The exciting news in astronomy is the discovery of water in the atmosphere of a relatively small planet, known as K2-18b, that happens to lie in what we often to as the habitable zone of its parent star. The result was reported in this Nature Astronomy paper, and also in this arXiv paper that appeared on the same day. I know some of the authors of both papers, have published with some of the authors of the second paper and am currently working with one of the authors of the first.
The reason why this is such a fascinating result is that it’s the detection of water in the atmosphere of an exoplanet, the exoplanet itself is actually quite small, and it lies in a region often referred to as the habitable zone, but that really simply means that conditions could be suitable for the existence of liquid water.However, some of the coverage has been less than satisfactory. It’s being presented as a planet that could support life. However, this paper reports that it has a mass of Earth masses and a radius of Earth radii. You can actually put this onto a mass-radius diagram, which I discovered I’d essentially already done. The figure on the right is a mass-radius diagram with all known exoplanets with masses below 20 Earth masses and radii below 2.75 Earth radii. You can clearly see the location of K2-18b, which I’ve also highlighted.
The dashed curves are composition curves. Those shown in the figure include that for a planet that would be 100% iron, one for a planet that would have an Earth-like composition (~25% iron, ~75% silicates), one for a planet that would be entirely silicates, and also includes composition curves for planets in which water would make up a substantial fraction of their mass, and one for a planet with a hydrogen-rich atmosphere that would be about 1% of the planet mass. You can see that K2-18b lies somewhere in the region where it would either have a substantial amount of water (~50%) or it would have a reasonably substantial (~1%, or more) hydrogen-rich atmosphere. In neither case would this be a planet that would typically be regarded as habitable.
Furthermore, the observations are transit observations at different wavelengths. In other words, what we’re observing is the planet as it passes between us and its host star, and comparing the amount blocked at different wavelengths. Given that these differences must be due the atmosphere blocking different amounts of light at different wavelengths, we can use this to say something about the atmospheric composition. However, the signal depends strongly on the scaleheight of the atmosphere, which is larger for a lighter atmosphere (one that contains substantial amounts of hydrogen) than it is for a heavier atmosphere (one that was pre-dominantly water vapour, for example). The paper itself makes clear that a non-negligible fraction of the atmosphere must be hydrogen and helium.
Additionally, a paper published in early 2019 has suggested that the radius of K2-18b is actually Earth radii, which would put K2-18b into a region of mass-radius space that suggests it almost certainly retains a substantial hydrogen-rich atmosphere; essentially, it’s a mini-Neptune. Maybe life could exist in the upper regions of such a planet’s atmosphere, but this is almost certainly not what most people would think of when they hear that a planet is potentially habitable.
You might think that this is mostly a problem with the media over-hyping a news story. But, no, it appears to be the narrative presented in the university press release and in the lead author’s The Conversation article. It would be good if the media were to talk to researchers not involved in the actual study, but it’s hard to blame them for presenting a story that is similar to what is being presented by the researchers themselves.
I think that this is all rather unfortunate. This kind of result is very exciting without needing to present a narrative that is probably not true. We also live in an era where it would seem important to not provide more ammunition for those who would like to undermine the public’s trust in experts.
Unfortunately, though, I suspect that these kind of over-hyped stories are likely to continue happening. In the context of habitability on an exoplanet, the public and the media should be dubious of any such claims for at least the next few years, if not longer. These kind of observations are very difficult and although the result presented here is impressive, we can still only do this for planets that are quite a bit larger than the Earth. This doesn’t mean that such planets can’t be habitable, but it’s almost certainly not going to be the kind of life that people think of when they hear such claims. I think it’s important to be clear about this when presenting such results.