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It was exciting, scientists announced water vapor in a super-Earth’s atmosphere. But, even as the announcement came, other scientists were cautioning that the planet – K2-18b – is probably less like a super-Earth and more like a mini-Neptune.

The discovery was outlined in two different papers, the first one published on arXiv on September 10, 2019, and the second in the peer-reviewed journal Nature Astronomy on September 11, 2019.

The papers detail the finding of water vapor in the atmosphere of K2-18b, an exoplanet in the habitable zone of its star – where temperatures could allow liquid water to exist – 110 light-years from Earth. It’s accurate that this is the first time that water vapor has been identified in the atmosphere of a smaller exoplanet (non-gas-giant) in the habitable zone of its star, but soon after the announcement, many planetary scientists critiqued how the discovery was covered in media and social media.

The water vapor detection itself is confirmed, but there is a lot of debate as to just what kind of planet K2-18b actually is, and how habitable it may be (or not).

Some scientists, including in the Nature Astronomy paper, have referred to the planet as a super-Earth. A super-Earth is larger than Earth but smaller than Neptune – typically up to about a maximum of twice the size of Earth – and many have been discovered already. Most are thought to be rocky, like Earth, but there is a transition point – starting about 1.6 to 2 times Earth’s radius – where a planet can become a mini-gas-giant, or a mini-Neptune as they are usually called. They are larger than super-Earths, but still smaller than Neptune. Most scientists now consider K2-18b to be 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 that of Earth, and a mass about nine times that of Earth. While some scientists would still consider that to be a possible super-Earth, most, it seems, would classify it as a mini-Neptune. All of this can be a bit confusing.