Do we have a sister planet, just 25 trillion miles away?
Big news in space this week: A planet has been discovered orbiting Proxima Centauri, the closest star to the solar system. Still bigger news is that the planet — named Proxima b – is strikingly similar to Earth: Its mass is estimated to be comparable to Earth’s — only about 30 percent higher — and it appears to orbit Proxima Centauri at roughly the right distance for liquid water to exist on its surface.
Everything we know about life tells us that water is the indispensable element; the possible presence of surface water means Proxima b may have evolved life. That its mass is comparable to Earth’s means that, even if it hasn’t, men may in future be able to live there comfortably, held down by gravity that’s more or less what we’re used to. There are, though, a preponderance of unknowns.
For one thing, nothing besides the planet’s existence is certain. It may in fact be much more massive than Earth, and it may actually orbit too close to its star for water not to boil, or too far for water not to freeze. Right now, the discovering scientists — led by Guillem Anglada-Escudé of Queen Mary University in London — are making well- educated guesses. But nothing can be pinned down until they have more and better data.
For another thing, even if the planet is placed correctly for liquid water to exist on its surface, liquid water might not exist on its surface. To liquefy, water needs the pressure provided by an atmosphere, and at the moment, there’s no way of knowing if Proxima b has an atmosphere.
Even if Proxima b has had an atmosphere in the past, that atmosphere may have been knocked away by solar radiation. While Proxima b is estimated to receive about the same amount of heat from Proxima Centauri as Earth does from the Sun, Proxima Centauri is a much smaller, cooler star — a red dwarf — and Proxima b is believed to orbit it at just 5 million miles. Earth orbits the Sun at 93 million miles. This means that Proxima b is certainly being bombarded with much more high-energy radiation than Earth is. If, like Earth, Proxima b has a molten core and an internal dynamo that produces a powerful magnetic field, its surface might be shielded from high doses of radiation. If, like Mars, it doesn’t, any life it tried to evolve would probably have been fried. (Though not definitely.)
Also because of Proxima b’s proximity to its star, it is likely to be in a tidally locked orbit — which would mean, as in the Moon’s orbit of Earth, that one side always faces inwards and the other always outwards. In Proxima b’s case, that would mean one side of the planet has been in daylight for about 5 billion years, while the other side has been in perpetual night. That means that one side is likely to be very, very hot and the other very, very cold. This might not be the case, though, if there’s an atmosphere to convey heat from one side to the other. And even if it is the case, there would probably be a pleasant temperate band of perpetual twilight where the two sides meet.
The only other problem with the planet is that it’s four and two-tenths light years away, which is about 25 trillion miles. That will make visiting difficult, at least until NASA works out a method of mass-conserving continual acceleration (which, happily, it is working on: if a spaceship could accelerate at a constant rate, without having to carry and expel immense quantities of propellant the way a rocket does, it might be able to reach Proxima b in, say, five or six years). In the shorter term, within about a decade, our telescopes should be capable of photographing Proxima b directly. (At the moment, we only know it’s there because of a slight wobble in Proxima Centauri’s rotation, which evinces the pull of a planet’s gravity.)
Uncertainty aside, the discovery of Proxima b is staggering and magnificent. There’s a planet which may be just like ours, orbiting the star closest to us. Let your imagination run wild. It’s a wonderful time to be alive.