On November 2nd, 2020, NASA announced that there are probably 300 million habitable planets in our galaxy. How did they find out? They used the Kepler space telescope data to estimate how many planets are rocky, and have surface temperatures suitable for liquid water. If a planet satisfied both criteria, it was deemed habitable. But is such a planet really habitable? If we were to hop into a spaceship and visit one such ‘habitable’ planet, could we actually live there?
Although there are purportedly 300 million habitable planets, astrophysicists have only located 60 (you can head to the Planetary Habitability Laboratory website to learn more). That doesn’t necessarily mean NASA’s estimate is wrong as rocky planets can be obscured from our telescopes in the glare of their host star, their orbit can be skewed in such a way as to hide them from Earth, or they could be too tiny to have gravitational effects that would give them away. So we can attribute the dearth of confirmed habitable planets to these pesky features of exoplanets: they’re small, they’re dim, they’re light.
Given the gnarly challenges of exoplanet hunting, it’s actually quite impressive that 60 habitable ones have been found. But to return to the million dollar question-just how habitable are they?
Proxima Centauri-b is Earth’s closest potentially habitable neighbour. It is in the Goldilocks region of its host star, which means its surface could harbour liquid water. But the existence of water is contingent on Proxima having an atmosphere, having a certain pressure of nitrogen and carbon dioxide in that atmosphere, and having gone through a formation process similar to that of the Earth. If these things do not exist or did not occur, then Proxima does not have water and hence cannot support life.
Even if it does have water, it is unlikely humans will settle there as Proxima Centauri-b faces 2000 times more solar wind pressure than the Earth. The solar wind is an amalgam of ions, superhot particles, plasma and radiation, so anyone on Proxima’s surface would be constantly bombarded by this solar wind and die within seconds.
Moreover models suggest Proxima is tidally locked. This means that the same side of the planet is always facing the host star. Over time, the star-facing side grows searing hot, while the opposite side becomes inhospitably cool. Hence the only habitable zone left would be the so-called ‘terminator zone’, which is the thin line at the border between the hot and cold halves of the planet.
Proxima Centauri-b is representative of all the discovered habitable planets. Analysis of these other planets reveal similar caveats and disappointments. While there is a slim chance that these planets can support life, it looks unlikely, though future data may change this appraisal.
Meanwhile, we shouldn’t lose hope of colonizing the galaxy. We could geoengineer planets to our liking, or engineer our own ‘Death Star’ type designer planet to live in. If there’s a way, it is certain humans will find it.
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