Thread by @JDLothringer

So we found water vapor in the atmosphere of a 9 Earth-mass exoplanet in the habitable zone! After observing 8 transits of K2-18b with HST/WFC3, we (led by Björn Benneke) find a significant water feature:

arxiv.org/abs/1909.04642

K2-18b receives only 5% more radiation than the Earth, leaving it with an equilibrium temperature of 265 K. This is the coolest exoplanet that we've detected water in. While not a true Earth-analogue due to its size, this bodes well for our exploration of small planets.

One of the most intriguing parts to me is how clear the atmosphere is! K2-18b appears cool enough that cloud species like KCl and ZnS don't form, while also being far enough away from its moderately active M-dwarf host star to not have significant haze production.

That's not to say there aren't *any* clouds: at pressures around 100 mbar, we find some water clouds: the first water clouds in an exoplanet! (Turns out we've found some water clouds in a chilly brown dwarf though: ui.adsabs.harvard.edu/abs/2016ApJ...…)

I should emphasize that it was mostly Björn doing the heavy lifting on this work. I just happen to be the highest co-author with a twitter account and wanted to share this exciting result!🔭🌌🛸

Keep your eyes peeled later today for results using some of the same data by a different team...!

Here's the other team's paper: nature.com/articles/s4155…