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Last week, two teams, one from the UK and one from across North America, independently reported a pretty cool discovery. They found water in the atmosphere of an exoplanet about 110 light-years away, called K2-18b.

Which has implications for our search for life on other worlds, but it's probably not time to pop the champagne cork just yet. See, this isn't the first time water's been found in an exoplanet's atmosphere. Water is a pretty common molecule in space, after all.

But what makes this discovery so interesting is that the planet is fairly small, and it's in its star's habitable zone, where it's likely that the water can be in liquid form. To make this discovery, the two teams both used the same data: observations of the planet's host star by the Hubble Space Telescope. They looked at the star's light signature, or spectrum, at different times: when the planet was in front of the star, and when it wasn't.

The obviously the difference between those two spectra gives you a spectrum of the planet's atmosphere. And that spectrum tells you what's in the atmosphere, because different atoms and molecules have characteristic features that let us identify them. So when they looked at that spectrum, they found the signature of good ol' H2O.

Sort of. Because these measurements aren't actually terribly precise. There's a bunch of statistical noise.

So the UK team applied some more math to the data and created a model of the planet's atmosphere. And depending on exactly how they modelled the atmosphere, they concluded it was anywhere from 0.01% to 50% water vapor. K2-18b is a type of exoplanet sometimes called a super-Earth, but also sometimes called a mini-Neptune.

And these are both bad names. This planet isn't really like Earth or Neptune. It's probably about 8x the mass of Earth, and 2/3 the density.

It might be more like a gas giant, or more like a rocky planet with a very thick atmosphere. We just don't know! Although K2-18b is technically in its star's habitable zone, we don't think anything lives there, water or no water.

It's really close to its star, taking only 33 days to orbit. But the star gives off a lot less light than our Sun. So the planet receives a similar amount of light to Earth.

That doesn't mean it's habitable, though. The star is a red dwarf, and those are known to frequently emit violent solar flares. And because it's so close, K2-18b is probably being bombarded with dangerous amounts of UV radiation and cosmic rays that are harmful to life as we know it.

This discovery does prove that current telescope tech is still capable of a lot of interesting science, even as astronomers race to build bigger and better ones. But we still want to learn more about what these planets are made of, and what else is in their atmospheres. For that, we still need to wait for the next generation of space telescopes, like NASA's James Webb Space Telescope and the European Space Agency's Ariel mission.

Moving a bit closer to home, our next piece of news is that we have a visitor! For only the second time, astronomers have found a rock from interstellar space flying through our solar system. You may remember ‘Oumuamua, which briefly visited in 2017.

Well, last week, astronomers announced a new object, given the temporary name of C/2019 Q4 (Borisov), which we'll just call Borisov for short right now. And it has been seen in the neighborhood. It was found at the end of August by an amateur astronomer in Crimea by the name of Gennady Borisov, hence the name.

It's actually not yet 100% confirmed to be from outside our solar system, but the evidence is looking good. Like ‘Omuamua, Borisov was probably made in a different star system, and was then flung out of the star's orbit. So now it's a chunk of rock that's just randomly floating through space between the stars, and which happened to come near the solar system.

Which means that by learning more about it, and other objects like it, we can maybe learn something about the formation of star systems beyond our own. For instance, Borisov is probably more comet-like than ‘Oumuamua. It looks fuzzy, so we think it probably has a thin, gaseous envelope, or coma, around it, just like a comet.

So now we can ask: what do comets from other stars look like? Another neat thing is that we should be able to observe Borisov for up to a year. ‘Oumuamua only stayed around for a few weeks after we detected it. But we've spotted Borisov as it's still getting closer to us.

And its closest approach is expected to be in December. At that point, it'll pass between Mars' and Jupiter's orbits. But it's on what astronomers call a hyperbolic orbit, meaning it's not actually orbiting the sun; it'll take a single big swing past it, and then depart.

Interstellar rocks like Borisov are rare, but not too rare. We think at any given time there's probably about one interstellar object like this in our solar system. But we rarely get a chance to actually watch one pass like this.

And the European Space Agency may try to send a mission to a future visitor like Borisov to learn even more about these objects, so any data we gather now may help it along. But maybe more importantly, it's a pretty cool reminder that amateur astronomers make awesome new discoveries! Thanks for watching this episode of SciShow Space News, and thanks especially to our patrons on Patreon without which we could not make this content!

If you're interested in becoming a patron, go to patreon.com/scishow. [♪ OUTRO].