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Exoplanets are being discovered in the habitable zone to sustain life as we know it. Could water be found on the closest exoplanet to us?

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When we talk about planets around other stars, it’s often by comparing them to the worlds in our own Solar System — like when you hear about a hot Jupiter or an Earth-like planet.

But sometimes things don’t fit into those boxes so neatly. Like HAT-P-26b, an exoplanet about 437 light-years away that orbits a star a little smaller than the Sun.

In a paper published last week in the journal Science, astronomers studying the planet's atmosphere discovered that this world is pretty different from what we’re used to: it’s a Neptune-sized planet, but it looks more like Jupiter, and it orbits closer to its star than Mercury does to the Sun. They’re calling it a warm Neptune, but it probably didn’t form the same way as our Neptune. Instead of forming far from its star, this planet probably formed closer in and later on.

HAT-P-26b is an example of a transiting exoplanet, meaning that you can see it pass in front of its star. And as it passes by, light from the star travels through the planet’s atmosphere on its way to Earth. Each atom and molecule leaves a signature in that light, so by studying how the starlight changes, researchers can figure out what’s in the exoplanet’s atmosphere.

HAT-P-26b is an especially good planet to study this way, because it’s … kind of fluffy? It has less than half of Earth’s gravity, which means it doesn’t have such a strong hold on its atmosphere, so the atmosphere extends pretty far above the planet's surface. And a more stretched-out atmosphere will have more of an effect on the parent star’s light.

The researchers found that this planet's atmosphere has lots of water vapor, but not too many clouds — and the clouds it does have are probably made of molecules with sulfur in them. They were also able to calculate the planet’s metallicity. Astronomers call any element that isn't hydrogen or helium a metal, so when they talk about metallicity, they're really just looking at how much heavier stuff there is.

They usually measure it relative to the Sun. Here's where HAT-P-26b gets weird: Uranus and Neptune have close to 100 times the metallicity of the Sun. But HAT-P-26b -- which, remember, is similar in size to Neptune -- has a metallicity of 4.8, much closer to Jupiter’s!

This suggests that the planet probably formed closer to its star than Neptune did in our solar system, where there isn't as much of those heavier elements. After it formed, the planet probably migrated to where it is today. HAT-P-26b isn’t the only exoplanet catching the attention of scientists right now.

There's also been a lot of buzz about Proxima Centauri b, the planet orbiting our nearest stellar neighbor just four light-years from Earth. When it was first discovered last year, scientists calculated that it should lie within the habitable zone, the region of space where the temperature is just right for liquid water to survive on the surface. And in a paper published this week in Astronomy & Astrophysics, researchers concluded that liquid water is even more likely to be able to survive on Proxima Centauri b is than we thought.

Since life as we know it needs liquid water to evolve, knowing whether liquid water could exist on Proxima Centauri b gets us one step closer to figuring out if it's a good place to look for alien life. Calculations like this are performed by simulating the planet’s atmosphere using what are called global climate models, or GCMs. Because each model makes different assumptions and approximates physics in different ways, two GCMs can often generate pretty different answers.

But in this case, the new results match incredibly well with what astronomers have found previously. The team set up the model using what we know about the physical characteristics of Proxima Centauri b, like its radius, then ran a bunch of simulations to try out different scenarios. They ran one set of simulations assuming the planet’s atmosphere is a lot like Earth’s, with a bunch of nitrogen and oxygen and small amounts of other chemicals.

They also ran another set based on a much simpler atmosphere that was almost entirely nitrogen with just a bit of carbon dioxide mixed in. For both categories, they also used a range of parameters corresponding to a bunch of slightly different orbits for Proxima Centauri b. They wanted to look at a lot of different orbits because we still don’t know exactly what path the planet takes around its star or how fast it rotates.

And those differences would affect conditions on the planet. In many of these simulations, the calculated surface temperature would mean liquid water could flow across the planet’s surface. In fact, it seems more likely that liquid water could survive on Proxima Centauri b than it did from the set of simulations published last year.

Since water is so important for life, this makes the planet seem like a great place to look for alien life. But one thing the authors didn’t try to account for is the violent nature of the planet’s parent star, Proxima Centauri. It’s what astronomers call an M dwarf, a class of stars much smaller and cooler than our Sun.

M dwarfs have super active surfaces and emit giant solar flares way more often than the Sun. Combine that with the fact that the planet orbits much closer to its star than any world in our Solar System and you have a recipe for a pretty bad day. Some previous studies have suggested that all this intense radiation would destroy many of the most critical molecules in the Earth’s atmosphere.

Without stuff like ozone, DNA would have been damaged beyond repair long before life managed to fully evolve. Still, at least the liquid water piece of the puzzle seems to be in place for Proxima Centauri b. So astronomers aren’t going to give up on the idea of searching for life on the planet just yet.

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