Comment: Exoplanets: Life on the Terminator

by Jacob Haqq-Misra
Thursday, August 20, 2015

Credit: Callan Bentley.

Small, cool, red dwarf stars are the most abundant type of star in the universe and compose most of our stellar neighborhoods. Red dwarfs also have the longest lifetime of any stellar type. Compared to larger, yellow dwarf stars like our sun, which typically have adult lifetimes of about 10 billion years, red dwarfs can sustain themselves for up to 100 billion years or longer, making them excellent candidates for hosting life-bearing exoplanets. On many of the exoplanets where we’d be looking for life, the habitable zone lies on the line between sunlight and shadow, which, ironically, is called “the terminator.”

Upcoming space-telescope missions have red dwarfs in their sights. These missions will allow astronomers to characterize the atmospheres and potential habitability of planets orbiting these stars. NASA’s Transiting Exoplanet Survey Satellite (TESS) will help elucidate the abundance of Earth-like planets around red dwarf stars. The TESS mission is a two-year sky survey scheduled to begin in 2017, which will survey the nearest 1,000 red dwarfs and observe changes in brightness resulting from planets passing in front of their host stars. This approach will provide astronomers with a list of potentially habitable systems for further investigation. Meanwhile, NASA’s James Webb Space Telescope (JWST), successor to the aging Hubble telescope, will provide one of the best opportunities for characterizing planets detected by TESS. JWST is currently scheduled for launch in 2018.

The habitable zone around a star describes the range of orbital distances at which a planet could sustain liquid water, so planets within this zone are the best candidates to search for signs of life. While Earth today resides in the habitable zone around the sun and maintains clement conditions suitable for life, Venus orbits too closely and features surface temperatures warm enough to melt lead. Mars today orbits just at the outer edge of this habitable zone, but its small size prevents the planetary processes that could make it habitable.

Because red dwarf stars are dimmer than the sun, the habitable zones for planets orbiting these stars are closer in, in comparison to the sun. This raises the potential problem that a planet within the habitable zone around a red dwarf star might experience such strong tidal forces that the planet would be forced into synchronous rotation. This means its period of axial rotation (its day) is equal to its period of revolution around its host star (its year), or, in other words, that the same side of the planet is always facing the star.

Earth’s moon resides in a state of synchronous rotation, so that we always see the same lunar face. In the case of planets, synchronous rotation means that one side of the planet is constantly baked by stellar radiation, whereas the opposing side would experience perpetual darkness. The border between these hemispheres is known as the terminator, a region of perpetual twilight that may be the only habitable surface area on such planets, with temperatures neither too hot nor too cold. If liquid water is in fact a requirement for life, then any biology that has developed around a red dwarf may be confined to living near planetary terminators.

However, the habitability of even these terminator regions could be compromised by another pitfall of synchronous orbital rotation: As the night sides of these planets receive no direct starlight, they are extremely cold, suggesting that gases, like water vapor or even nitrogen, in their atmospheres could condense out as frozen deposits on the surface. In other words, planets orbiting red dwarf stars are at risk of losing their entire atmospheres as massive glaciers form on the night side, leaving airless balls of rock and ice, with no possibility of habitability.

Fortunately, large-scale atmospheric dynamics might self-correct this problem. On Earth, an east-west atmospheric pattern known as the Walker Circulation transports energy in response to heating from the sun. On exoplanets, Walker-like circulation could provide an impetus for transporting energy from the hot day-side to the cold night-side, leaving the terminator hospitable after all.

The combined efforts of ground-based surveys and missions like JWST and TESS may lead to the discovery of potentially habitable Earth-like planets nearby. Perhaps the first signs of extraterrestrial life will be detected around one of our dim red dwarf neighbors.

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