Planet habitability around small stars has been a hot topic of discussion amongst astronomers. Small stars are more numerous in the galaxy than stars like our Sun, so if they can host life on their planets, they open up a new realm of worthy observations. Recent discoveries in the exoplanet field have made this topic even more prevalent, most notably with our very own solar system neighbor.
At only 4.22 light years away, Proxima Centauri is the nearest solar system to our Sun. And it turns out, Earth may have a new friend on the block to play with. In August 2014, astronomers discovered a planet revolving around our solar neighbor; that planet is called Proxima Centauri b (Prox b). Prox b is about 1.3 times the mass of Earth and probably rocky, which would mean it’s capable of maintaining liquid water on its surface. Interestingly, Prox b is also located within its host star’s habitable zone, and all these factors together make it sound like a great candidate for potential life.
An artist’s conception of a planet orbiting a red dwarf star, with two moons. Credit: D. Aguilar/Harvard-Smithsonian Center for Astrophysics
Unfortunately, its parent star isn’t holding up its end of the bargain. Proxima Centauri is a red dwarf–a star much smaller than our Sun. It has only 14% the diameter of our Sun and 12% of its mass. This makes Proxima Centauri much less bright than our Sun, and so its habitable zone is located much closer to its surface. And this is where extreme life-altering issues can arise.
Proxima Centauri likely spent the first billion years of its life cooling down to its current temperature, which means it was initially much too hot to give life a chance to grab hold on Prox b–unless Prox b was located in the star’s original habitable zone. But there’s one complication–even if Prox b was once lucky enough to be there, it might have been left out in the cold as Proxima Centauri cooled, taking its habitable zone with it.
An angular size comparison of how Proxima will appear in the sky seen from Proxima b, compared to how the Sun appears in our sky on Earth. Proxima is much smaller than the Sun, but Proxima b lies very close to its star. Credit: ESO/G. Coleman
There’s another possible problem for life on Prox b–the weather. When planets are located in the habitable zones surrounding small stars (like Proxima Centauri), they are often tidally locked to their hosts, similar to how our moon is tidally locked to us. In the case of Prox b, one day is equal to the length of its year, which means half of the planet experiences nonstop life-giving sunlight, while the other half is shrouded in inhospitable darkness. This situation can cause extreme weather conditions across the boundaries between the day side and the night side. Although it’s possible that ocean and wind currents crossing from the day to night side (and vice versa) could more evenly distribute heat around the planet, it’s also possible that the day side’s constant warmth could evaporate any water.
This artist’s impression shows the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. The double star Alpha Centauri AB also appears in the image between the planet and Proxima itself. Proxima b is a little more massive than the Earth and orbits in the habitable zone around Proxima Centauri, where the temperature is suitable for liquid water to exist on its surface. Credit: ESO/M. Kornmesser
But before we give up on the possibility of life on Prox b–some good news. The lifetimes of smaller stars like Proxima Centauri are significantly longer than the lifetimes of bigger stars because small stars burn through their fuel much more slowly. The longer the star’s lifetime, the longer its planets have to settle their climates, allowing for the required stability for life’s evolution. So even though life on Prox b may have to deal with extreme weather, at least it has time on its side.
There’s another catch, though–stars with long lifetimes are usually more active, especially when they’re young. Proxima Centauri likely flared up in its youth, which could potentially have incinerated Prox b’s atmosphere–that is unless Prox b had a massively strong magnetic field to protect it. The day (sun-facing) side of Prox b would be most vulnerable to these flares. Sure, life could migrate to the night side to avoid the contact of the flares, but it would be really cold. (And the electric bill would be shockingly high.)
The bottom line:
Living on a planet orbiting a small star can be very problematic. Eternal darkness on half of the planet due to tidal locking plus strong rotational winds due to extreme differences in surface temperature plus intense UV radiation from solar flares do not make for a comfortable living situation.
But on the other hand, those warm winds could make the night side cozier; the long lifetime of a red dwarf star could allow life more time to evolve, and a sufficient magnetic field could protect it from being destroyed by frequent solar flares. So while no one knows for sure yet if there’s life on Prox b, astronomers will be keeping their eyes on it.