution of known comets appears to continue smoothly across this boundary.

With P < 200 yr, SPCs are guaranteed to have at least one perihelion passage in modern history, with many being observed multiple times. This contrasts with the situation for the long-period comets (LPCs; P > 200 yr), which can be detected only if their perihelion passage coincides with the present epoch.

LD97 opted, instead, to use the Tisserand invariant of the circular restricted threebody problem (Tisserand 1889), which conveniently combines the comet’s orbital period (or, equivalently, the semimajor axis a) and inclination into a single expression.

Most JFCs (P < 20 yr) are ECs (2 < TJ < 3), and vice versa, and most HTCs (20 < P < 200 yr) are NICs (TJ < 2). The NIC category is broad, however, and includes LPCs as well.

several known comets with P < 20 yr have large orbital inclinations, or even retrograde orbits; appear to be a low-period extension of NICs

Neptune’s migration into a massive planetesimal disk (Mdisk ' 15-20 Earth masses, M⊕) between ∼22 and 30 au.

Most inner SDOs ('80%) are fossilized, meaning that their (barycentric) semimajor axis did not change by more than 1.5 au over the last 1 Gyr. This includes objects that interacted with Neptune’s orbital resonances in the past and subsequently decoupled from Neptune by various dynamical processes (Kaib & Sheppard 2016, Nesvorn´y et al. 2016). The remaining '20% of inner SDOs are being actively scattered by Neptune (hereafter the scattering SDOs; Gladman et al. 2008). Nearly all outer SDOs, on the other hand, are on scattering orbits (i.e., their semimajor axis changes by more than 1.5 au in the last 1 Gyr). Thus, even though the inner scattered disk is more massive than the outer one, the number of scattering objects in each population is roughly the same.

The outer part of the Oort cloud forms first and is present in our simulations already in the first 10 Myr. By checking on the orbital histories of outer Oort-cloud bodies we found that most of them reached a > 20, 000 au after having encounters with Saturn (and typically without having encounters with Jupiter; Dones et al. 2004). In addition, a significant fraction of outer Oort-cloud bodies reached their distant orbits by being scattered by Uranus or Neptune (and without having encounters with Jupiter or Saturn). The inner Oort cloud formed as a ‘wave front’ of orbits in our simulations that was moving from outside in as time advanced. Most bodies that ended up in the inner Oort cloud were scattered to a > 1000 au by Neptune (some after having encounters with Uranus,but rarely with Jupiter/Saturn).

the ratio of the Oort cloud to scattered disk should be OC/SD ∼ 20

Np(2.5) ' 500 is required to match the inclination distribution of known ECs, while Np(2.5) > 1000 is required to match the number of known large HTCs. and HTCs presumably formed in the same region, in the original planetesimal disk at < 30 au, there is no a priori reason to think that their internal structures, and thus their physical lifetimes, should be different. Also, Np(2.5) appears to be an adequate parametrization of the physical lifetime: the results for other parametrizations,

1. The orbital distribution of ECs is well reproduced in our models without P9. With P9, the inclination distribution of model ECs is wider than the observed one. Models with q9 > 300 au could resolve this issue, but it is not clear whether they could also help to match other constraints (such as the orbits of extreme KBOs and the solar obliquity). 2. We find that known HTCs have a nearly isotropic inclination distribution, and appear in the model as an extension of the population of returning LPCs to shorter orbital periods. The contribution to HTCs from the P9 cloud, if real, would be relatively minor. 3. The nominal model estimate of the number of large ECs falls short by a factor of ∼2-4 when compared to observations. This problem can be resolved if large comets have longer physical lifetimes (see below). The number of large HTCs obtained in the model from the Oort cloud agrees well with observations. 4. We demonstrate that the physical lifetime of active comets depends on their nuclear size and explain how this can help to produce the correct number of large ECs in the model. Combining the analysis of ECs, HTCs and LPCs, we estimate that comets a few hundred meters in size should only survive several perihelion passages, ∼1-km class comets should be active for hundreds of perihelion passages, and ∼10-km class comets should live for thousands of perihelion passages. [Previously, Di Sisto et al. (2009) and Rickman et al. (2017) considered the dependence of physical lifetime of comets on size in their models.] 5. The inner scattered disk at 50 < a < 200 au should contain ∼ 1.5 × 107 D > 10 km bodies. The Oort cloud should contain ∼ 3.8 × 108 D > 10 km comets. These estimates can be extrapolated to smaller or larger sizes using the size distribution of Jupiter Trojans (Fig. 14b).

rickman

comets split because of proximity to large bodies (shoemaker levy 9)

comet hartley 2 had a near-ocean like isotope ratio

oort cloud vs scattered disc comet, differences in formation

it is very difficult to determine a comet nucleus's size from the ground- bright dust might skew the absolute size

use of hubble and other spaceborne telescopes has helped

the lack of data on the sizes of comets prevents us from gaining an accurate measurement of the mass of the oort cloud

albedo of nyucli is generally assumed

1.5 to 2.5 squares with the impact record on Pluto and Charon, which have an alpha of 2.3

the bright spot at the centre of a coma is called the central condensation

ice sublimation due to heating by absorption of sunlight

in scientific literature, comet and nucleus are synonymous

An object is only ever called a comet if it generates a coma

it must have ice near the surface and a perihelion close enough for it to sublimate

if a comet had its perihelion yanked out of solar proximity, it would probably be called an asteroid, unless it was observed to form a coma before then.

However, cometary activity is the only definitive method to determine cometary status from the ground until we can probe an object's interior

There are objects that are difficult to classify- the boundary between comets and asteroids will be somewhat fluid

as per iau 1994: a periodic comet is one which has an orbital period of less than 200 years and more than one confirmed perihelion passage

in 1999: IAU defines single apparition comets as periodic if its orbital period is less than 30 years

As of 2017, there are 347 known periodic comets

D designation, those that have disappeared (3/D Biela)

Names are not unique, and so are treated as secondary in the cometary nomenclature

numerical endings (wild 2, tempel 1) are now considered redundant, though they are still used in the popular litdrature

ambiguities in naming (asteroid 4015 Wilson-Harrington; 95P Chiron and 174P echeclus)

less than ten sungrazers in recorded history prior to 1979

usually called great comet as they were exceptionally bright. Now observed constantly, usually the children of larger parents colliding with the sun, by SoHO, stereo and SOLWiND spacecraft

long period/short period division of 200 years arbitrary, basically based on whether a human had a chance to see it twice

comet capture by jupiter can decrease orbital period

Tisserand parameter of 2 separates the Jupiters from the Halleys

There is almost no overlap in orbital period; Jupiters with a period of more than 20 years are almost nonexistent, as are Halleys with periods of less than 20

Oort 1977

It is still well known in Holland because of the Planetarium, which the wool comber Eyse Eysinga completed in 1785.

When I was three years old my parents had moved to the neighborhood of Leiden and that's where I got my education.

I suppose it was by reading books by Jules Verne who was very popular in those days.

My father was a medical director in a sanitorium for nervous illnesses.

My father was interested in science. He made some simple brain experiments and was one of the first in the Netherlands to use psychological tests.

They were very liberal. They thought that if you really liked the subject very deeply you must follow your calling and try to study that subject. But when at Groningen I wasn't quite decided yet whether I would specialize more in physics or astronomy. It was the personality of Professor Kapteyn which decided me entirely. He was quite an inspiring teacher and especially his elementary astronomy lectures were fascinating.

No, they were left very free in this. My own earliest research was on the phenomenon of stars of high velocity which certainly fell outside the regular domain I would say of research of the Observatory at that time.

DeVorkin: But it was certainly close to Kapteyn's interests.

because before one realized sufficiently how serious the interstellar absorption was there was no sufficient reason to have great doubts about the general size of the galactic system. Although at that time we thought that the galactic system would be like the spiral galaxies we didn't know either where the large spirals were and how large they were because we didn't know the distances. But from the earliest days of which I can remember my own thoughts, I think I was convinced that they were of a similar nature.

But this only became really clear by Lindblad's research which pointed out that these high velocity objects formed a system of much slower rotation. He had the idea that the galactic system was very much larger, it contained on one hand objects like, the globular clusters which had only very slow if any rotation around the center of the galaxy and that there were other intermediate groups of high velocity stars which also had a very much slower rotation than the bright stars around us.

But I realized at the time that it would be difficult to have any real system in which the mass would not be concentrated toward the center so that the inner part should move around more rapidly than the outer parts. This effect showed up very clearly in these O-type stars and distant early B-type stars. And so in that way one was able to give a very direct confirmation of this rotation hypothesis of Lindblad's.

But Lindblad's articles were a very large step; a step which also Shapley had not made at all to go from the idea of the limited swarm of stars which was the Kapetyn system — to go from that to the idea that this is only just an arbitrary section of a very much larger system. It was a revolutionary step at that time. One didn't at all realize at that time that the absorption was so strong, that the size of the Kapteyn System was entirely limited by that.

No I didn't work out a real model, I mean I was satisfied to get one parameter, which in essence was a concentration of mass towards the center of the galaxy and the rotational velocity itself.

I know that Leiden is well known for cloudy skies like Princeton.

Oort: Yet, the polarization of Crab Nebula was first investigated in the middle of the town of Leiden with many city lights around and quite successfully investigated at that.

medium something on the order of 3 x 10-24 grams per cubic centimeter has been called by Struve and others the famous Oort limit.

Most of the members of this group were put in hostage camps very soon after the speech by Meyers. I wasn't and so we went down to live in the country for the rest of the war,

Could we talk about that Board and the development of support for radio instrumentation in Holland for a little while? How did the Board initially get formed? Who was the instigator? How were the contacts made?

Oort: Well, my idea for this large project was quite favorably received. After the World War we were very poor and there were, of course, many more important things to think of, such as building bridges and harbors. But the first Prime Minister after the war was a man I knew well, who was very much interested in science.

that you had very quickly grasped the value of the 21 cm line in mapping out the structure of the Galaxy. Did you feel the most effective way to go at general galactic structure

There is not the slightest relation between my interests in the comets and the interest in the solid particles in interstellar space, although now-a-days some people think that the comets can be formed in interstellar space; I think this is very doubtful.

Oort 1950

"We may conclude that a sensible fraction of the long-period comets must have come from a region of space extending from a distance 2a = 20 000 to distance at least 15 000 AU from the Sun, that is, almost to the nearest star. That does not mean that they are interstellar; they belong very definitely to the Solar System, because they share accurately the Sun's motion."

22 comets

"So far no comet has been found for which the eccentricity exceeds one by an amount large enough to be considered as real".

A preponderance of hyperbolic comets would exist if the comets were drawn in by the planets from interstellar space- Van Woerkom

"They must then form a huge cloud, extending, according to the numbers cited above, to distances of at least 150 000 AU and possibly still further."

Oort suggested it may have been a disrupted planet.

"It is evident from Van Woerkom's study that within one or two million years of their first perihelion passage practically all long-period comets will have disappeared."

For new comets to continue to appear after 20 million years, they would have to be continuously perturbed. A resisting medium is impossible in dynamical grounds, so Oort suggested the motions of stars.

Fred Whipple alerted Oort to Opiks work before the 1950 paper was completed.

Opik never really believed in a cloud; he believed stellar perturbations would limit the aphelia of comets to 2000 AU.

The difference between comets and asteroids is that asteroids have been affected by solar radiation. Oorts calculations showed that a cloud could remain stable over the age of the solar system, the edge is not 2000 AU, but 200 000 AU

Because stars are unlikely to pass closer than 20 000 AU, we are unlikely to see comets from closer in, which agrees with observations

Oort concluded that the comets could not have accreted in situ, and must have formed among the planets.

They instead were perturbed by the giant planets into their current orbits.

The asteroids could not have comets

oort

In the early years under Kapteyn, galactic centre was viewed as close to the Solar System and in the direction of cygnus

through his work with globular clusters, Shapley had shown that the centre of the galaxy was ~10 kpc away and in the direction of Sagittarius

In the 1920s. Oort published 2 papers confirming the hypotheiss of Lindblad that the galaxy in fact rotates, and that our stellar neighborhood was just "an arbitrary section" of the galactic system

Oort was one of the astronomers who determined the galaxy's spiral structure and in a 1951 paper, connected its structure to other "spiral nebulae".

Hobbies: ice-skating, rowing and comets

In the 1940s, Oort became the advisor of a graduate student who wished to work on comet orbits

It aroused his interest likely through the gravitational influence of stellar encounters

"the piece of investigation that has given me the most satisfaction is soemthing rather outside my regular field. It is my work on the origin of the comets in the Solar System. It is the only investigation that has been [proberly rounded off. All other researches result in soemthing of which, after all, one understands only half."

though he didn't perform much cosmological research himself directly, the structure and origin of the universe was Oort's overriding interest (Herschel)

In 1958, he published a paper showing that the mass of the universe was too little to close it

When oort began, dutch astronomy was dominated by Kapteyn, who was Oort's mentor through much of his early work

Unlike Herschel, Oort was never a dragon chaser. He was strict in drawing lines between speculation and outright fantasy, a sobriety dutch astronomy has inherited from him

His papers were known for being dense, consise and literary, with very few formulae

Oort's pressure is largely responsible for hydrogen-line radio astronomy

Oort was never particularly interested in solar physics

Damasso Feb 2020

Maximum stable orbit of 1700 AU, about 1 of the Suns, because of A and B

There are unlikely to be planets around Proxima of Saturn's size or larger out to 10 AU

Neptune mass out to 1 AU (RADIAL v)

Red dwarfs have a higher instance of smaller planets (1-3 Earth radii) than main sequence stars

Radial velocity studies of HARPS and UVES data sets spanning 17 years

We found a clear peak with the highest power at P∼1907 days

semi-major axis ac = 1.48 ± 0.08 AU, minimum mass mc sin ic = 5.8 ± 1.9 M⊕, and equilibrium temperature Teq=39+16−18 K.

82.1 days proxima rotation

It would also be the first at a distance from the parent star much larger than the expected original location of the snowline in the protoplanetary disk, which was within 0.15 AU

The formation of a super-Earth well beyond the snowline challenges formation models according to which the snowline is a sweet spot for the accretion of super-Earths, due to the accumulation of icy solids at that location

The planet is massive enough relative to the central star to have opened a relatively deep gap in the protoplanetary disk and have migrated in type II mode. According to Kanagawa et al. (33), its migration timescale would have been 1 million years

We speculate that this inner ring may be due to dust produced by planetesimals clustered in one of the planet’s inner mean motion resonances.

Benedict June 2020

Fritz Benedict an emeritus Senior Research Scientist with McDonald Observatory at The University of Texas at Austin

1990s made with Hubble Space Telescope. For that study, he had used Hubble’s Fine Guidance Sensors (FGS).

astrometry

Indeed, Benedict found a planet with an orbital period of about 1,907 days buried in the 25-year-old Hubble data. This was an independent confirmation of the existence of Proxima Centauri c.

Fritz Benedict has used data he took over two decades ago with Hubble Space Telescope to confirm the existence of another planet around the Sun's nearest neighbor, Proxima Centauri, and to pin down the planet's orbit and mass.

Thus Benedict decided to re-visit his studies of Proxima Centauri from the 1990s made with Hubble Space Telescope. For that study, he had used Hubble's Fine Guidance Sensors (FGS).

Though their primary role is to ensure accurate pointing of the telescope, Benedict and others routinely used FGS for a type of research called astrometry: the precise measurement of the positions and motions of celestial bodies. In this case, he used FGS to search for Proxima Centauri's motion on sky caused by tugging from its surrounding—and unseen—planets.

When Benedict and research partner Barbara MacArthur originally studied Proxima Centauri in the 1990s, he said, they only checked for planets with orbital periods of 1,000 Earth days or fewer. They found none. He now revisited that data to check for signs of a planet with a longer orbital period.

Woods, May 2020

SPHERE - Spectro-Polarimetric High-contrast Exoplanet REsearch

Raffaele Gratton Image of Proxima C

However, the detection is not fully certain because the candidate’s motion does not fit with the current best measurement of the astrometric motion of the host star.

7.2 ± 2.2 M⊕.

However, the average contrast of the peak seen in the combined SPHERE images suggests an object that is approximately five times the size of Jupiter. A possible solution for the discrepancy is that the planet candidate is surrounded by dust clouds or a system of expansive rings much like Saturn’s.

To reflect enough light to explain the brightness of the images, that disk or set of rings would have to be more than twice as wide as the bright part of Saturn’s ring system. “It needs to take up a lot of real estate to be bright enough,” says Mark Marley at NASA’s Ames Research Center in California.

Last, but not least, they found that the candidate had an unexpectedly high apparent brightness (aka. flux) a planet orbiting a red dwarf star. Because of this, the team could not say with any confidence whether or not what they observed was indeed Proxima c.

However, this last item raised another possibility that the team had to consider, that the unusual brightness may be the result of a circumplanetary material.

Proxima c is there, its size and orbit suggest that it is a relatively old planet, and we aren’t sure how such an old planet would retain rings for a long time – in contrast, Saturn’s rings are probably relatively young at just 100 million years old.

Astronomers May Have Captured the First Ever Image of Nearby Exoplanet Proxima C View of the Alpha Centauri system. The bright binary star Alpha Centauri AB lies at the upper left. The much fainter red dwarf star Proxima Centauri is barely discernible towards the lower right of the picture. Credit: Digitized Sky Survey 2; Acknowledgement: Davide De Martin and Mahdi Zamani Little is more enticing than the prospect of seeing alien worlds around other stars—and perhaps one day even closely studying their atmosphere and mapping their surface. Such observations are exceedingly difficult, of course. Although more than 4,000 exoplanets are now known, the vast majority of them are too distant and dim for our best telescopes to discern against the glare of their host star. Exoplanets near our solar system provide easier imaging opportunities, however. And no worlds are nearer to us than those thought to orbit the cool, faint red dwarf Proxima Centauri—the closest star to our sun at 4.2 light-years away.

In 2016 astronomers discovered the first known planet in this system: the roughly Earth-sized Proxima b. But because of its star-hugging 11-day orbit around Proxima Centauri, Proxima b is a poor candidate for imaging. Proxima c, by contrast, offers much better chances. Announced in 2019, based on somewhat circumstantial evidence, the planet remains unconfirmed. If real, it is estimated to be several times more massive than Earth—a so-called super Earth or mini Neptune—and to orbit Proxima Centauri at about 1.5 times the span between Earth and the sun. Its size and distance from its star make the world a tempting target for current and near-future exoplanet-imaging projects. Now, in a new preprint paper accepted for publication in the journal Astronomy & Astrophysics, some astronomers say they might—just might— have managed to see Proxima c for the first time.

“This planet is extremely interesting because Proxima is a star very close to the sun,” says Raffaele Gratton of the Astronomical Observatory of Padova in Italy, who is the study’s lead author. “The idea was that since this planet is [far] from the star, it is possible that it can be observed in direct imaging. We found a reasonable candidate that looks like we have really detected the planet.”

Last year Gratton and his team were first alerted to the possibility of imaging the planet by Mario Damasso of the Astrophysical Observatory of Turin in Italy, who was the lead author of the original paper on Proxima c’s possible discovery. Damasso and his colleagues had presented evidence for Proxima c’s existence based on its star’s telltale wobbling, which they inferred was caused by the pull of an unseen orbiting planet. Confirming a world’s existence in this way requires seeing the same wobble occur again—and again—in a process that often takes many months or even years. Damasso wondered if there might be another way. Thus, he asked Gratton and his team to look through data from the SPHERE (Spectro-Polarimetric High-Contrast Exoplanet Research) instrument on the European Southern Observatory’s Very Large Telescope (VLT) in Chile to see if they could actually see the planet. “As soon as our paper on Proxima c was considered for publication, I contacted [Gratton] to discuss the possibility of pushing SPHERE to its limits,” Damasso says. “The [planetary] system is potentially so cool that it is worthy to try other techniques.”

If you squint a bit while staring at the SPHERE data, a picture of the mysterious planet seems to swim into view. By focusing on Proxima c’s predicted position and separation from its star within multiple, stacked infrared images from SPHERE, Gratton and his colleagues were able to pick out 19 potential appearances of the planet across several years of routine observations. Of these candidate detections, one stood out as being particularly enticing: it appeared in the images about six times brighter than their “noise”—that is, unwanted light from artifacts or background stars. “It’s a possible candidate that has a low probability of being a false alarm,” says Emily Rickman of the Geneva Observatory, who is a co-author of the paper.

The object also appears to be 10 to 100 times brighter than a planet of its mass should be. This luminosity, the study authors reason, could arise from a large amount of dust surrounding the planet, perhaps in a vast ring system that is three to four times larger than that of Saturn. To some, that situation seems too strange to be true.

“It’s certainly possible for things like this to exist. But for your first detection of something like this to have that massive ring system, you’d have to postulate a universe in which most Neptune-sized planets have massive ring systems enormously bigger than Saturn’s. And that seems like an unlikely universe to live in.”

Thanks to an INAF-led team, a second exoplanet (a super-Earth) was found early this year around Proxima Centauri using the Radial Velocity Method. Based on the separation between the two planets, another INAF-led team attempted to observe this planet using the Direct Imaging Method. While not entirely successful, their observations raise the possibility that this planet has a system of rings around it, much like Saturn.

For the sake of their study, which recently appeared in the journal Astronomy & Astrophysics, the team relied on data obtained by the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument on the ESO's Very Large Telescope (VLT). This extreme adaptive optics system and coronagraphic facility is dedicated to the characterization of exoplanet systems at optical and near-infrared wavelengths.

Jankins May 2019

Proxima Centauri b is not a transiting exoplanet

48 hours with the Spitzer telescope

Lin and Kaltinegger

(i) a 1 bar surface pressure atmosphere assuming Earth-like mixing ratio; (ii) eroded atmospheres with 0.5 bar and 0.1 bar surface pressures assuming Earth-like mixing ratio, and (iii) an anoxic atmosphere (trace levels of O2; 3 x 10-3 CO2) with 1 bar surface pressure that mimics Earth’s atmosphere before the Great Oxidation Event

To maintain surface temperatures above freezing for the 1 bar oxic cases of Proxima b, we assume a constant mixing ratio of 100 times present atmospheric levels of CO2 (3.65 x 10-2) for both planets and a CH4 mixing ratio of 1.6 x 10-6 at the surface of Trappist-1e (M8.0V host), and 1.6 x 10- 4 at the surface of Proxima b (M5.5V host), which 1 http://archive.stsci.edu/iue/ receives slightly less incident radiation due to its orbital separation.

We divide the exoplanet atmospheres into 35 layers for our models up to an altitude of at least 60 km, with smaller spacing towards the ground. The atmospheric species, which account for the most significant spectral features are H2O, CO2, O2, H2, CH4, CO, N2O, CH3CL, OH, O3 for the oxic cases, and H2O, CO2, O, O2, H, OH, HO2, H2O2, O3, H2, CO, HCO, H2CO, CH4, CH3, C2H6, NO, NO2, HNO, SO, SO2, H2SO4 for the anoxic cases. In our models we assume an Earth-like surface albedo with 70% ocean, 2% coast, and 28% land with 50% cloud coverage. The land surface is divided into 30% grass, 30% trees, 9% granite, 9% basalt, 15% snow, and 7% sand

Therefore, protective mechanisms that allow organisms to survive such environment would be essential for maintaining surface habitability. For the eroded and anoxic atmospheres, due to low optical depth of UV shielding gas, such mechanisms are of greater importance. Studies of extremophiles on Earth identified several strategies organisms apply to survive high energy radiation. Protective pigments, for example, could attenuate incoming radiation, and DNA repair pathways can reduce or even prevent damages due to radiation (see e.g. Neale & Thomas 2016; Sancho et al 2007; Onofri et al., 2012; Cockell et al., 1998). Living subsurface, such as under a layer of rock, soil, sand, or water, can significantly reduce the exposure to detrimental UV radiation, and therefore increase habitability (e.g. Ranjan & Sasselov 2016; Cockell et al 2000,2009, O'MalleyJames & Kaltenegger 2017). However, this strategy would make remote detection of such life difficult. Biofluorescence is an alternative protective mechanism. Widely observed in nature, biofluorescence can also convert UV light into less energetic wavelengths, therefore protecting the organisms from damage and increasing the detectability of such a biosphere (O'Malley-James & Kaltenegger 2018, 2019b). Despite its damaging effects to biological molecules, UV light has been shown to be crucial to increase efficiency in prebiotic chemistry. Macromolecular building blocks of life likely require certain levels of surface UV radiation to form (Ranjan & Sasselov 2016; Rimmer et al. 2018). Therefore, whether high UV levels on the surface of planets in the HZ of M stars are a concern or a prerequisite for surface habitability and life for planets orbiting M stars is an open question. Furthermore, models have shown while UV surface levels for both Proxima b and Trappist-1e are higher than modern Earth, they are lower than the levels early Earth received, even for eroded atmospheres for active input star models

Because of Proxima b’s large apparent angular separation, it can be resolved by planned ground-based telescopes like the ELT, increasing the achievable contrast ratio by about 103 compared to unresolved planets. While Trappist1e cannot be resolved with the ELT, chemical signatures in the atmosphere of unresolved exoplanets have already been observed using highresolution spectra at the Very Large Telescope, making both planets intriguing targets for future atmospheric characterization in reflected light.

ESPRESSO Feb 2020

Previously, scientists thought that this exoplanet, which lies in the habitable zone of its star, harbored a minimum of about 1.3 Earth masses. The new measurement indicates that Proxima b could be even more like our home planet, at least in size, than previous observations led scientists to think.

The research team studied Proxima b using the Echelle Spectrograph for Rocky Exoplanet and Stable Spectroscopic Observations, or ESPRESSO for short. ESPRESSO is a Swiss spectrograph that is currently mounted on the European Southern Observatory's (ESO) Very Large Telescope in Chile. Spectrographs observe objects and split the light coming from those objects into the wavelengths that make it up so that researchers can study the object in closer detail.

Proxima b was first detected four years ago by an older spectrograph, HARPS ("High Accuracy Radial Velocity Planet Searcher"), which is installed on a scope at ESO's La Silla Observatory in Chile But with these newer observations, scientists have an updated, ultra-precise view of the planet.

We confirm the presence of Proxima b independently in the ESPRESSO data and in the combined ESPRESSO+ HARPS+UVES dataset. The ESPRESSO data on its own shows Proxima b at a period of 11.218 ± 0.029 days, with a minimum mass of 1.29 ± 0.13 M⊕. In the combined dataset we measure a period of 11.18427 ± 0.00070 days with a minimum mass of 1.173 ± 0.086 M⊕. We get a clear measurement of the stellar rotation period (87 ± 12 d) and its induced RV signal, but no evidence of stellar activity as a potential cause for the 11.2 days signal. We find some evidence for the presence of a second short-period signal, at 5.15 days with a semi-amplitude of only 40 cm s−1. If caused by a planetary companion, it would correspond to a minimum mass of 0.29 ± 0.08 M⊕. We find that forthe case of Proxima, the full width half maximum of the cross-correlation function can be used as a proxy for the brightness changes and that its gradient with time can be used to successfully detrend the RV data from part of the influence of stellar activity. The activity-induced RV signal in the ESPRESSO data shows a trend in amplitude towards redder wavelengths. Velocities measured using the red end of the spectrograph are less affected by activity, suggesting that the stellar activity is spot dominated. This could be used to create differential RVs that are activity dominated and can be used to disentangle activity-induced and planetary-induced signals. The data collected excludes the presence of extra companions with masses above 0.6 M⊕ at periods shorter than 50 days.

The ESPRESSO data on its own shows Proxima b at a period of 11.218 ± 0.029 days, with a minimum mass of 1.29 ± 0.13 M⊕. In the combined dataset we measure a period of 11.18427 ± 0.00070 days with a minimum mass of 1.173 ± 0.086 M⊕. We get a clear measurement of the stellar rotation period (87 ± 12 d) and its induced RV signal, but no evidence of stellar activity as a potential cause for the 11.2 days signal.

This breakthrough was possible thanks to radial velocity measurements of unprecedented precision using ESPRESSO, the Swiss-manufactured spectrograph, the most accurate currently in operation, which is installed on the Very Large Telescope in Chile. Proxima b was first detected four years ago by means of an older spectrograph, HARPS, also developed by the Geneva-based team, which measured a low disturbance in the star's speed, suggesting the presence of a companion.

flares

Numerical models have shown that Proxima Centauri b probably lost a large amount of its water in its early life stages—an amount comparable to an ocean on Earth—but despite this, it is still possible that some liquid water remained in warmer regions of the planet, maybe in a tropical belt or at the hemisphere facing the central star in case of locked rotation. This makes other factors affecting habitability, such as the magnetic activity of the host star, particularly important, as activity-related phenomena (flares, coronal mass ejections, strong UV flux) can erode a planet's atmosphere, rendering it uninhabitable in the long term.

The strong flaring activity of Proxima Centauri has already been known to astronomers, and several superflares were observed previously. During such eruptions, extremely large amounts of energy are released that may reach 1033 ergs, or 10 times the Carrington event in 1859, the strongest flare ever seen on the sun—consider such a flare from a much smaller star. In 2016, during one these superflares, the brightness of Proxima Centauri increased by a factor of 70 compared to its quiescent state —it became the only cool red dwarf visible to the naked eye, albeit only for a few minutes.

In the ~50 day-long time series, the researchers identified 72 flares: The star spent about 7 percent of its time flaring. The researchers found signs of oscillations in the light curves of the two largest flares with a time scale of a few hours. These may be due to oscillation of the radiating plasma, or due to periodic reconnections of the magnetic field. The estimated energy of the eruptions was between 1030 and 1032 ergs. These do not reach superflare level, but according to the distribution of the observed events, flares with an energy of 1033 ergs are expected to occur three times a year, while eruptions of one magnitude larger would happen every two years.

The study, titled "The First Naked-Eye Superflare Detected from Proxima Centauri", recently appeared online. The team was led by Howard Ward, a Ph.D. candidate in physics and astronomy at the UNC Chapel Hill, with additional members from the NASA Goddard Space Flight Center, the University of Washington, the University of Colorado, the University of Barcelona and the School of Earth and Space Exploration at Arizona State University.

As they indicate in their study, solar flare activity would be one of the greatest potential threats to planetary habitability in a system like Proxima Centauri. As they explain:

"[W]hile ozone in an Earth-like planet's atmosphere can shield the planet from the intense UV flux associated with a single superflare, the atmospheric ozone recovery time after a superflare is on the order of years. A sufficiently high flare rate can therefore permanently prevent the formation of a protective ozone layer, leading to UV radiation levels on the surface which are beyond what some of the hardiest-known organisms can survive."

In the 53 day-long light curve of Proxima Cen obtained by TESS in Sectors 11 and 12 we found 72 flare events;

• The flare rate was 1.49 events per day, with 7.2% of the data being marked as flaring. The flares had an energy

output in the order of 1029 − 1032 ergs, originating from at least ≈ 4 − 30% of the stellar surface;

• A fit to the cumulative flare frequency distribution yields α = 1.81±0.03, while a maximum likelihood estimator

gives α = 1.52, in good agreement with previous findings;

• Most of the flares were multiple/complex events, two of the events showed quasiperiodic post-flare oscillations

with the time scale of a few hours, probably caused by periodic motions of the emitting plasma or oscillatory reconnection;

• Superflares (events with energy output over 1033 ergs) are expected ≈ 3 times per year, flares a magnitude

larger (with 1034 ergs) every second year – this could reduce the chances of Proxima Cen b being habitable, as the planet is only 1/20th AU from the host star, and the fluence of radiation and particles increase inversely proportional to the square of the (decreased) distance; 10 Vida et al.

• Long-term trends can be seen in both the short-cadence SAP and the long-cadence full-frame images (FFI), that

can be compatible with the ≈ 83 day-long rotation period measured earlier. Unfortunately the length of the dataset does not allow to safely confirm this period;

• No obvious signs of planetary transits were detected in the light curve.

Researchers of the Konkoly Observatory of the MTA CSFK (Budapest, Hungary), led by Krisztián Vida, investigated Proxima Centauri using the newest data from the Transiting Exoplanet Survey Satellite (TESS) space telescope. The primary task of the TESS is to search for Earth-like exoplanets around nearby brighter stars. In its initial two-year mission, it will cover almost the entire sky, spending about a month at each region. TESS observed Proxima Centauri in two sectors between April and June this year.

"In March 2016 the Evryscope detected the first-known Proxima superflare. The superflare had a bolometric energy of 10^33.5 erg, ~10× larger than any previously-detected flare from Proxima, and 30×larger than any optically measured Proxima flare. The event briefly increased Proxima's visible-light emission by a factor of 38× averaged over the Evryscope's 2-minute cadence, or ~68× at the cadence of the human eye. Although no M-dwarfs are usually visible to the naked-eye, Proxima briefly became a magnitude-6.8 star during this superflare, visible to dark-site naked-eye observers."

The superflare coincided with the three-month Pale Red Dot campaign, which was responsible for first revealing the existence of Proxima b. While monitoring the star with the HARPS spectrograph – which is part of the 3.6 m telescope at the ESO's La Silla Observatory in Chile – the campaign team also obtaining spectra on March 18th, 08:59 UT (just 27 minutes after the flare peaked at 08:32 UT).

Lin and Kaltinegger The surface pressure of rocky exoplanets is unknown. In addition the extent of atmospheric erosion Proxima b and Trappist-1e have experienced is difficult to quantify without information on the planet's magnetic field, the stellar wind pressure at the planet's orbit, and the atmospheric composition. Therefore we included models for different surface pressure, accounting for atmosphere erosion and different initial surface pressures in O’Malley-James & Kaltenegger (2019a), which is the base of our spectra shown here. While M-stars can be active (e.g. West et al. 2011) and higher amounts of UV can hit their planets, several teams have made the case that planets in the HZ of M stars can remain habitable, despite periodic high UV fluxes (see e.g. discussion in O'Malley-James & Kaltenegger 2019a, Rimmer et al. 2018, Segura et al. 2010, Scalo et al. 2007, Buccino et al. 2007, Tarter et al. 2007, Heath et al. 1999). Note that recent studies suggest that high UV surface flux may even be necessary for prebiotic chemistry to occur (see Ranjan & Sasselov 2016; Rimmer et al. 2018). like the James Webb Space Telescope (JWST) and the Extremely Large telescopes (ELTs), such as the Giant Magellan Telescope (GMT), Thirty Meter Telescope (TMT), and the Extremely Large Telescope (ELT) Here we use the 39m diameter ELT as an example for near-future ground-based telescopes. With its expected inner working angle of 6 milliarcsecond (mas) for visible wavelengths, it will be able to resolve Proxima b, with an apparent angular separation of 37 mas from its host star.

TESS Sep 2019

The age of the system (≈ 6Gyr) would also make it a good target for the search of life. Numerical models suggest that the planet could have lost about an ocean’s worth of water due to the early irradiation in the first 100–200 million years of its life (Ribas et al. 2016; Turbet et al. 2016), although the amount of initial water on the planet is unknown. After this period Proxima Cen b could either end up as a dry, atmosphereless planet by further loss its atmospheric gases, or it could keep most of the atmosphere preserving liquid water on the surface. In the latter scenario the authors concluded that liquid water may be present over the surface of the planet in the hemisphere of the planet facing the star, or in a tropical belt. According to these models, it cannot be ruled out that Proxima Cen b could be considered a viable candidate for a habitable planet – this makes the effect of external factors, like flaring activity of the host star, even more interesting. Unusually energetic flare events are often referred as ’superflares’. The exact threshold for naming an eruption superflare is somewhat arbitrary in the literature, but Kielkopf et al. (2019) suggested 1033 ergs – roughly ten times the energy output of the Carrington event on the Sun – as a reasonable threshold. Currently two of such events were observed on Proxima Cen: Howard et al. (2018) observed an eruption with a flux increase of ≈ 68 and a bolometric energy of 1033.5 ergs; and Kielkopf et al. (2019) found an other event with an estimated energy in the order of ≈ 1032 − 1033 ergs in Sloan i 0 band. According to Davenport et al. (2016), the frequency of such outbreaks Flaring activity of Proxima Centauri from TESS observations 9 is ≈ 8 times per year. From the TESS data we estimate roughly 3 superflares per year with an energy of 1033 ergs, and about one event in every two year with an energy output of 1034 ergs (note, that the TESS data were obtained roughly at the minimum of the magnetic activity cycle of Proxima Cen). These numbers are much higher, than the estimated superflare frequency for G-type stars (Shibayama et al. 2013), roughly one per thousand year (1034 − 1035 ergs), therefore, such events could have a more serious effect on their surroundings as in the case of solar-like stars, especially since the planets orbit much closer to their hosts in late-type stars as in solar-like objects. The effects of flares on exoplanetary habitability is strongly debated (cf. Lingam & Loeb 2017). UV radiation can modify, ionize, and even erode planetary atmospheres over time - leading to the photodissociation of important molecules such as water and ozone (Khodachenko et al. 2007; Yelle et al. 2008). On the other hand, planets orbiting M dwarfs may not receive enough UV flux for abiogenesis (i.e, the process by which life can arise from non-living simple organic compounds), which could be remedied by frequent flares (Ranjan et al. 2017). While Segura et al. (2010) showed that single, large flare events do not threat habitability around M-dwarfs, strong, frequent flares, can cause the planetary atmospheres to be continuously altered, making them less suitable for habitability (see Vida et al. 2017; Roettenbacher & Kane 2017, and references therein)

when UV radiation is absorbed byclose proximity of planets to their host star in the HZs of cool stars can cause planetary magnetic fields to be compressed by stellar magnetic pressure, reducing a planet’s ability to resist atmospheric erosion by the stellar wind

Certain radiation-tolerant species have demonstrated an ability to survive full solar UV in space exposure experiments (e.g. Sancho et al. 2007; Onofri et al. 2012); however, they achieve this by entering a dormant state. Therefore, although life may be able to survive on highly UV-irradiated surfaces like this, it would likely not be able to actively metabolize and complete a life cycle.

O’Malley-James Kaltenegger, Jan 2019

At the location of their planets in the HZ, the active M stars in our sample – Proxima Centauri and TRAPPIST-1 – have UV fluxes that equal, or exceed, present-day solar UV flux during flare events, while the inactive M stars have UV fluxes orders of magnitude weaker (Fig. 1). A planet’s atmospheric composition influences the surface UV environment, with thinner low-density atmospheres enabling more UV radiation to penetrate a planet’s surface due to lower column-integrated number densities of UV-absorbing gases compared to denser atmospheres of the same composition. The surface UV flux estimates for the planets (Fig. 3) show that more high-energy UV radiation reaches the ground as atmospheric thickness and ozone levels decrease. However, even though these planets in the HZs of active star systems receive higher UV fluxes than present-day Earth, their UV surface flux is lower than that of the early Earth 3.9 billion years ago (see Rugheimer et al. 2015) due to the lower top-of-atmosphere (TOA) UV flux for wavelengths larger than 200 nm from M stars compared to the Sun, even during flares. In our atmosphere models, ozone filters out the most biologically harmful UV wavelengths shortwards of about 300 nm, as on present-day Earth, decreasing in effectiveness with decreasing ozone concentration. Shortwards of about 200 nm, absorption by atmospheric CO2 filters out biologically harmful UV flux. Thus even if planets around active M stars have eroded atmospheres or do not contain ozone (anoxic), the resulting surface UV flux is still approximately an order magnitude lower than on the early Earth even for the planet orbiting the most active star in our sample, Proxima-b. The incident UV surface flux is about two orders of magnitude lower for TRAPPIST-1e. For planets orbiting inactive M stars the surface UV flux is even lower, which might result in a different concern for habitability, i.e. the question of whether such low UV surface levels could produce the macromolecular building blocks of life, assuming these require a certain minimum UV levels (Ranjan & Sasselov 2016; Rimmer et al. 2018). 3.2 Habitable surface UV environments and early Earth The high-energy surface UV (UV-B and UV-C) cut-off occurs at shorter, more biologically harmful wavelengths for similar TOA UV fluxes as atmospheric pressure decreases and ozone concentration decreases. When UV radiation is absorbed by biological molecules, especially nucleic acids, harmful effects such as mutation or inactivation can result, with shorter UV wavelengths having the most damaging effects (see e.g. Kerwin & Remmele 2007). Even though we cannot anticipate what kind of life could evolve on other worlds – assuming life could emerge on these worlds – we can explore the surface habitability of the closest potentially habitable planets in regard to known life on Earth. We use biological action spectra to show the relative biological effectiveness, i.e. a measure of biological damage at a given UV wavelength based on life as we know it, caused by a particular UV surface radiation environment (see two biological action spectra in panel A in Fig. 4). One shows the relative mortality rates at different UV wavelengths of the radiation-tolerant extremophile Deinococcus radiodurans (Setlow & Boling 1965; Calkins & Barcelo 1982). Deinococcus radiodurans is one of the most radiation-resistant organisms known on Earth

Proxima c

In the Solar system, Saturn sets the ν6 secular resonance (Ito & Malhotra 2006; Minton & Malhotra 2011), which controls the inner edge of the asteroid belt and therefore the rate of impacts from near-Earth asteroids (Morbidelli et al. 1994; Bottke et al. 2000). The relation between Saturn’s location relative to the asteroid belt and mass and the impact rate have been explored through numerical simulation (Smallwood et al. 2018). If an asteroid belt exists between Proxima b and Proxima c, Proxima c could control the rate of asteroid impacts on Proxima b. Here, we consider the risks that an asteroid belt located between Proxima b and Proxima c would pose for life on Proxima b.

Proxima b

November

Astrophysicists at the Georgia Institute of Technology modeled a theoretical twin of Earth into other star systems called binary systems because they have two stars. They concluded that 87% of exo-Earths one might find in binary systems should have axis tilts similarly steady to Earth's, an important ingredient for climate stability that favors the evolution of complex life. "Multiple-star systems are common, and about 50% of stars have binary companion stars. So, this study can be applied to a large number of solar systems," said Gongjie Li, the study's co-investigator an assistant professor at Georgia Tech's School of Physics.Single-star solar systems like our own with multiple planets appear to be rarer. In Alpha Centauri AB, star B, about the size of our sun, and the larger star, A, orbit one another at about the distance between Uranus and our sun, which is a very close for two stars in a binary system. The study modeled variations of an exo-Earth orbiting either star but concentrated on a modeled Earth orbit in the habitable zone centered around B, with A being the orbiting star. A's orbit is very elliptical, passing close by and then moving very far away from B and slinging powerful gravity, which, in the model, overpowered exo-Earth's own dynamics. Its tilt and orbit varied widely; adding our moon to the model didn't help. "Around Alpha Centauri B, if you don't have a moon, you have a more stable axis than if you do have a moon. If you have a moon, it's pretty much bad news," Quarles said. Even without a moon and with mild axis variability, complex, Earthlike evolution would seem to have a hard time on the modeled exo-Earth around B. "The biggest effect you would see is differences in the climate cycles related to how elongated the orbit is. Instead of having ice ages every 100,000 years like on Earth, they may come every 1 million years, be worse, and last much longer," Quarles said. But a sliver of hope for Earthlike conditions turned up in the model: "Planetary orbit and spin need to precess just right relative to the binary orbit. There is this tiny sweet spot," Quarles said.

June

a UC Riverside–led team discovered that a buildup of toxic gases in the atmospheres of most planets makes them unfit for complex life as we know it. "To sustain liquid water at the outer edge of the conventional habitable zone, a planet would need tens of thousands of times more carbon dioxide than Earth has today," said Edward Schwieterman, the study's lead author and a NASA Postdoctoral Program fellow working with Lyons. "That's far beyond the levels known to be toxic to human and animal life on Earth." The new study concludes that carbon dioxide toxicity alone restricts simple animal life to no more than half of the traditional habitable zone. For humans and other higher order animals, which are more sensitive, the safe zone shrinks to less than one third of that area. What is more, no safe zone at all exists for certain stars, including two of the sun's nearest neighbors, Proxima Centauri and TRAPPIST-1. The type and intensity of ultraviolet radiation that these cooler, dimmer stars emit can lead to high concentrations of carbon monoxide, another deadly gas. Carbon monoxide binds to hemoglobin in animal blood—the compound that transports oxygen through the body. Even small amounts of it can cause the death of body cells due to lack of oxygen. Carbon monoxide cannot accumulate on Earth because our hotter, brighter sun drives chemical reactions in the atmosphere that destroy it quickly.

May

A new study by researchers based at the University of Vienna and at the Space Research Institute of the ÖAW in Graz has shown that young stars can rapidly destroy the atmospheres of potentially-habitable Earth-like planets, which is a significant additional difficulty for the formation of life outside our solar system. When orbiting young stars with high activity levels, the thermospheres of planets are heated to much higher temperatures which, in extreme cases, can cause the gas to flow away from the planet. These results have significant implications for the early evolution of the Earth and for the possibility of Earth-like atmospheres forming around M-dwarfs. For the Earth, the most likely explanation for why the atmosphere was not lost is that the early atmosphere was dominated by carbon dioxide, which cools the upper atmosphere by emitting infrared radiation to space, thereby protecting it from the heating by the early Sun's high activity. The Earth's atmosphere could not have become nitrogen dominated, as it is today, until after several hundred million years when the Sun's activity decreased to much lower levels. More dramatically, the results of this study imply that for planets orbiting M-dwarf stars, they can only form Earth-like atmospheres and surfaces after the activity levels of the stars decrease, which can take up to several billion years. More likely is that many of the planets orbiting M-dwarf stars have very thin or possible no atmospheres. In both cases, life forming in such systems appears less likely than previously believed.

March

Edward W. Schwieterman, a NASA postdoctoral program fellow at the University of California, Riverside; the carbonate silicate cycle- the closer a planet is to the inner edge of the habitable zone, the less co2 is required for it. 1000 times as much co2

Jan

Harvard University, where postdoctoral researcher Manasvi Lingam and Professor Abraham Loeb Mdwarfs 400 to 750 nm Not habitable; unable to exceed the minimum UV flux that is required to ensure a biosphere similar to that of Earth.

Rocky planets orbiting red dwarf stars may be bone dry and lifeless.. a rapidly eroding dust-and-gas disk encircling the young, nearby red dwarf star AU Microscopii (AU Mic) by VLT. Fast-moving blobs of material appear to be ejecting particles from the AU Mic disk. If the disk continues to dissipate at this rapid pace, it will be gone in about 1.5 million years. In that short time, icy material from comets and asteroids could be cleared out of the disk. One theory is that powerful mass ejections from the turbulent star expelled them.

However, something important happens when planets decrease in size: As they warm, their atmospheres expand outward, becoming larger and larger relative to the size of the planet. These large atmospheres increase both the absorption and radiation of heat, allowing the planet to better maintain a stable temperature. The researchers found that atmospheric expansion prevents low-gravity planets from experiencing a runaway greenhouse effect, allowing them to maintain surface liquid water while orbiting in closer proximity to their stars. The researchers found that the critical size is about 2.7 percent the mass of Earth. If an object is smaller than 2.7 percent the mass of Earth, its atmosphere will escape before it ever has the chance to develop surface liquid water, similar to what happens to comets today. To put that into context, the moon is 1.2 percent of Earth mass and Mercury is 5.53 percent.

Numerical models have shown that Proxima Centauri b probably lost a large amount of its water in its early life stages—an amount comparable to an ocean on Earth—but despite this, it is still possible that some liquid water remained in warmer regions of the planet, maybe in a tropical belt or at the hemisphere facing the central star in case of locked rotation. This makes other factors affecting habitability, such as the magnetic activity of the host star, particularly important, as activity-related phenomena (flares, coronal mass ejections, strong UV flux) can erode a planet's atmosphere, rendering it uninhabitable in the long term.

The strong flaring activity of Proxima Centauri has already been known to astronomers, and several superflares were observed previously. During such eruptions, extremely large amounts of energy are released that may reach 1033 ergs, or 10 times the Carrington event in 1859, the strongest flare ever seen on the sun—consider such a flare from a much smaller star. In 2016, during one these superflares, the brightness of Proxima Centauri increased by a factor of 70 compared to its quiescent state —it became the only cool red dwarf visible to the naked eye, albeit only for a few minutes. Researchers of the Konkoly Observatory of the MTA CSFK (Budapest, Hungary), led by Krisztián Vida, investigated Proxima Centauri using the newest data from the Transiting Exoplanet Survey Satellite (TESS) space telescope. The primary task of the TESS is to search for Earth-like exoplanets around nearby brighter stars. In its initial two-year mission, it will cover almost the entire sky, spending about a month at each region. TESS observed Proxima Centauri in two sectors between April and June this year. In the ~50 day-long time series, the researchers identified 72 flares: The star spent about 7 percent of its time flaring. The researchers found signs of oscillations in the light curves of the two largest flares with a time scale of a few hours. These may be due to oscillation of the radiating plasma, or due to periodic reconnections of the magnetic field. The estimated energy of the eruptions was between 1030 and 1032 ergs. These do not reach superflare level, but according to the distribution of the observed events, flares with an energy of 1033 ergs are expected to occur three times a year, while eruptions of one magnitude larger would happen every two years. Such frequent, high-energy eruptions almost certainly have a severe impact on the atmosphere of Proxima Centauri b: The atmosphere probably cannot relax to a steady state between eruptions, and is continuously altered. This scenario is similar to observations in the TRAPPIST-1 system, another cool red dwarf that hosts exoplanets.

Lead author and Rice graduate student Alison Farrish and her research adviser, solar physicist David Alexander, led their group's first study to characterize the "space weather" environment of stars other than our own to see how it would affect the magnetic activity around an exoplanet. It's the first step in a National Science Foundation-funded project to explore the magnetic fields around the planets themselves. In the study published in The Astrophysical Journal, the researchers expand a magnetic field model that combines what is known about solar magnetic flux transport—the movement of magnetic fields around, through and emanating from the surface of the sun—to a wide range of stars with different levels of magnetic activity. The model is then used to create a simulation of the interplanetary magnetic field surrounding these simulated stars. In this way they were able to hypothesize the potential environment experienced by such "popular" exoplanet systems as Ross 128, Proxima Centauri and TRAPPIST 1, all dwarf stars with known exoplanets. "Depending on where it is within the extended magnetic field of the star, it is estimated that some of these habitable zone exoplanets could lose their atmospheres in as little as 100 million years," Alexander said. "That is a really short time in astronomical terms. The planet may have the right temperature and pressure conditions for habitability, and some simple lifeforms might form, but that's as far as they're going to go. The atmosphere would be stripped and the radiation on the surface would be pretty intense.

August

GJ1061, which is the 20th-closest star system, approximately 17.5 light-years away. It is classified as a small, low-mass (M dwarf) star with low volatility, suggesting it might have habitable planets. The group reports that they found evidence of three planets and possibly a fourth circling GJ1061. All three of the planets were found to be slightly larger than Earth and all three orbit close to the star—each takes just days to make its way around. The researchers focused on one planet in particular, which they named planet d. They found it took only 13 days for it to make its way around its star. The researchers calculated that such a distance puts it in the Goldilocks zone. They also note that, unfortunately, M dwarf stars tend to have a volatile history. If planet d was blasted with radiation for millions of years, it is not likely suitable to harbor life now.

November

in collaboration with researchers at the University of Colorado Boulder, NASA's Virtual Planet Laboratory and the Massachusetts Institute of Technology, discovered that only planets orbiting active stars—those that emit a lot of ultraviolet (UV) radiation—lose significant water to vaporization. Planets around inactive, or quiet, stars are more likely to maintain life-sustaining liquid water. The researchers also found that planets with thin ozone layers, which have otherwise habitable surface temperatures, receive dangerous levels of UV dosages, making them hazardous for complex surface life. "For most of human history, the question of whether or not life exists elsewhere has belonged only within the philosophical realm," said Northwestern's Howard Chen, the study's first author. "It's only in recent years that we have had the modeling tools and observational technology to address this question." "Still, there are a lot of stars and planets out there, which means there are a lot of targets," added Daniel Horton, senior author of the study. "Our study can help limit the number of places we have to point our telescopes."