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K2-18b: first water vapour detection in a habitable-zone planet

Presentation #1145 in the session “Open Engagement Session B”.

Published onMar 17, 2021
K2-18b: first water vapour detection in a habitable-zone planet

Today, more that 4000 exoplanets have been detected, with super-Earths being the most common in our galaxy. We still know very little about these planets, with their basic parameters such as radius and mass — when available — suggesting a great variety among them. However, the density alone does not reveal the chemical composition and climate of these planets, nor casts light into their formation history. To answer those questions, we need to observe their atmospheres. Currently, the WFC3 camera on-board the Hubble Space Telescope is the most powerful instrument to perform infrared transit spectroscopy of exoplanets. Atmospheric characterisation of super-Earths is within reach of the WFC3 but such observations have been very limited so far, with no confirmed detection of molecules.

In this talk, I will present the first detection of a molecular signature from the atmosphere of a super-Earth. HST transit observations of K2-18b, a planet of eight Earth masses orbiting an M2.5 red dwarf, the have revealed a strong signature of water vapour. In addition, K2-18 b is orbiting within the habitable zone of its star, providing the first opportunity to study the nature of a temperate planetary body beyond the mass-radius relationship.

More specifically, we analysed here eight transits of K2-18 b obtained with the WFC3 camera onboard the Hubble Space Telescope, using our specialised, publicly available, tools — Iraclis and TauRex — to perform the end-to-end analysis from the raw HST data to the atmospheric parameters. Our analysis resulted in the detection of an atmosphere around K2-18 b with an ADI (a positively defined logarithmic Bayes Factor) of 5.0, or approximately 3.6 sigma confidence, making K2-18 b the first habitable-zone planet in the mass regime 1-10 M⊕ with an observed atmosphere around it. We modelled the atmosphere following three approaches: a cloud-free atmosphere containing only H2 O and H2 /He, a cloud-free atmosphere containing H2O, H2/He and N2 (N2 acted as proxy for “invisible” molecules not detectable in the WFC3 bandpass but contributing to the mean molecular weight), and a cloudy (flat-line model) atmosphere containing only H2 O and H2 /He.

However, the current data are still very limited, proving only the existence of the atmosphere and the presence of water vapour. These results make K2-18b one of the prime targets for future characterisation studies with the next generation of space telescopes such as the JWST and Ariel. Such observations will help us reveal the presence of additional molecules, such as methane, understand the thermal structure of the atmosphere, and, ultimately, assess the potential habitability of this planet.

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