Huge exoplanet news! The very nearest star to our solar system, Proxima Centauri — kind of like Fort Lauderdale to our Miami, cosmically — apparently has a planet orbiting it, according to a report published Wednesday in Nature. The newly discovered planet "candidate" (a word that gives the astronomers some wiggle room in case it turns out to be a misapprehension), Proxima b, appears to be roughly the size of Earth, and in an orbit around the star that would let the planet possibly have liquid water at the surface. Which means this planet is potentially habitable. It could have life on it right this minute! This could be the planet where the flying saucers are coming from! It's all very exciting to contemplate.
But we should not get ahead of the facts.
When scientists say the planet is in the "habitable zone," they are not saying that it actually is inhabited. They don't know a thing about this planet except for its approximate mass and distance from the star. They don't know if it has an atmosphere.
"Until we know about the atmosphere of a planet, any discussion about life if pure speculation," Paul Butler, a Carnegie Institution astronomer who is part of the team that made the new discovery, told me by email.
No one knows how life originates — whether it's a freak event or something fairly common. We don't even know how life originated here on Earth.
Speaking of which: We already know of two other planets, much closer to Earth, that are in a star's habitable zone. We call them Mars and Venus.
The precise boundaries of a habitable zone are a matter of debate, and you have to remember that stars and planets aren't static entities. Stellar luminosity changes over time. Planets migrate. But Mars and Venus cut a nice profile, relatively speaking. They are rocky, "Earthlike" planets that could have had life at some point.
Mars was probably just a bit too small to hang on to its atmosphere; Venus suffered a runaway greenhouse effect and is broiling at the surface. If you were an astronomer on a planet orbiting Proxima Centauri, and were searching for exoplanets, and were studying the yellow star just down the pike, you'd likely conclude that it had three, count 'em three, rocky planets in the habitable zone.
“We should probably be wary of making too many predictions on what happens on other planetary systems based on our limited experience with our own solar system," Alycia Weinberger, an astronomer at the Carnegie Institution, told The Washington Post. "When we only had to extrapolate from the solar system we thought we knew a lot."
Butler says we have more mountains to climb in the hunt for life on other worlds:
Finding Proxima b is a baby step. It has taken 20 years to go from finding any planet to finding potentially habitable planets. My hope is that in another 20 years technology advances to the point where we can directly image these planets, and take spectra of the planets. The spectra would reveal what the atmosphere is made of. If we find planet atmospheres with water vapor and oxygen, or other gases that are far out of thermodynamic equilibrium, then we can start jumping up and down in our excitement about an actual living world.
It doesn't seem likely that any of us are going to go zooming off to Proxima b to live anytime soon. First, there's the distance. It's 25 trillion miles, roughly. Even if you could accelerate quickly to a tenth of the speed of light (going much faster than any spacecraft has ever gone) you'd need upward of 40 years to get there.
More plausible would be a tiny, chip-sized interstellar probe designed to take direct measurements of Proxima b's atmosphere. We asked astronomer Johanna Teske of the Carnegie Institution how such a probe might work. She writes:
If we were to send a probe, with the purpose of sending us information back (at the speed of light), we'd probably want it to have a detector specifically tuned to wavelengths corresponding to molecules we would want to detect in the atmosphere. These might include water, methane, oxygen, ozone; we'd want to focus on anything that wouldn't be expected in an equilibrium atmosphere, because that would indicate some kind of process changing the atmosphere from equilibrium (like geologic activity, or perhaps plant life).
If you, personally, did make it to Proxima b in your very fast interstellar spaceship, you'd better be prepared for some harsh conditions. You'd really be roughing it. You might not be a happy camper.
Human beings have evolved to survive and thrive in conditions special to the Earth. Our bodies are designed to operate with a certain level of gravity, and in a certain kind of solar radiation, and with a day lasting 24 hours, and of course we like an atmosphere that fits our definition of "air."
Proxima Centauri is a red dwarf star, much smaller and cooler than our sun. The problem with red dwarfs is that they can be unruly. It's not the heat, it's the radiation. They emit violent flares.
Astronomer Seth Shostak of the SETI Institute explains by email:
The problems with being on a world in orbit around a red dwarf have to do with its tendency to act like a teenager and produce violent outbursts that could flood a nearby world with sterilizing UV radiation. But if you evolved on such a planet, maybe you’ve already developed defense mechanisms against that (like getting under some shelter … I mean, rain drops can be lethal for insects, but they know enough to protect themselves). The other uncertainty is that there’s some worry that the radiation might strip away part of your atmosphere, and that could be good or bad, depending on … your atmosphere!
Weinberger told us that Proxima Centauri is fairly mellow these days:
Proxima Centauri itself seems to be a relatively modest flarer, but early in its history it probably had a significant ultraviolet emission. We don’t know what that does to the atmosphere of a planet, particularly one so close to its parent star.
Of course, if we did want to move to Proxima b or some other planet that orbited a red dwarf star, we could genetically engineer ourselves to be more comfortable there. I asked Sara Seager, an astrophysicist at M.I.T., what we'd have to do to accomplish this. She responded by email:
If the planet orbiting the red dwarf star has the same surface gravity and identical atmosphere, we wouldn’t need any modification. We’d have to live on the dark side of the planet to avoid the much higher UV and Xray fluxes, and possibly only venture to the day side with special safety suits. Or genetically engineer a rapid repair to DNA to avoid mutations. If the planet has a different atmosphere, that is no oxygen, we’d have to generate oxygen or terraform. If the planet has a higher surface gravity (after all, radial velocity measures minimum mass) we’d want to be shorter and more stocky to make moving around easier.
Incidentally, the new planet, though calculated to be 1.3 times Earth mass, could have the same gravity at the surface, depending on its size, according to Seager. She calculates that a planet that's 1.14 Earth diameters and 1.3 times Earth mass would have the same surface gravity as Earth. We trust her math on this.
Alan Boss, an exoplanet expert at the Carnegie Institution, notes that the very short "year" of Proxima b — just 11 days — means that the night sky would look very different every night. During the day, the red dwarf sun would look huge in the sky. "Seasons" would potentially be only a few days long.
Here at The Post, we've been writing about the search for exoplanets for more than two decades, and it's been an incredible running story. The big picture is stunning: We now know the universe must be lousy with planets, including rocky, Earth-sized planets.
If Proxima Centauri, the very nearest star to our own sun, has a rocky, Earth-sized planet, chances are such "Earths" are commonplace in the universe. There's a lot of real estate out there upon which life could potentially originate and evolve.
A generation ago, people speculated about such things. But now we know.