Habitability of red dwarf systems

An artist's impression of a planet in orbit around a red dwarf.

This artist's concept illustrates a young red dwarf star surrounded by three planets.

Determining the habitability of red dwarf systems could help reveal the likelihood of extraterrestrial life, as red dwarfs make up most stars in the Milky Way Galaxy. While the relatively little energy output, small habitable zones, probability of tidally locked planets, and high stellar variation are postulated impediments to habitability, the ubiquity and longevity of red dwarfs are possible positive factors.

As of 2012, many factors appear to indicate that many red dwarfs, smaller than 30% of Sun's mass, have a very low probability for hosting indigenous life. Planets in the habitable zone of most red dwarfs would experience such a strong tidal heating that the hydrogen necessary for water and all known life would be 'baked out' of the planets before a stable orbit could be achieved, creating so-called 'Tidal Venuses'. Combined with other problems, such as those created by tidal locking, the variable radiation of red dwarfs, lack of planetary axial tilts, small habitable zones due to low energy output, different spectral energy distribution than the Sun (lacking ultraviolet and visible light), etc., this would indicate that the probability of red dwarf stars hosting life as we know it is very low compared to other star types. However, this does not mean that the planets around red dwarfs are 'uninhabitable' (could not be inhabited by future humans or other non-indigenous life).

A recent estimate put the number of super-Earth-sized planets in the habitable zones of red dwarf stars in our galaxy to number in the tens of billions. So although very improbable, the sheer number of red dwarf planets might overcome the limitations of red dwarf systems, and further scientific research and insights into astrobiology may also increase the odds of finding life around M-class stars.

Red dwarf characteristics

Red dwarfs are the smallest, coolest, and most common type of star. Estimates of their abundance range from 70% of stars in spiral galaxies to more than 90% of all stars in elliptical galaxies, an often quoted median figure being 73% of the stars in our Milky Way galaxy (known since the 1990s from radio telescopic observation to be a barred spiral). Red dwarfs are either late K or M spectral type. Given their low energy output, red dwarfs are never visible by the unaided eye from Earth; neither the closest red dwarf star to the Sun when viewed individually, Proxima Centauri (which is also the closest star to the Sun), nor the closest solitary red dwarf, Barnard's star, is anywhere near visual magnitude.

Research

Light emission and tidal lock

Relative star sizes and photospheric temperatures. Any planet around a red dwarf, such as the one shown here, would have to huddle close to achieve Earth-like temperatures, probably inducing tidal lock. See Aurelia.

For many years, astronomers ruled out red dwarf systems as potential abodes for life. Their small size (from 0.1 to 0.6 solar masses) means that their nuclear reactions proceed exceptionally slowly and they emit very little light (from 3% of that produced by the Sun to as little as 0.01%). Any planet in orbit around a red dwarf would have to orbit very close to its parent star to attain Earth-like surface temperatures; from 0.3 AU (just inside the orbit of Mercury) for a star like Lacaille 8760, to as little as 0.032 AU for a star like Proxima Centauri (such a world would have a year lasting just 6.3 days).

Planets that are close enough to red dwarfs to receive a sufficient amount of radiation for liquid water are most likely to have long been tidally locked to their respective stars so that the planet rotates only once for every time it completes an orbit; this means that one face always points at the star (creating perpetual day) and one face always points away (creating perpetual night). Potential life could be limited to a ring-like region, known as the terminator, where the sun would always appear on the horizon.

Should the planet have a moon massive enough to maintain an Earthlike atmosphere, however, the moon could be tidally locked with the more massive planet instead of with the star, and as such it could have a day-and-night cycle, increasing chances of habitability on the moon. Tidal forces between the two bodies would also keep the centers of both the planet and its moon liquid, which should result in a strong enough magnetic field to protect the planet and its moon from outbursts coming from the parent star.

The tidally locked planet would probably need an atmosphere thick enough to transfer some of the star's heat from the day side to the night side; this would prevent the colder, night side's atmospheric temperature from dropping below the condensation point, causing a drop in atmospheric pressure that would draw more of the atmosphere towards the night side until all of the atmosphere gets frozen on the night side. It was long assumed that an atmosphere would need to be so thick as to impede photosynthesis from any plants on the day side surface. However, more recent research has suggested otherwise. A 2010 study concluded that Earth-like aquaplanets tidally locked to their stars would still have temperatures above -33 Celsius on the night side. Studies by Robert Haberle and Manoj Joshi of NASA's Ames Research Center in California have shown that a planet's atmosphere (assuming it included greenhouse gases CO₂ and H₂O) need only be 100 mb, or 10% of Earth's atmosphere, for the star's heat to be effectively carried to the night side. This is well within the levels required for photosynthesis on the day side, though some of their models still had water frozen on the dark side. Martin Heath of Greenwich Community College has shown that seawater, too, could effectively circulate without freezing solid if the ocean basins were deep enough to allow free flow beneath the night side's ice cap. Geothermal heat might also help keep the lower parts of any ocean liquid.^{[citation needed]} Further research—including a consideration of the amount of photosynthetically active radiation—has suggested that tidally locked planets in red dwarf systems might at least be habitable for higher plants.

Compared to Sun-like stars, red dwarfs produce more wavelengths of light that are absorbed by water ice, which would increase the temperature of icy bodies, thereby extending the previously thought HZ around Red dwarfs.

However, a 2012 study at University of Washington revealed that planets in the habitable zone of red dwarfs would experience such a strong geological tidal heating that the hydrogen necessary for water and all known life would be 'baked out' of the planets before a stable orbit could be achieved, creating so-called 'Tidal Venuses'. This holds for red dwarfs with masses less than 30% of the Sun's mass. Tidal heating shifts the habitable zone outward. Orbit instability complicates the issue. Combined with the other problems of tidal locking, variable radiation, axial tilts, etc., this would make the probability of many red dwarf stars hosting life as we know it very low compared to other star types.

Photosynthesis

Size and brightness are not the only factors in making red dwarfs potentially unsuitable for life. If the planet is tidally locked, on the day side, because the sun does not rise or set, areas in the shadows of mountains would remain so forever. Photosynthesis as we understand it would be complicated by the fact that a red dwarf produces most of its radiation in the infrared, and on the Earth the process depends on visible light. Photosynthesis on a red dwarf planet would require additional photons to achieve excitation potentials comparable to those needed in Earth photosynthesis for electron transfers, due to the lower average energy level of near-infrared photons compared to visible. Having to adapt to a far wider spectrum to gain the maximum amount of energy, foliage on a habitable red dwarf planet would probably appear black if viewed in visible light. Because water strongly absorbs red light, less energy is available for possible ocean life.

Weather conditions and habitability

Due to differential heating, a tidally locked planet would experience fierce winds blowing continually towards the night side^{[citation needed]} with permanent torrential rain at the point directly opposite the local star^{[citation needed]}, the solar pole. In the opinion of one author this makes complex life improbable. But the scientists who worked on the Aurelia and Blue Moon series disagree.

Plant life would have to adapt to this constant gale, e.g. by anchoring securely into the soil and sprouting long flexible leaves that do not snap. Plants would probably be less productive in the dim red sunlight, so consequently there would be less oxygen in the atmosphere and animal life would be constrained in size. Animals would probably rely on infrared vision as signaling by calls or scents would be difficult over the din of the planet-wide gale. Underwater life would, however, be protected from fierce winds and flares, and vast blooms of black photosynthetic plankton and algae could support the sea life.

There may not even be enough water for habitable planets around many red dwarf stars.

Variability

Red dwarfs are far more variable and violent than their more stable, larger cousins. Often they are covered in starspots that can dim their emitted light by up to 40% for months at a time. On Earth life has adapted in many ways to the similarly reduced temperatures of the winter. Life may survive by hibernating and/or by diving into deep water where temperatures could be more constant. More serious is that the oceans could perhaps freeze over during cold periods. After the cold has ended the planet’s albedo would be higher causing light from the red dwarf to be reflected. This could cause conditions similar to Snowball Earth so cold could last millions of years.^{[citation needed]}

At other times, red dwarf stars emit gigantic flares that can double their brightness in a matter of minutes. Indeed, as more and more red dwarfs have been scrutinized for variability, more of them have been classified as flare stars to some degree or other. Such variation in brightness could be very damaging for life. Flares might also produce torrents of charged particles that could strip off sizable portions of the planet's atmosphere. If the planet had a magnetic field, though, it would deflect the particles from the atmosphere. And even the slow rotation of a tidally locked M-dwarf planet—it spins once for every time it orbits its star—would be enough to generate a magnetic field as long as part of the planet's interior remained molten.

However, the violent flaring period of a red dwarf's lifecyle is estimated to only last roughly the first 1.2 billion years of its existence. If a planet forms far away from a red dwarf so as to avoid tidelock, and then migrates into the star's habitable zone after this turbulent initial period, it is possible that life may have a chance to develop.

Life could initially protect itself from radiation by remaining underwater until the star had passed through its early flare stage, assuming the planet could retain enough of an atmosphere to produce liquid oceans. The scientists who wrote Aurelia believed that life could survive on land despite a red dwarf star flaring. Once life reached onto land, the low amount of UV produced by a quiescent red dwarf means that life could thrive without an ozone layer, and thus never need to produce oxygen.

Other scientists disagree that red dwarf stars could sustain life. See Rare Earth hypothesis. Tidal-locking would probably result in a relatively low planetary magnetic moment. Active red dwarfs that emit coronal mass ejections would bow back the magnetosphere until it contacted the planetary atmosphere. As a result, the atmosphere would undergo strong erosion, possibly leaving the planet uninhabitable.

Abundance

There is, however, one major advantage that red dwarfs have over other stars as abodes for life: they live a long time. It took 4.5 billion years before humanity appeared on Earth, and life as we know it will see suitable conditions for as little as half a billion years more. Red dwarfs, by contrast, could live for trillions of years, because their nuclear reactions are far slower than those of larger stars, meaning that life both would have longer to evolve and longer to survive. Further, while the odds of finding a planet in the habitable zone around any specific red dwarf are unknown, the total amount of habitable zone around all red dwarfs combined is equal to the total amount around sun-like stars given their ubiquity. The first super-Earth with a mass of a 3 to 4 times that of the Earth's found in the potentially habitable zone of its star is Gliese 581 g, and its star, Gliese 581, is indeed a red dwarf. Although tidally locked, it is thought possible that at its terminator liquid water may well exist. The planet is thought to have existed for approximately 7 billion years and has a large enough mass to support an atmosphere.

Another possibility could come in the far future, when according to computer simulations a red dwarf becomes a blue dwarf as it's exhausting its hydrogen supply. As this kind of star is more luminous than the previous red dwarf, planets orbiting it that were frozen during the former stage could be thawed during the several billions of years this evolutionary stage lasts (5 billion years, for example, for a 0.16 solar mass star), giving life an opportunity to appear and evolve.

In fiction

In Olaf Stapledon's 1937 science fiction novel Star Maker, one of the many alien civilizations in our galaxy he describes is one in the terminator zone of a tidally locked planet of a red dwarf system. This planet is inhabited by intelligent plants that look like carrots with arms, legs, and a head that "sleep" part of the time by inserting themselves in soil on plots of land and absorbing sunlight by photosynthesis, and that are awake part of the time, emerging from their plots of soil as locomoting beings who participate in all the complex activities of a modern industrial civilization. Stapledon also describes how life evolved on this planet.

In Larry Niven's "Draco Tavern" stories, the highly advanced Chirpsithra aliens evolved on a tide-locked Oxygen world around a Red Dwarf. However, no detail is given beyond that it was about 1 terrestrial mass, a little colder, and used red dwarf sunlight.

Superman's home, Krypton, was in orbit around a red star called Rao which in some stories is described as being a red dwarf.

See also

• Astrobiology
• Aurelia and Blue Moon
• Gliese 581 g
• Habitable zone
• Habitability of orange dwarf systems
• Planetary habitability
• SETI

Learning materials from Wikiversity:

• • How Life could Evolve in a Red Dwarf Star System

External links

• Red Star Rising