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Exoplanet Proxima b is 'potentially habitable' — so how would we find life if it's there?

ESO

Yesterday, the European Southern Observatory dropped a bombshell announcement: an Earth-sized planet orbits around Proxima Centauri, the closest star to our Solar System. And as an added bonus, it sits in the habitable zone of its star — the place where temperatures are just right for liquid water to exist. That means the planet, called Proxima b, is potentially habitable and may support life.

It’s not going to be an easy process

However, "potentially habitable" doesn’t mean "habitable," and it definitely doesn’t mean "inhabited." It’s possible that this world is a barren wasteland similar to Mars or a toxic planet like Venus. But life on Proxima b isn’t completely out of the question just yet.

So let’s say there is life there — how would we even find it? It’s not going to be an easy process, and it may be a while before we have any solid answers.

Why is finding life on exoplanets so damn difficult?

Well, seeing an exoplanet at all is tough. For one, these worlds are super far. Even Proxima b — the closest exoplanet we may ever find — is 4.2 light years away. That’s about 25 trillion miles. So finding one of these places with just a conventional telescope is out of the question. We need super powerful telescopes large enough to peer into deep space.

Then there’s the problem of light. Exoplanets are relatively faint, since they don’t emit light by themselves. They only reflect the light coming from the stars they orbit around. And those stars also muck things up. Their light completely washes out any orbiting exoplanets nearby. That makes it really tough for astronomers to distinguish between a star’s light and the light being reflected off a nearby planet.

What it's like to look for an exoplanet next to its star.

That’s why astronomers rarely see an exoplanet directly; they usually discover these worlds through crafty, indirect methods. One way is to look for transiting exoplanets — planets that pass directly between Earth and their parent stars at some point during their orbits. When a planet transits, it blocks out a very small portion of the star’s light, causing the star to dim slightly. Looking for stars that dim periodically is one way to find exoplanets.

Astronomers rarely see an exoplanet directly

In a similar way, astronomers also have to get creative with how they look for life on these worlds. "Certainly in the case of this discovery and in general, we don’t get to see a direct picture or image of the planet that has been discovered," David Kipping, an astronomer at Columbia University, tells The Verge. "So it’s not like we can see continents and plants and life and direct evidence for life just by taking a photo."

So then what’s the best way to look for life on an exoplanet?

The main thing astronomers want to know is what’s in an exoplanet’s atmosphere. Specifically, they’re looking for gases in the atmosphere that could point to signs of life below — so-called bio-signatures. "These are chemicals that life produces, such as oxygen, methane, and other organic molecules that you might be able to see in the atmosphere of the planet," says Kipping.

Finding an atmosphere like Earth's would be a big sign of life. (NASA)

For instance, the amount of oxygen in our atmosphere is a big indicator of how much life is on the surface of Earth. "Oxygen for example doesn’t stick around in an atmosphere," says Kipping. "Oxygen rusts. So if we turned off all the life on the Earth tomorrow — if all life went extinct — all the oxygen would very quickly disappear. It needs a continuous supply source, and it’s life that does that."

So if we see molecules like oxygen, does that mean there’s life there?

Well, not exactly. Sometimes chemicals stay in the atmosphere even though they shouldn’t really be there for very long — a phenomenon called chemical disequilibria. In other words, something must be producing these chemicals, but that "something" isn’t necessarily living organisms. For instance, chemical disequilibria has been seen on Mars, where methane has been detected, according to Kipping. But that doesn’t mean Mars certainly has life. The methane may be produced by some geological effects or something else we don’t know about yet.

What would be really convincing is seeing an exoplanet atmosphere that looks a lot like our own

What would be really convincing for astronomers is seeing an exoplanet atmosphere that looks a lot like our own. "If we see signs of life that look like what we have on Earth, it’s more of a slam dunk because we know it and we’re familiar with it," says Kipping. "If we see chemical disequilibria it’s a good sign, but it’s probably not a slam dunk. It’s always hard to rule out there might be some other thing that we don’t understand."

Okay fine. But how do we find these molecules anyway if we can’t take a picture of an exoplanet?

Our best bet is to study an exoplanet that transits in front of its star. Whenever a planet transits a small amount of starlight passes through the world’s atmosphere, and different molecules in the atmosphere change the wavelengths of the light that’s passing through. So studying this filtered starlight can tell us a lot about what surrounds an exoplanet. "We can actually work out the fingerprint of how light is filtered through the atmosphere and that tells us which molecules are inside the atmosphere," says Kipping.

Cool! Can we do that with Proxima b?

It depends. The astronomers don’t know if Proxima b transits yet. They actually found this planet using another technique called the radial velocity method. That involves studying the very tiny movements in the planet’s host star, Proxima Centauri. When Proxima b orbits around Proxima Centauri, the planet’s gravity actually tugs on the star and causes it to wobble slightly. Astronomers confirmed Proxima b’s existence by studying these wobbles.

Proxima Centauri (ESA / Hubble)

It’s still possible that Proxima b transits in front of Proxima Centauri, but we don’t know for sure. Kipping and his team at Columbia University have been looking for transiting exoplanets in front of Proxima Centauri and may have answers for us soon.

But even if this planet does transit, there’s an issue…

Oh man, what now?

Proxima Centauri is a fickle star. It’s a type of red dwarf known as a flare star. That means every so often, Proxima Centauri flares and increases in brightness. This flaring is going to make it very hard to figure out where Proxima b is, even if it does transit.

Does that mean we’re out of luck with Proxima b?

No! The flaring issue may not be insurmountable. Kipping says astronomers could try to observe a transiting Proxima b in the infrared — a wavelength of light outside of the visible spectrum. "Generally stars look more boring in infrared light than they do in visible light, where they’re very active," says Kipping. "The hope is that maybe we observe with an infrared telescope, the star might quiet down enough for us to make this very precise measurements that we need."

If Proxima b doesn’t transit, it's not game over

And even if Proxima b doesn’t transit, it's not game over. Astronomers will try to directly image the planet by blocking out the light of Proxima Centauri in order to find Proxima b. The problem is most of our current telescopes probably aren’t capable of doing this just yet. More powerful ground and space-based telescopes are in the works, and they might be able to pull it off. But those won’t be up and running for a few years.

Let’s say everything goes right and we somehow find strong signatures of life on Proxima b. Then what?

Kipping says it best: "We’d have to go there!"

But about that... remember that whole 25 trillion miles number? Yeah, any spacecraft using modern propulsion technology would need tens of thousands of years to reach Proxima Centauri. So getting a probe to this planet is probably not something that’s going to happen in our lifetime. That doesn’t mean it’s not a worthwhile endeavor. "You’re servicing the next many, many generations ahead of human beings," says Kipping. "Hopefully our civilization will still be around in 10,000 years, so someone will be there to hear the data on the other end."

Of course, there is an effort to visit Proxima Centauri on a much shorter timescale. The Starshot project, for instance, wants to propel a nano-satellite to the system using a giant laser. That would theoretically get the probe there in just 20 years. But there are a whole host of problems that need to be worked out before that can happen, such as miniaturizing a power and communications system that can transmit photos over 4.2 light years of space.

Yeah, but wouldn’t it be cool to find alien life on Proxima b?

Oh it would be huge news — especially because Proxima b is an exoplanet. That means that any alien life we find probably got started there on its own. "If we discovered life on Mars and Europa, for example, it’d be of course a fantastic discovery, but it would be very difficult to convince the scientific community that this was a completely independent start to life," says Kipping. "It could have been just something that got knocked off the Earth and got started on Mars. Or vice versa. So we could be related to the Martians or the Europans we find."

"It would kind of close the loop in how common life is in the Universe."

But it would be nearly impossible to link Earth life to any life we find on Proxima b. "It would mean that life starts completely independently in at least two examples," says Kipping. "And if it happens in two places, it probably happens everywhere. It would kind of close the loop in how common life is in the Universe."

In that case, I’d like to hype check Proxima b, please.

I’d give it moderate to significant hype. First, we need to have some much needed perspective: other researchers need to look at the raw data and make sure the planet is actually there. "It’s a lot of fun talking about life and speculating about the beaches and things on these planets, but it’s worth stepping back," says Kipping. "There have been Earth-sized planets announced in the past which subsequently disappeared when other teams looked at that data and couldn’t independently discover it."

Plus, there’s still a ton we don’t know about this place. Proxima b is way closer to its star than we are to the Sun, so it gets blasted with a whole lot more solar radiation. And researchers only think this thing is rocky based on its size. But we really don’t know for sure, nor do we know the planet’s exact radius and density.

It’s still probably the most significant exoplanet discovery to date

But despite this lack of knowledge and all the issues of finding life there, it’s still probably the most significant exoplanet discovery to date. It’s both the closest exoplanet ever found and it’s potentially habitable. That’s just crazy fortunate. It confirms that there are probably a lot of planets out there that may also have liquid water and support life. And since this potentially habitable planet is so close to us, it’s going to be a very popular target for astronomers. We’re going to be studying this planet many years to come.