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, still referred to as exoplanet candidate EPIC 201912552 in our work, received the name K2-18b. 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."

Four years later, astronomers found traces of water in the atmosphere K2-18b --- the first such discovery for an exoplanet in the habitable zone, i.e., allowing for liquid water (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). I was very happy to be part of this team.