Proxima Centauri b: Difference between revisions

Proxima Centauri b
	Artist's conception of Proxima Centauri b as a rocky-like exoplanet, with Proxima Centauri and the Alpha Centauri binary system in the background. The actual appearance of the planet is unknown.
Discovery
Discovered by	Anglada-Escudé et al.
Discovery site	European Southern Observatory
Discovery date	24 August 2016
Detection method	Doppler spectroscopy
Orbital characteristics
Semi-major axis	0.0485+0.0041; −0.0051 AU
Orbital period (sidereal)	11.186+0.001; −0.002 Earth day
Argument of periastron	310 ± 50
Semi-amplitude	1.38 ± 0.21
Star	Proxima Centauri
Physical characteristics
Mean radius	1.30+1.20; −0.62R🜨
Mass	≥1.27+0.19; −0.17 M🜨
Temperature	Teq: 234 K (−39 °C; −38 °F)

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Revision as of 16:40, 4 December 2021

Proxima Centauri b (or Proxima b^[3]) is an exoplanet orbiting in the habitable zone of the red dwarf star Proxima Centauri, which is the closest star to the Sun and part of a triple star system with Alpha Centauri. It is approximately c. 4.2 ly from Earth in the constellation Centaurus, making it and Proxima c the closest known exoplanets to the Solar System.

Proxima Centauri b orbits the star at a distance of roughly 0.0485+0.0161
−0.0051 au with an orbital period of approximately 11.186+0.001
−0.002 Earth day. Its other properties are only poorly understood but it is believed to be an Earth-like planet with a mass of 1.27+0.19
−0.17 M_🜨, although this is a lower boundary. Whether it is actually habitable is a complex function of a number of unknown properties, such as whether it has an atmosphere. Proxima Centauri is a flare star with intense emission of electromagnetic radiation that could rip an atmosphere right off the planet. The planet's proximity to Earth offers an opportunity for robotic space exploration, for example with the Breakthrough Starshot project.

Research history

Proxima Centauri had become a target for exoplanet searches already before the discovery of Proxima Centauri b, but initial studies in 2008 and 2009 ruled out the existence of larger-than-Earth exoplanets in the habitable zone.^[4] Planets are very common around dwarf stars, with on average 1-2 planets per star,^[5] and about 20-40% of all red dwarfs have one in the habitable zone.^[6] Additionally, red dwarfs are by far the most common types of stars.^[7]

Before 2016, observations with instruments^[a] at the European Southern Observatory in Chile had identified anomalies in Proxima Centauri^[8] which could not be satisfactorily be explained by flares^[b] or chromospheric^[c] activity of the star. Anglada-Escudé et al. 2016 proposed that an exoplanet in the habitable zone of Proxima Centauri could explain these anomalies.^[11] In 2020, another planet Proxima Centauri c was discovered,^[12] while the existence of a dust belt around Proxima Centauri and of a third planet were as of 2021^[update] unconfirmed.^[13] The discovery of Proxima Centauri b, a planet at habitable distances from the closest star to the Solar System, was a major discovery in planetology^[14] and has drawn interest to the Alpha Centauri star system that Proxima is a member of.^[15]

Physical properties

Proxima Centauri b is the closest exoplanet to Earth,^[16] being at a distance of c. 4.2 ly.^[3] It orbits Proxima Centauri every 11.186+0.001
−0.002 Earth day at a distance of 0.0485+0.0161
−0.0051 au,^[11] over 20 times closer to Proxima Centauri than Earth is to the Sun.^[17] as of 2021^[update] it is unclear if it actually has an eccentricity^[d]^[20] but Proxima Centauri b is unlikely to have any obliquity.^[21] The age of the planet is unknown;^[22] Proxima Centauri itself may have been captured by Alpha Centauri and thus not necessarily of the same age as the latter, which are about 5 billion years old.^[13] Proxima Centauri b is unlikely to have stable orbits for moons.^[23]

The estimated mass of Proxima Centauri b is 1.27+0.19
−0.17 M_🜨 as estimated by the original discoverers;^[11] more recent estimates as of 2020^[update] are similar^[24] but all estimates are dependent on the inclination of the planet's orbit and may be underestimates.^[13] This makes it similar to Earth, but the radius of the planet is unknown and hard to determine^[25] and the mass borders on the cutoff between Earth-type and Neptune-type planets.^[5] Depending on the composition, Proxima Centauri b could either be a Mercury-like planet with a large core - which would require particular conditions early in the planet's history - to a very water-rich planet. Observations of the Template:Iron-Template:Silicon-Template:Magnesium ratios of Proxima Centauri may allow a determination of the composition of the planet^[26] since they are expected to roughly match these of the planets; various observations have found Solar System-like ratios of these elements.^[27]

Relatively little is known about Proxima Centauri b as of 2021^[update] - mainly its distance from the star and its orbital period -^[28] but a number of simulations of its properties have been made.^[13] A number of simulations and models have been created that assume Earth-like compositions^[29] and include predictions of the galactic environment, internal heat generation from radioactive decay and magnetic induction heating^[e], planetary rotation, the effects of stellar radiation, the amount of volatile species the planet consists of and the changes of these parameters over time.^[27]

Proxima Centauri b likely developed under different conditions than Earth, with less water, stronger impacts and an overall faster development assuming that it formed at its current distance from the star.^[31] Proxima Centauri b probably did not form at its current distance to Proxima Centauri, as the amount of material in the protoplanetary disk would be insufficient. Instead, it or fragments formed at larger distances and then migrated to the current orbit of Proxima Centauri b. Depending on the nature of the precursor material, it may be rich in volatiles.^[11] A number of different formation scenarios are possible, many of which depend on the existence of other planets around Proxima Centauri and which would result in different compositions.^[32]

Tidal locking

Proxima Centauri b is likely to be tidally locked to the host star,^[23] which for an 1:1 orbit would mean that the same side of the planet would always face Proxima Centauri.^[22] It is unclear if habitable conditions can arise under such circumstances^[33] as an 1:1 tidal lock would lead to an extreme climate with only part of the planet habitable.^[22]

However, the planet may not be tidally locked. If the eccentricity of Proxima Centauri b was higher than 0.1^[34]-0.06, it would tend to enter a Mercury-like 3:2 resonance^[f] or higher-order resonances such as 2:1.^[35] Additional planets around Proxima Centauri and interactions^[g] with Alpha Centauri could excite higher eccentricies.^[36] If the planet isn't symmetrical (triaxial), a capture into a non-tidally locked orbit would be possible even with low eccentricity.^[37] A non-locked orbit however would result in tidal heating of the planet's mantle, increasing volcanic activity and potentially shutting down a magnetic field-generating dynamo.^[38] The exact dynamics are strongly dependent on the internal structure of the planet and its evolution in response to tidal heating.^[39]

Star

Proxima Centauri is a red dwarf^[35] with a mass equivalent to 0.120±0.015 Solar masses and a radius of 0.141±0.021. With an effective temperature^[h] of 3050+100
−100 kelvin, it has a spectral type^[i] of M5.5V and a luminosity 0.00155±0.00006 of the Sun.^[11] Proxima Centauri is a flare star and its luminosity varies by a factor of 100 over a timespan of hours.^[42] The magnetic field of Proxima Centauri is considerably stronger than that of the Sun, with an intensity of 600±150 Gauss;^[1] it varies in a 7-year long cycle.^[43]

It is the closest star to the Sun^[j], with a distance of 4.2426 ± 0.0020 light-years (1.3008 ± 0.0006 pc). Proxima Centauri is part of a multiple star system, whose other members are Alpha Centauri A and Alpha Centauri B which form a binary star subsystem.^[44] The dynamics of the multiple star system could have caused Proxima Centauri b to move closer to its host star over its history.^[45] The detection of a planet around Alpha Centauri in 2012 is considered questionable.^[44] Despite its proximity to Earth, Proxima Centauri is too faint to be visible to the naked eye^[4] with the exception of an instance where a flare made it visible to the naked eye.^[46]

Atmosphere and climate

Proxima Centauri b is located within the classical habitable zone of its star;^[47] it receives about 65% of Earth's irradiation. Its equilibrium temperature is about 234+6
−14 K.^[11] Various factors, such as the orbital properties of Proxima Centauri b, the spectrum of radiation emitted by Proxima Centauri^[k] and the behaviour of clouds^[l] and hazes influence the climate of an atmosphere-bearing Proxima Centauri b.^[52]

There are two likely scenarios for the atmosphere of Proxima Centauri b, one rich in oxygen and/or carbon dioxide if large amounts of water were converted to oxygen during the early phases of Proxima Centauri. and the hydrogen lost. Another when the planet initially featured a hydrogen-rich atmosphere or originated farther away from Proxima Centauri;^[53] this would have reduced the escape of water and allowed it to persist on the planet.^[45] If an atmosphere exists, it is likely to contain oxygen-bearing compounds such as oxygen and carbon dioxide. Together with the star's magnetic activity, they would give rise to aurorae that could be observed from Earth^[54] if the planet has a magnetic field.^[55]

Climate models including general circulation models used for Earth climate^[56] have been used to simulate the properties of Proxima Centauri b's atmosphere. Depending on its properties such as whether it's tidally locked, the amount of water and carbon dioxide a number of scenarios are possible: Planets partially or wholly covered with ice, planet-wide or small oceans or only dry land, combinations between these^[57] or scenarios with one or two "eyeballs"^[m]^[59] or lobster-shaped areas with liquid water.^[60] Additional factors are the nature of convection,^[61] the distribution of continents, which can sustain a carbonate-silicate cycle and thus stabilize the atmospheric carbon dioxide concentrations,^[62] ocean heat transport which broadens the space for habitable climates, salinity variations that alter the properties of an ocean,^[59] the rotational period of the planet which determines Rossby wave dynamics^[63] and sea ice dynamics which could cause a global ocean to freeze over.^[64]

Stability of an atmosphere

The stability of an atmosphere is a major issue for the habitability of Proxima Centauri b:^[65]

Strong irradiation by UV radiation and X-rays from Proxima Centauri constitutes a challenge to habitability.^[16] Proxima Centauri b receives about 10-60 times as much of this radiation as Earth^[47] with a particular increase in the X-rays^[66] and might have received even more in the past,^[67] adding up to 7-16 times as much cumulative XUV radiation than Earth.^[68] UV radiation and X-rays can effectively evaporate an atmosphere^[17] since hydrogen readily absorbs the radiation and does not readily lose it again, thus warming until the speed of hydrogen atoms and molecules is sufficient to escape from the gravitational field of a planet.^[69] They can remove water by splitting it into hydrogen and oxygen and heating the hydrogen in the planet's exosphere until it escapes. The hydrogen can drag other elements such as oxygen^[70] and nitrogen away.^[71] Nitrogen and carbon dioxide can escape on their own from an atmosphere but this process is unlikely to substantially reduce the nitrogen and carbon dioxide content of an Earth-like planet.^[72]
Stellar winds and coronal mass ejections are an even bigger threat to an atmosphere.^[17] The amount of stellar wind impacting Proxima Centauri b may amount to 4-80 times that impacting Earth.^[68] The more intense UV and X-rays radiation could lift the planet's atmosphere to outside of the magnetic field, increasing the loss triggered by stellar wind and mass ejections.^[73]
At Proxima Centauri b's distance from the star, the stellar wind is likely to be denser than around Earth by a factor of 10-1000 depending on the strength of Proxima Centauri's magnetic field.^[74] As of 2018^[update] it is unknown whether the planet has a magnetic field^[16] and the upper atmosphere may have its own magnetic field.^[73] Depending on the intensity of Proxima Centauri b's magnetic field, it can penetrate deep into the atmosphere of the planet and strip parts of it off,^[75] with substantial variability over daily and annual timescales.^[74]
If the planet is tidally locked to the star, the atmosphere can collapse on the night side.^[76] This is particularly a risk for a carbon dioxide-dominated atmosphere although carbon dioxide glaciers could recycle.^[77]
Unlike the Sun, Proxima Centauri's habitable zone would have been farther away early in the star's (and Proxima Centauri b's) existence^[78] when the star was in its pre-main sequence^[n] stage.^[79] In the case of Proxima Centauri, assuming that the planet formed in its current orbit it could have spent (169±13)×10⁶ y too close to the star for water to condense.^[45] Thus the planet could have ended up inside of it and overheated and desiccated like Venus;^[80] the water of Proxima Centauri b would evaporate, forming steam,^[81] which would then be cleaved by UV radiation into oxygen and hydrogen and the hydrogen subsequently lost.^[45]
While the characteristics of impact events on Proxima Centauri b are currently entirely conjectural, they could destabilize the atmospheres^[82] and boil off oceans.^[12]

Even if Proxima Centauri b loses its original atmosphere, volcanic activity could rebuilt it after some time. A second atmosphere would likely contain carbon dioxide,^[33] which would form a more stable atmosphere than an Earth-like atmosphere would be.^[27] In the case of Earth, the amount of water contained within the mantle might approach that of one Earth ocean.^[38] Additionally, impacts of exocomets could resupply water to Proxima Centauri b, if they are present.^[83]

Delivery of water to Proxima Centauri b

A number of mechanisms can deliver water to a developing planet; how much water Proxima Centauri b received is unknown.^[31] Modelling by Ribas et al. 2016 indicates that Proxima Centauri b would have lost no more than one Earth ocean equivalent of water^[16] but later research suggested that the amount of water lost could be considerably larger^[84] and Airapetian et al. 2017 concluded that an atmosphere would be lost within ten million years.^[85] The estimates are strongly dependent on the initial mass of the atmosphere, however, and are thus highly uncertain.^[38]

Life

In the context of exoplanet research, "habitability" is usually defined as the possibility that liquid water exists on the surface of a planet.^[53] As normally understood in the context of exoplanet life, liquid water on the surface and an atmosphere are prerequisites for habitability - any life limited to the sub-surface of a planet,^[78] such as in a subsurface ocean like in Europa in the Solar System, would be difficult to detect from afar^[79] although it may constitute a model for life in a cold ocean-covered Proxima Centauri b.^[86]

The habitability of red dwarfs is a controversial subject,^[22] with a number of considerations:

Both the activity of Proxima Centauri and tidal locking would hinder the establishment of these conditions.^[11]
Unlike XUV radiation, UV radiation on Proxima Centauri b is redder (colder) and thus may interact less with organic compounds^[87] and may produce less ozone.^[88] Conversely, stellar activity could deplete an ozone layer sufficiently to increase UV radiation to dangerous levels.^[38]^[89]
Depending on its eccentricity, it may partially lie outside of the habitable zone during part of its orbit.^[22]
Oxygen^[90] and/or carbon monoxide may built up in the atmosphere of Proxima Centauri b to toxic quantities.^[91] High oxygen concentrations may however aid in the evolution of complex organisms.^[90]
If oceans are present, the tides could alternately flooding and drying coastal landscapes, triggering chemical reactions conducive to the development of life,^[92] favour the evolution of biological rhythms such as the day-night cycle which otherwise would not develop in a tidally locked planet without a day-night cycle,^[93] mix oceans and supply and redistribute nutrients^[94] and stimulate periodic expansions of marine organisms such as red tides on Earth.^[95]

On the other hand, red dwarfs like Proxima Centauri have lifespans much longer than these of the Sun, up to many times the estimated age of the Universe, and thus give life plenty of time to develop.^[96] The radiation emitted by Proxima Centauri is ill-suited for oxygen-generating photosynthesis but sufficient for anoxygenic photosynthesis^[97] although it is unclear how life depending on anoxygenic photosynthesis could be detected.^[98] One study in 2017 estimated that the productivity of a Proxima Centauri b ecosystem based on photosynthesis may be about 20% that of Earth's.^[99]

Observation and exploration

As of 2021^[update], Proxima Centauri b has not yet been directly imaged, as its separation from Proxima Centauri is too small.^[100] It is unlikely^[o] and to pass across the disk of Proxima Centauri.^[101] The star is monitored for the possible emission of technology-related radio signals by the Breakthrough Listen project which in April-May 2019 detected the blc1 signal; later investigations indicated it is probably of human origin.^[102]

Future large ground-based telescopes and space-based observatories such as the James Webb Space Telescope and the Wide-Field Infrared Survey Telescope could directly observe Proxima Centauri b, given its closeness to Earth,^[17] but disentangling the planet from its star would be difficult.^[33] Possible traits observable from Earth are the reflection of starlight from an ocean,^[103] the radiative patterns of atmospheric gases and hazes^[104] and of atmospheric heat transport^[p].^[105] Efforts have been done to determine how Proxima Centauri b would look like to Earth if it has particular properties such as atmospheres of a particular composition.^[28]

Even fast spacecraft take a long time to travel interstellar distances; the probe Voyager 2 would take about 75,000 years. Among the proposed technologies to reach Proxima Centauri b in human lifespans are solar sails that could reach speeds of 0.2 of the speed of light; problems would be how to decelerate a probe when it arrives in the Proxima Centauri system^[106] and collisions of the high-speed probes with interstellar particles.^[107] Among the projects of travelling to Proxima Centauri b are the Breakthrough Starshot project, which aims to develop instruments and power systems that can reach Proxima Centauri in the 21st century.^[108]

View from Proxima Centauri b

Looking towards the sky around Orion from Alpha Centauri with Sirius near Betelgeuse, Procyon in Gemini, and the Sun between Perseus and Cassiopeia generated by Celestia

From Proxima Centauri b, Alpha Centauri would be considerably brighter than Venus is from Earth.^[109]

Diagrams

An angular size comparison of how Proxima will appear in the sky seen from Proxima b, compared with how the Sun appears in our sky on Earth. Proxima is much smaller than the Sun, but Proxima b is very close to its star.
The relative sizes of a number of objects, including the three stars of the Alpha Centauri triple system and some other stars for which the angular sizes have also been measured. The Sun and Jupiter are also shown for comparison.
This chart shows the large southern constellation of Centaurus (the Centaur) and shows most of the stars visible with the naked eye on a clear dark night. The location of the closest star to the Solar System, Proxima Centauri, is marked with a red circle. Proxima Centauri is too faint to see with the unaided eye but can be found using a small telescope.
This picture combines a view of the southern skies over the ESO 3.6-metre telescope at the La Silla Observatory in Chile with images of the stars Proxima Centauri (lower-right) and the double star Alpha Centauri AB (lower-left) from the NASA/ESA Hubble Space Telescope. Proxima Centauri is the closest star to the Solar System and is orbited by the planet Proxima b.

Videos

A numerical simulation of possible surface temperatures on Proxima b performed with the Laboratoire de Météorologie Dynamique's Planetary Global Climate Model. Here it is hypothesized that the planet possesses an Earth-like atmosphere and that it is covered by an ocean (the dashed line is the frontier between the liquid and icy oceanic surface). Two models were produced for the planet's rotation. Here the planet is in a so-called 3:2 resonance (a natural frequency for the orbit), and is seen as a distant observer would do during one full orbit.
A numerical simulation of possible surface temperatures. Here it is hypothesized that the planet possesses an Earth-like atmosphere and that it is covered by an ocean (the dashed line is the frontier between the liquid and icy oceanic surface). Here the planet is in synchronous rotation (like the Moon around the Earth), and is seen as a distant observer would do during one full orbit.

Notes

^ The Ultraviolet and Visual Echelle Spectrograph and the High Accuracy Radial Velocity Planet Searcher.^[8]
^ Flares are presumably magnetic phenomena during which for minutes and hours parts of the star emit more radiation than usual.^[9]
^ The chromosphere is an outer layer of a star.^[10]
^ Proxima Centauri b's eccentricity is constrained to be less than 0.35^[11] and later observations have indicated eccentricities of 0.08+0.07
−0.06,^[18] 0.17+0.21
−0.12 and 0.105+0.091
−0.068^[19]
^ Tides may result in internal heating in Proxima Centauri b; depending on the eccentricity Io-like values with intense volcanic activity or Earth-like values could be reached.^[30] The magnetic field of the star can also induce intense heating of the planet's interior.^[27]
^ A 3:2 ratio of the planet's rotation and its orbit around the star.^[22]
^ The tides excited by Alpha Centauri could have induced an eccentricity of 0.1.^[30]
^ The effective temperature is the temperature a black body that emits the same amount of radiation would have.^[40]
^ A spectral type is a scheme to categorize stars by their temperature.^[41]
^ Hence the name "Proxima".^[15]
^ The radiation of a red dwarf is much less effectively reflected by snow, ice^[35] and clouds^[48] although - in the case of ice - the formation of salt-bearing ice (hydrohalite) could offset this effect.^[49] It also does not as readily degrade trace gases like methane, dinitrogen monoxide and methylchloride as the Sun's.^[50]
^ For example, cloud accumulation below the star in the case of a tidally locked planet^[37] stabilizes the climate by increasing the reflection of starlight.^[51]
^ One or multiple areas of liquid water surrounded by ice.^[58]
^ During the pre-main sequence, the star is brighter than before it settles into the main sequence.^[45]
^ The probability is about 1.5%.^[28]
^ If there is an atmosphere or ocean and Proxima Centauri b is tidally locked, an atmosphere or an ocean would tend to redistribute heat from the day-side to the night-side and this would be visible from Earth.^[105]

References

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External links

A search for Earth-like planets around Proxima Centauri
The habitability of Proxima Centauri b – Pale Red Dot website for future updates
"ESOcast 87: Pale Red Dot Results" – via YouTube.
"Interviews with Pale Red Dot scientists" – via YouTube.
"Press Conference at ESO HQ" – via YouTube.

[9] The Ultraviolet and Visual Echelle Spectrograph and the High Accuracy Radial Velocity Planet Searcher.^[8]

[11] Flares are presumably magnetic phenomena during which for minutes and hours parts of the star emit more radiation than usual.^[9]

[13] The chromosphere is an outer layer of a star.^[10]

[23] Proxima Centauri b's eccentricity is constrained to be less than 0.35^[11] and later observations have indicated eccentricities of 0.08+0.07
−0.06,^[18] 0.17+0.21
−0.12 and 0.105+0.091
−0.068^[19]

[35] Tides may result in internal heating in Proxima Centauri b; depending on the eccentricity Io-like values with intense volcanic activity or Earth-like values could be reached.^[30] The magnetic field of the star can also induce intense heating of the planet's interior.^[27]

[40] A 3:2 ratio of the planet's rotation and its orbit around the star.^[22]

[42] The tides excited by Alpha Centauri could have induced an eccentricity of 0.1.^[30]

[48] The effective temperature is the temperature a black body that emits the same amount of radiation would have.^[40]

[50] A spectral type is a scheme to categorize stars by their temperature.^[41]

[53] Hence the name "Proxima".^[15]

[61] The radiation of a red dwarf is much less effectively reflected by snow, ice^[35] and clouds^[48] although - in the case of ice - the formation of salt-bearing ice (hydrohalite) could offset this effect.^[49] It also does not as readily degrade trace gases like methane, dinitrogen monoxide and methylchloride as the Sun's.^[50]

[63] For example, cloud accumulation below the star in the case of a tidally locked planet^[37] stabilizes the climate by increasing the reflection of starlight.^[51]

[71] One or multiple areas of liquid water surrounded by ice.^[58]

[92] During the pre-main sequence, the star is brighter than before it settles into the main sequence.^[45]

[115] The probability is about 1.5%.^[28]

[121] If there is an atmosphere or ocean and Proxima Centauri b is tidally locked, an atmosphere or an ocean would tend to redistribute heat from the day-side to the night-side and this would be visible from Earth.^[105]

[FOOTNOTEAnglada-EscudéAmadoBarnesBerdiñas2016439-1] Anglada-Escudé et al. 2016, p. 439.

[FOOTNOTETaskerMatthieu202041-2] Tasker & Matthieu 2020, pp. 41.

[FOOTNOTETurbetLeconteSelsisBolmont20161-3] Turbet et al. 2016, p. 1.

[FOOTNOTEKippingCameronHartmanDavenport20171-4] Kipping et al. 2017, p. 1.

[FOOTNOTEKippingCameronHartmanDavenport20172-5] Kipping et al. 2017, p. 2.

[FOOTNOTEWandel2017498-6] Wandel 2017, p. 498.

[FOOTNOTEMeadowsArneySchwietermanLustig-Yaeger2018133-7] Meadows et al. 2018, p. 133.

[FOOTNOTEAnglada-EscudéAmadoBarnesBerdiñas2016437-8] Anglada-Escudé et al. 2016, p. 437.

[FOOTNOTEGüdel20149-10] Güdel 2014, p. 9.

[FOOTNOTEGüdel20146-12] Güdel 2014, p. 6.

[FOOTNOTEAnglada-EscudéAmadoBarnesBerdiñas2016438-14] ^ ^a ^b ^c ^d ^e ^f ^g ^h Anglada-Escudé et al. 2016, p. 438.

[FOOTNOTESirajLoeb20201-15] Siraj & Loeb 2020, p. 1.

[FOOTNOTENoackKislyakovaJohnstoneGüdel20211-16] Noack et al. 2021, p. 1.

[FOOTNOTEMascareñoFariaFigueiraLovis20201-17] Mascareño et al. 2020, p. 1.

[FOOTNOTEQuarlesLissauer20181-18] Quarles & Lissauer 2018, p. 1.

[FOOTNOTESchulze-MakuchIrwin2018240-19] Schulze-Makuch & Irwin 2018, p. 240.

[FOOTNOTEGarraffoDrakeCohen20161-20] Garraffo, Drake & Cohen 2016, p. 1.

[FOOTNOTEWalterováBěhounková202013-21] Walterová & Běhounková 2020, p. 13.

[FOOTNOTEMascareñoFariaFigueiraLovis20208-22] Mascareño et al. 2020, p. 8.

[FOOTNOTENoackKislyakovaJohnstoneGüdel20219-24] Noack et al. 2021, p. 9.

[FOOTNOTEGarraffoDrakeCohen20162-25] Garraffo, Drake & Cohen 2016, p. 2.

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v t e The Alpha Centauri system
Stars	Rigil Kentaurus (A) Toliman (B) Proxima Centauri (C)
Planets	Ab? Bb^† Bc? Cd Cb Cc?
Proposed mission concepts	2069 Alpha Centauri mission AIMStar Breakthrough Starshot Project Longshot Starlight
^† - Disproven Category

v t e 2016 in space
« 2015 2017 »
Space probe launches	Hitomi (X-ray observatory; Feb 2016) ExoMars Trace Gas Orbiter (Mars orbiter; Mar 2016) OSIRIS-REx (asteroid sample-return mission; Sep 2016)
Impact events	DN160822 03
Selected NEOs	Asteroid close approaches (85990) 1999 JV6 1994 WR12 2013 TX68 2016 EU85 469219 Kamoʻoalewa 2016 PQ (164121) 2003 YT1
Exoplanets	38 Virginis b atmospheric composition of 55 Cancri e BD+20 594b Gliese 536 b HD 19467 b HD 131399 Ab (retracted in 2022) HD 164922 c HIP 41378 b c d e f K2-33b K2-332 b K2-72 b c d e KELT-9b KELT-11b Kepler-20g Kepler-56d Kepler-737b Kepler-1229b Kepler-1520b Kepler-1625b Kepler-1638b Kepler-1647b OGLE-2007-BLG-349(AB)b OGLE-2012-BLG-0950Lb Proxima Centauri b Pr0211 c Qatar-3b Qatar-4b Qatar-5b TRAPPIST-1 b c d TW Hydrae b V830 Tauri b 2MASS J11193254–1137466 AB 2MASS J2126–8140 as planet
Discoveries	GW150914 (announced) IDCS 1426 Planet Nine (hypothesized) GN-z11 SDSS J1240+6710 Crater 2 GW151226 (announced) 2015 RR245 (announced) BOSS Great Wall GRB 160625B (471325) 2011 KT19 (announced) 2014 UZ224 (announced) WISE J080822.18-644357.3 Virgo I
Novae	ASASSN-15lh SN 2016aps ASASSN-16kt ASASSN-16ma
Comets	252P/LINEAR 460P/PanSTARRS 53P/Van Biesbroeck C/2016 R2 (PANSTARRS) 81P/Wild 9P/Tempel 144P/Kushida 43P/Wolf–Harrington 45P/Honda–Mrkos–Pajdušáková
Space exploration	Juno (entered Jupiter orbit; Jul 2016) Rosetta / Philae (end of mission to comet 67P; Sep 2016) Schiaparelli EDM (Mars lander; Oct 2016)
Outer space portal Category:2015 in outer space — Category:2016 in outer space — Category:2017 in outer space