Though it’s been less than two weeks since its discovery was announced, more than a year has passed on Proxima Centauri b – the probably rocky, potentially habitable planet orbiting Proxima Centauri, our nearest star beyond the sun.
Back on Earth, NASA astrobiologists crunched all the available data on Proxima b to figure out how likely it is to host life.
The result? Not very, it seems.
The team envisaged a range of possible scenarios for the planet. Most of these render it uninhabitable, although the team does admit a certain “narrow range” of possibilities that would admit life. So life on Proxima b is a long shot, but not out of the question.
The planet made headlines because it has a similar mass to the Earth, and also orbits in the “goldilocks zone” (not too hot and not too cold) around its star, where the temperature might be mild enough to harbour liquid water.
But this new modelling study is a reminder that there are many factors, other than size and distance from the star, which determine whether a planet can support life.
The study, performed by Rory Barnes and his team from NASA’s Astrobiology Institute, considered different possibilities for loads of variables that are currently unknown, such as variability in the parent star’s brightness, the composition of Proxima b’s atmosphere and so on.
They then simulated what kind of planet Proxima b would be in each case. They also worked out how future instruments (such as the James Webb Space Telescope) might be able to tell down the track.
One of the biggest question marks over the habitability of Proxima b is the fact that its parent star, Proxima Centauri, is a red dwarf. Though very slow-burning, and with low energy output compared to the sun, red dwarfs can be highly variable in their brightness and prove boisterous and moody guardians.
Proxima Centauri has been measured dropping its energy output by up to 17% for months on end. The sun’s variability, on the other hand, is only about 0.1% (and that’s already enough to cause an ice-age or two every 100,000 years).
Besides, red dwarfs are also known to produce mega-flares, 10,000 times more powerful than those from the sun. When a cantankerous sun hurls a solar flare at us, the torrent of charged particles might cause a few blackouts and spectacular aurorae. But a red-dwarf mega-flare could strip away a planet’s atmosphere entirely – unless it has a strong enough magnetic field to deflect the onslaught.
Another key question is how Proxima b’s close orbit has affected its evolution. Whizzing around its star at a distance of fewer than seven million kilometres (that’s much closer than the planet Mercury is to the sun), Proxima b is probably tidally locked.
This means there are no sunrises or sunsets; the same side of the planet always faces its star, just like the same side of the moon always faces Earth. If you were standing on the Proxima-Centauri-facing surface of Proxima b, the huge red disc of its parent star would be a permanent fixture in the sky.
Astronomers in the 1990s found that tidally locked planets inevitably lose their atmospheres as gases freeze out on the cold side. But many later studies disagreed, finding winds could hurl heat to the back side of the planet, as long as the atmosphere was at least 0.3 bars (about a third as thick as Earth’s).
In the new study, Barnes and his team consider these aspects and many others to predict what conditions are like on Proxima b today.
The bottom line is, despite seeming Earth-like at first glance, Proxima b could yet turn out to something else entirely – perhaps boiling hot on one side and freezing cold on the other (like Mercury), enveloped in a runaway greenhouse atmosphere (like Venus) or a temperate but dry world (a bit like a warmer version of Mars).
There’s even another possibility where the planet contains both water and oxygen but is still uninhabitable (because too much oxygen could actually hinder complex biomolecules from forming). This means the detection of both oxygen and water is still not proof of life. A more definitive marker could be detection of methane – produced by organisms such as methanogenic bacteria.
Among the myriad different possible scenarios for Proxima b, the researchers do admit a precious few where the planet evolves into a planet just like Earth. But these only transpire when conditions are just right: such as starting out with an extremely high water content as well as a hydrogen-rich atmosphere.
So what is Proxima b really like? Only more time – and data – will tell. With the James Webb Space Telescope due for launch in 2018, an answer could be less than three Earth years away.
In the meantime, the Proxima b discover has added new impetus to the Breakthrough Starshot project, which plans to use lasers to blast tiny rice-grain sized spacecraft into the cosmos at 20% the speed of light.
Originally aimed at the Alpha Centauri binary star systems, Starshot will now likely shoot for Proxima instead.
Harvard astronomer Avi Loeb, an advisor for the Starshot project, has said that Proxima b, if habitable, might yet one day save the human species from extinction. The exoplanet is “the most natural location to where our civilisation could aspire to move” before the sun balloons into a red giant and destroys the Earth in roughly five billion years.
The slow-burning Proxima Centauri should last another four trillion years (or 300 times the current age of the universe). Time enough for us to rest easy and consider our next move.
Cathal O'Connell is a science writer based in Melbourne.
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