Are the Universe’s Smallest Stars Promising Places for Life?
The Perils of a Red Sun
There’s a catch: Not everything is rosy under a red sun. Because red dwarfs are much cooler and dimmer than Sun-like stars, their habitable zones lie much closer. Proxima b, for instance, orbits an average 8 times closer to its star than Mercury.
This proximity presents formidable challenges for life: First, much like the Moon shows only one side to Earth, a red dwarf star’s gravity quickly slows down the rotation of a nearby planet to the point where only one side of the planet always faces its star—a phenomenon known as tidal locking. The dayside gets hot enough to boil water and the nightside cold enough to freeze it. Having an atmosphere like Earth or Venus can level out these temperature differences.
Red dwarf stars also flare more frequently than our Sun, unleashing waves of ultraviolet and X-ray radiation that is harmful to life as we know it. If the Sun can strip off Mars’ water and dense atmosphere from a huge distance, planets like Proxima b lying about 30 times closer to furiously flaring stars could lose their atmospheres in just a hundred million years. To make matters worse, such planets being tidally locked means their molten interiors likely churn slowly, generating feeble or no magnetic fields. This deprives the planets from the kind of radiation protection that is so effective here on Earth and contributes to its proliferation of life.
In 2014, NASA’s Swift mission caught a red dwarf emitting a flare 10,000 times more powerful than any ever expelled by our Sun. Most newly born red dwarfs may emit such mega-flares, perhaps in revolt against us having judged them on size. The intense radiation from these mega-flares could prevent life from ever arising on nearby planets.
Taken together, these factors paint red dwarf exoplanets as water-poor wastelands baking under dull red skies. Is there any hope for life around these stars?
Cloudy With a Chance of Life
If an exoplanet forms far enough away from a flaring red dwarf and starts off with significantly more water than Earth, it could retain enough water in the long run despite the flaring. This could be the case for TRAPPIST-1: Hubble’s ultraviolet observations of the 8-billion-year old system hinted the three habitable-zone planets may have some surface water.
With surface water comes water clouds like on Earth. In 2013, extensive simulations of the atmospheres of red dwarf planets showed that exoplanets with surface water and clouds could reflect enough sunlight to prevent a planet from becoming too hot and thus unconducive to life.
Furthermore, just like in our own solar system, exoplanet moons in red dwarf solar systems could host subsurface water oceans despite being much further away than the Sun. But it will be some time before telescope technology advances to the point where we can take a look.