Our search for planets around other stars, called exoplanets, has exploded within the last decade. We’ve now found exoplanets in the thousands. While the initial discoveries were mostly of Jupiter-like giant planets, the more recent ones have unraveled several Earth-sized worlds as well.
Earth-sized planets would mean the planet has a rocky surface and enough mass to hold on to an atmosphere. By that definition, Venus and Mars are also Earth-sized. What scientists are more interested in are Earth-like worlds, where life, as we know it, has the potential to exist.
The promise of another Earth
The stars that these exoplanets orbit play an important role in determining habitability — and red dwarf stars among them seem particularly promising. Red dwarf stars are the most common type of stars in the galaxy, making up 75% of the stellar population. They are expected to live for more than a trillion years easily, outliving sun-like stars by more than a hundred times!
As a result of their ubiquity and longevity, red dwarf stars are of great interest to planetary scientists, who have since been asking: What are the odds that there are planets orbiting red dwarf stars? How many of them are habitable? If so, can they sustain life for longer cosmological periods, like Earth has? And what are the chances that intelligent life exists on such planets?
In 2016, scientists discovered the exoplanet Proxima b orbiting a red dwarf Proxima Centauri, the star closest to our Sun, located only 4.3 light years away. What makes this discovery particularly interesting is that Proxima b is a potentially habitable Earth-like planet and so life in another star system may not be that far away. In 2017, astronomers discovered seven Earth-sized planets around another red dwarf TRAPPIST-1, about 40 light years away. Three of these planets are in the star’s habitable zone. More such planets kept getting discovered.
In 2020, NASA announced the discovery of another Earth-like world, Kepler-1649c, going around another red dwarf 300 light years away. While this planet is much further away, what makes it particularly exciting is its striking similarity to Earth. Kepler-1649c is almost exactly the same size as Earth and may have temperatures and conditions similar to our planet as well.
But for all these Earth-like planets, there is a catch: their host stars.
Houston, we have a problem
Red dwarf stars are smaller, cooler and emit much less energy than sun-like stars. So if a planet has to be in its red dwarf’s habitable zone and have surface temperatures suitable for life, it needs to be much closer than Earth is from the Sun. As such, all the habitable-zone exoplanets mentioned above are too close to their stars and such proximity has critical implications. Here is a comparison of how close Proxima b is to its star compared to how close Mercury is to our Sun.
First, it means that these planets are likely tidally locked: one side of the planet always faces its star while the other side is in constant darkness. The star-facing side could get hot enough to boil water — while the ‘dark side’ could get cold enough to freeze it. The only hope for life on such planets would then be the twilight zone — along the line dividing day and night, where the temperatures could be moderate. However, large temperature differences between one side of the planet and the other could give rise to strong winds, with potentially massive storms blowing from warmer to cooler regions.
And these might just be the least of these (hypothetical) aliens’ problems. Between 2003 and 2012, NASA’s GALEX mission observed multiple red dwarf stars in the ultraviolet part of the electromagnetic spectrum. Scientists churning this data found that these stars ceaselessly produce flares like our Sun, except they are much stronger. The flares are accompanied by the emission of large amounts of high-energy ultraviolet and X-ray radiation. Such flaring could strip away a planet’s atmosphere in fewer than a billion years.
Because the exoplanet has to be closer to its red dwarf star to be in its habitable zone, the atmosphere loss is accelerated. It has been estimated that these planets could lose their hydrogen and oxygen, and thus water, in about 10 to 100 million years. Additionally, ultraviolet radiation can penetrate a planet’s atmosphere and damage any existing, terrestrial life forms.
Red dwarf stars are also known to produce mega-flares thousands of times more powerful than their solar counterparts. They are common when a red dwarf star is just born. The adverse surface conditions as a result of the radiation from these mega-flares might even prevent life from arising in the first place.
All these factors considered together suggest that the planets orbiting red dwarf stars are likely to be water-poor hellscapes. Fortunately, there have been some silver linings. For example, data from the Hubble space telescope has indicated to scientists that the outer, more massive planets in the TRAPPIST-1 system may have retained their water instead of losing it all.
In 2013, researchers at the University of Chicago and Northwestern University suggested that planets tidally locked around red dwarf stars could have substantial cloud cover on their star-facing sides. Clouds reflect starlight and can keep planets cooler while also caging the planets’ infrared radiation to keep things warm enough to support life. So if the planet has a substantial atmosphere, the heat can be distributed more evenly among both hemispheres, avoiding problems cause by tidal locking.
We have a way to check this. If an exoplanet has clouds, the temperature of the star-facing side would be lower compared to that of a cloudless planet, and vice versa. NASA’s most expensive telescope till date, the James Webb Space Telescope (JWST), due to be launched in 2021, will be able to characterize exoplanetary atmospheres. The recently launched Transiting Exoplanet Survey Satellite has begun finding planets in the habitable zones of red dwarf stars. JWST will then be able to study them further.
The long lifespans of red dwarf stars also mean that life on planets around them will have more time to develop. For example, the TRAPPIST-1 system is older than our Solar System. The long-term evolution of planets around red dwarfs also plays a role, including in the form of changing geological conditions and orbital dynamics.
So, whether potentially habitable planets around red dwarf stars are truly habitable remains an open question, one that will take a long time to be settled scientifically. And whichever way it is answered, there are bound to be profound implications. When all sun-like stars have died in few tens of billion years from now, red dwarfs will be the only remaining stars in the galaxy. And exoplanets orbiting them might be the only options for life to take root on.
The stage is also set for the much-awaited launch of JWST, and for its follow-up observations of multiple exoplanets, helping determine their habitability and looking for biomarkers in their atmospheres. One can only hope that it is put up in space soon.
Originally published at The Wire, updated in 2020 to accommodate a new discovery.