We wrote about Proxima Centauri B last year, and now another terrestrial-sized exo-planet in a nearby star’s habitable zone has been revealed. This one also orbits a small reddish star—such stars make up the vast majority of stars in our universe—but the star isn’t as prone to stellar outbursts as Proxima Centauri:
Co-discoverer Nicola Astudillo-Defru from the Geneva Observatory in Switzerland told BBC News: "Just because Proxima Centauri blasts its planet with strong flares and high energy radiation, yes, I think Ross 128 is much more comfortable for the development of life.
"But we still need to know what the atmosphere of Ross 128 b is like. Depending on its composition and the reflectivity of its clouds, the exoplanet may be life friendly with liquid water as the Earth, or sterile like Venus."
Ross 128 B? Well, maybe we need a better name. But it’s an interesting find nevertheless, especially coming as it does on the heels of Prox B. Another possible Earth-like planet around another red dwarf. It orbits so close that the planet is almost certainly tide-locked, the same side facing its small red sun during its short, 10 day “year.”
We have no idea what the composition of this planet might be. The mass and orbital estimates are exactly that: educated guess informed by faint signs in a trickle of photons stretching back 11 light years. The planet could be too hot or too cold, it might be composed of a lot more gas or some other substance than we think or have ever seen on a more familiar planet. Ross 128 B could be a giant toxic drop of scalding hot ammonia or frozen hydrocarbon compounds for all we know. There’s even a slim chance it's nothing but an anomaly in the data and does not exist at all.
Regardless, tidally locked terrestrial planets will be dominated by several dynamics completely alien to us. One is the permanent sub-stellar and anti-stellar points on the surface of the planet. Earth and Mars rotate every 24 hours; even Venus and Mercury present a different side to the sun over time. But the surface of tidally locked planets would always have one point in the daylight side that is closest to the sun, and one on the night side that is farthest away. Since we’ve never seen such a planet close up, we have no idea how this mix of thermodynamics acting on an unknown atmospheric composition and geography might manifest itself in terms of weather and climate.
In the illustration above, originally modified for Prox B, we made the following assumptions:
[A] deep atmosphere composed largely of nitrogen, a respectable amount of carbon-dioxide, streaked with smaller amounts of water vapor, and hydrocarbons like methane and traces of argon, at about twice the pressure of our own. Since the planet spins in sync with its “year,” the molten core still provides a respectable magnetic field protecting the upper layers of the atmosphere from stellar stripping. We assumed the planet orbits at the outer edge of the system’s habitable zone, making one orbit every 10 to 11 Earth-days, putting it on the colder side of comfortable when the star isn’t flaring, and made it on average drier than Earth, giving us a surface scoured by wind lit in cool reds and browns ...
One thing we do know is that Ross 128 is over nine billion years old, about twice the age of our sun and solar system. Our sun will be at the end of its life after nine billion years, but for a little red dwarf, that age is barely out of primary school. Small stars burn their fuel slowly, and thanks to the endlessly churning convection deep inside they do not accrue shells of heavier matter around a central core. They don’t go nova, they don't blow up into red giants or cast off their outer layers in a gorgeous planetary nebula. They just burn quietly away on the main sequence eon after eon. The larger ones like Ross 128 might last over 100 billion years, the smallest possible stars like Trappist 1 may shine for a trillion years or more! And, while we don’t know these things for sure, yet, so far these small red stars seem to be richer in smaller, more terrestrial-like worlds than the planets around other, larger, hotter stars.
Red dwarfs already make up the majority of stars in the universe. But they are greatly outshone by their far brighter, more massive and shorter-lived peers. When the universe loses its brightest show-off stars, and starts to dim as the end of the great stellar era finally looms, billions and billions of red dwarfs will still be shining away, marking the remaining super-galaxies, lighting and heating their retinue of planets.
If life were to arise on such planets in that far distant future, it would still have all the time in the universe to evolve. If life has already arisen on such a place, it might be old and wise (and dangerous) far beyond human comprehension already. And if some of civilizations that grew up around an old red dwarf became or have become space-faring, they will find a cosmos riddled with familiar red stars and, probably, lots of habitable small, rocky planets huddled close around the many dim reddish light-sources that reminds them of home.