ESA/Hubble, M. Kornmesser - https://www.spacetelescope.org/news/heic1916 - CC-BY 4.0 https://creativecommons.org/licenses/by/4.0

In 2013, I spent a sabbatical in the Center for Cosmology and Particle Physics at NYU. With David Hogg, Dan Foreman-Mackey and Dun Wang, we got interested in data from NASA's Kepler space telescope. Launched in 2009, it initially observed 150000 stars over four years, in search of exoplanet transits. We came up with a causal machine learning method to analyze Kepler data that we referred to as ``half-sibling'' regression.

Meanwhile, the Kepler spacecraft suffered a technical failure, which left it with only two functioning reaction wheels, insufficient for the precise spatial orientation required by the original Kepler mission. NASA decided to use the remaining fuel to make further observations, however the systematic errors were significantly larger than before --- a godsend for our method designed to remove exactly these errors. We augmented it with models of exoplanet transits and an efficient way to search light curves, leading to our discovery of 36 planet candidates (Foreman-Mackey et al. 2015), of which 21 were subsequently validated as bona fide exoplanets (Montet et al. 2015).

One of them, listed as a candidate exoplanet of the star EPIC 201912552 in our work, received the name K2-18b upon confirmation. As later described by Benneke et al. (2017): "The two transit events of the planet candidate K2-18b were originally discovered by analyzing the Campaign 1 data from the extended Kepler Space Telescope ("K2") mission (Foreman-Mackey et al. 2015). Modern seeing-limited images and adaptive optics imaging subsequently ruled out background eclipsing binaries as a possible source for the detected transit events (Montet et al. 2015). Radial velocity measurements further eliminated the possibility that the apparent transit events were caused by non-planetary companions co-moving with K2-18b."

K2-18b orbits the red dwarf K2-18, at a distance of 124 light-years from Earth. The planet is about two and a half times as large as Earth, nine times as massive, and its orbital period is a mere 33 days, i.e., shorter than that of Mercury. However, since K2-18 is less bright than the sun, this ends up being nicely within the star's habitable zone, i.e., allowing for the possibility of liquid water.

In 2019, astronomers found evidence of water in the atmosphere K2-18b --- the first such discovery for an exoplanet in the habitable zone (Benneke et al., Tsiaras et al., 2019). The planet turned out to be one that had been first been detected in our work (Foreman-Mackey et al. 2015). This was a major stroke of luck, and it made the news in many countries (e.g., USA, UK, Germany).

Addendum: the story did not end here. In 2023, carbon dioxide and methane was detected on K2-18b using the James Webb space telescope. K2-18b led to the notion of a hycean world, with abundant liquid water and a hydrogen envelope, and is an intriguing object for astrobiology (Sky at Night, NASA, The Guardian).

I am grateful that I was able to be part of this team.