In case you haven't heard, scientists think they've found an exciting new planet — one right in our back yard, closer than any exoplanet ever found before. But as with any exciting science story, the hype is drowning out the facts. Here's what you need to know about this fabulous finding.
1. Proxima b is probably a real planet, and it orbits our closest neighboring star
This new planet isn't a hoax or a bad study. This one checks out. It's quite likely that scientists have detected the presence of a planet orbiting Proxima Centauri, our closest neighboring star. It's just 4.25 light years away, which is pretty darn close on the cosmic scale.
2. Proxima b is not "another Earth" and probably isn't particularly "Earth-like"
Sorry Internet-hype machine, but saying that Proxima b is an Earth-like planet is way out of line.
(Not cool, guys. Not cool.)
The truth is, we know very little about this world. For starters, it hasn't been directly detected. No one has seen it in a telescope. We'll get back to that in a minute, but for now, we'll just say that while the planet is very, very likely to exist, it's not like we can be 100 percent certain it's there.
And even if the planet exists, we definitely don't know that it's anything like Earth. For starters, it orbits a completely different type of star — a tiny, cool red dwarf. To be in a red dwarf's habitable zone (the distance from the star where the planet could theoretically hold liquid water), a planet has to be so close that it's likely to become tidally locked. That means it has one side in the sun and one in the dark all the time, forever.
3. The method that was used to detect Proxima b means that we know very little about it
Like, really. Very little.
Most of the exoplanets we talk about these days are found using the Kepler space telescope, which detects worlds by tracking the way they make their stars dim as they pass in front of them. When a planet passes in front of its host star from the perspective of one of our telescopes, we call that event a transit.
Transiting planets are great, especially if they transit often, because we can use transits to study their characteristics. When a planet passes in front of a star, we can analyze the light that passes around the planet to puzzle out the composition of the planet's atmosphere. We can do this to detect water vapor and other molecules necessary for life, allowing us to deem a planet truly "Earth-like."
But that's not how Proxima b was found.
Instead, scientists used the so-called Doppler or radial velocity method. They still looked at the light of the host star (Proxima Centauri), but they didn't see Proxima b pass in front of it and cause it to dim. Instead, they saw the light signature wobble back and forth as the star pushed and pulled against the gravitational force of its orbiting planet.
While the Doppler method is great for detecting planets, it's not great at telling us anything about them. Scientists can predict the distance of the planet from its host star, they can estimate its orbital period, and they can figure out its minimum mass. That's it. So for now, that's all we know: Proxima b is at least 1.3 times as massive as Earth, it orbits its sun every 11.2 days, and it's technically in its star's habitable zone.
Based on what we know about red dwarf stars, it's likely that a planet at this distance and of this size is a rocky world. But there are rocky worlds that aren't Earth-like at all — just look at Mars, Mercury and Venus.
The researchers who found Proxima b are going to try to catch it transiting so they can learn more about it, but lots of planets never happen to cross in front of their stars from our perspective. They estimate a 1.5 percent probability of a transit, which is obviously a long shot.
4. Proxima b is in the "habitable zone," but that's kind of misleading
Proxima b is technically the right distance from Proxima Centauri to be able to hold liquid water. But there are a lot of caveats here. Without an atmosphere, they estimate a surface temperature of abouut minus-40 degrees Fahrenheit. That's warm, in the grand scheme of possible planet temperatures, but it's still pretty chilly.
That doesn't mean liquid water is impossible. Europa, a moon of Jupiter considered a prime candidate in the hunt for microbial alien life, has a surface temperature of minus-260 degrees Fahrenheit at its equator — but it maintains a salty subsurface ocean. But for a planet to really be considered Earth-like, we want to see liquid water that can exist regularly on the surface, Earth-style.
Proxima b could actually be as warm as Earth, assuming it has an atmosphere to help it trap heat. But does it have an atmosphere?
Like we said before, there's no way to tell with the current data. And there's reason to think that even if the planet did once have an atmosphere, it's been stripped away. Proxima Centauri is a flare star, one that shoots out destructive ropes of radiation every few minutes. Proxima b would constantly be bombarded by these flares, so it's easy to imagine any atmosphere being stripped away (much as Mars had its atmosphere stripped away by our own sun's powerful solar flares).
5. Life on Proxima b would be pretty rough — even if it had an atmosphere and tons of water
Proxima b is far from the most habitable looking exoplanet we've ever seen. It's certainly not going to be a place humans want to colonize, what with all the radiation. It's possible that something has evolved to fit the conditions there, either by some crazy adaptation or by hiding deep below the ground or in an ocean. But even if Proxima b is "Earth-like," relatively speaking, it's still a very alien world. Scientists aren't sure how easily life could evolve around a star like Proxima Centauri, and it's an intriguing question — but you shouldn't hold your breath for signs of alien life on this new exoplanet.
6. We're probably not going to visit (at least not anytime soon)
4.25 light years is still way too far for us to send a probe with current technology, and we certainly can't get a spaceship there. We can't even get to Mars, guys. We're not going to Proxima b.
Sending a probe to the next star over would obviously be really cool, and knowing there's a planet there to study would be great motivation. There's at least one well-funded effort to make this happen, but it's going to be a long shot for the foreseeable future. Here's some perspective: NASA's New Horizons probe had to travel 3 billion miles to get to Pluto and took nearly a decade to do so. At around 25 trillion miles away, a trip to Proxima b would be more than 8,000 times as long, and require new technology to keep the probe in contact with Earth (and to keep its batteries running).
Proxima b's existence is a great motivator, but such an effort would still take loads of time, money and ingenuity. It's more likely that scientists will be able to study it with the next generation (or two) of space telescopes.
7. As cool as this is, it isn't surprising — because planets are everywhere
Scientists now believe that nearly every star hosts at least one planet. Not all of them are big, or habitable, or even particularly interesting, but they're out there. It's no surprise that Proxima Centauri has a planet orbiting it. In fact, it could easily have a few others as well. And its neighboring binary stars Alpha Centauri A and B (stars much more like our sun than Proxima is) probably have some orbiting them as well.
A recent study estimated that the Milky Way has about 1 billion "Earths" — rocky planets in the habitable zones of sunlike stars, ones much more like us than the nearby Proxima b. And then you have all of the planets orbiting red dwarfs like Proxima Centauri, which could conceivably host life. The galaxy is rich with potentially habitable planets, and we're going to keep finding them. Proxima b is just one of many exciting finds in a golden age of exoplanet detection.