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Is K2-18b really a habitable super-earth? | space

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  Patchy tan-orange planet with atmosphere; roiling reddish star in the distance.

The artist's concept of K2-18b, as well as another planet in this system, K2-18c, with the parent star, a red dwarf, in the background. Image via Alex Boersma / iREx.

A few days ago, EarthSky reported that for the first time ever, water vapor has been detected in the atmosphere in a potentially habitable external planetary earth. We were not alone in our report. As one would expect, the finding received much attention from the media. But it turns out that the story may not be true as first reported and to some extent was incorrectly characterized.

The discovery was described in two different newspapers, the first published on arXiv on September 1

0, 2019, and the second in the peer-reviewed journal Nature Astronomy on September 11, 2019.

The newspapers describe the discovery of water vapor in the atmosphere of K2-18b, an exoplanet in the star zone of its star – where temperatures can allow liquid water to exist – 110 light-years from Earth. It is true that it is the first time water vapor has been identified in the atmosphere of a smaller exoplanet (the non-gas giant) in the star zone of its star, but shortly after the announcement, many planetary scientists criticized how the discovery was covered in media and social media.

The water vapor detection itself is confirmed, but much is discussed about what type of plane K2-18b really is, and how habitable it can be (or not).

  Semi-light blue planet with white bands in its atmosphere and distant sun.

Another artist's concept of super-earth K2-18b. Scientists have detected water vapor in the atmosphere, but is it proper? Most researchers say it is unlikely. Image via ESA / Hubble, M. Kornmesser / UCL News.

Some scientists, including in Nature Astronomy have referred to the planet as a super-earth. A super Earth is larger than Earth but smaller than Neptune – usually up to about twice the size of Earth – and many have already been discovered. Most are believed to be rocky, like Earth, but there is a transition point – from about 1.6 to 2 times the Earth's radius – where a planet can become a mini-gas giant, or a mini-Neptune as they are commonly called. They are larger than super-Earths, but still smaller than Neptune. Most scientists now consider that K2-18b is a mini-Neptune, not a super-earth, with a deep atmosphere of hydrogen and / or helium, and possibly no solid surface at all.

K2-18b has a radius of about 2.7 times the earth and a mass about nine times the earth. While some scientists still consider it to be a possible super-earth, most seem to classify it as a mini-Neptune. All this can be a bit confusing.

The 2017 study previously referred to considered that K2-18b may be either large and stony or covered with water and / or ice. But that study did not account for atmospheric constraints, only mass and radius. As exoplanet scientist Erin May told me on Twitter:

My graduate student focused in part on the difference between these classes of planets. Many studies show that it is extremely difficult to create a planet> 2 radii without a large atmosphere. Mass and radius (density) alone are actually not very useful here. I would also like to point out that this planet from mass and radius should never have been considered a super-earth. I think there is a tendency to throw this term because it is more "exciting", but we as astronomers have to keep our terminology straight.

Néstor Espinoza, an astronomer at the Max Planck Institute for Astronomy (MPIA), also told me:

If you believe in the water function, you have to believe that it is an H / Han dominated atmosphere so yes. And the source you cite is not "outdated" – at that time we just didn't have atmospheric limitations, just mass and radius. Also: the fact that we see a water feature * implies * an H / He dominated atmosphere. There is no road around it.

There's a good Twitter thread about all this here from Jessie Christiansen, a researcher at NASA Exoplanet Science Institute (NExScI):

Also from Nature correspondent Alexandra Witze:

And thread from Marina Koren on Atlantic :

So what is said about housing? Since the planet – considered by most scientists – is a mini-Neptune, this greatly reduces the chance. The actual water vapor, or even rain (which is still considered possible in this planet's atmosphere), is good, but longevity requires a rocky surface / interior for chemical nutrients and liquids. There may indeed be planets out there with life forms in a gaseous atmosphere, but for the Earth's kind of life at least K2-18b seems inappropriate for this.

  Earth, super-earth and mini-Neptunes show depth of water and gas envelopes.

There has been much debate about whether K2-18b is a super-earth or mini-Neptune. Most researchers now agree that it is a mini-Neptune, which makes the home much less likely. Image via Patterson Clark / Washington Post / Quora.

Finding evidence of water vapor on a distant exoplanet in its star-inhabited zone is intriguing but is not in itself proof that it is common. There are many factors that need to be considered, including the composition of the planet and its atmosphere. However, K2-18b is the smallest exoplanet so far found to have water vapor in its atmosphere, which is a good sign: it supports the researchers' claim that even smaller planets with water vapor and / or liquid water will exist, worlds that are more earth-like in both size and composition. Upcoming space-based telescopes such as the James Webb Space Telescope (JWST) will be able to study the atmospheres of planets like this and less, in more detail than ever before, and even search for biosignatures, which can be proof of life.

Bottom line: The exoplanet K2-18b has water vapor in its atmosphere, but the planet itself is probably very earth-like.

Source: Water vapor in the atmosphere of the Eight-Earth-Easier Zone K2-18 b

Source: Water Vapor on the Habitable-Zone Exoplanet K2-18b

Via UCL News

  Paul Scott Anderson [19659039]</pre>
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