Published in the 'Astrophysical Journal Letters', a study led by Caroline Piaulet-Ghorayeb, a Ph.D. candidate at the Universite de Montreal's Trottier Institute for Research on Exoplanets (IREx), has unveiled significant findings regarding the exoplanet GJ 9827 d. Earlier in 2024, researchers at IREx detected water in the atmosphere of GJ 9827 d using data from the Hubble Space Telescope (HST), marking it as the smallest known exoplanet, approximately twice the size of Earth, to possess a confirmed atmosphere. The latest observations with the James Webb Space Telescope (JWST) provide crucial insights, revealing a water-rich atmosphere and opening up exciting possibilities for understanding exoplanetary atmospheres.
Detecting atmospheres around smaller, Earth-sized planets has posed a considerable challenge. Most atmospheric studies have focused on larger gas giants or mini-Neptunes, which have hydrogen-dominated atmospheres similar to Jupiter or Neptune in our Solar System. According to Piaulet-Ghorayeb, 'For now, all the planets we've detected that have atmospheres are giant planets, or at best mini-Neptunes.' In contrast, GJ 9827 d features a distinctive atmosphere rich in heavier molecules, including water vapor, rather than being dominated by hydrogen.
The research team utilized transmission spectroscopy to analyze light passing through the planet's atmosphere as it transited its star. By integrating JWST data with previous HST observations, they confirmed that the spectral features detected originate from the planet's atmosphere, ruling out contamination by the star. Two possible atmospheric types for GJ 9827 d were identified: one with a cloudy, hydrogen-dominated atmosphere and trace water, and another with a denser, water-rich atmosphere.
Despite its high surface temperature of approximately 350 degrees Celsius making it unlikely to support life, the discovery is a significant milestone in the quest for Earth-like exoplanets. Piaulet-Ghorayeb remarked, 'This is a huge step towards the goal of searching for atmospheres around smaller, terrestrial-like planets.' The identification of a water-rich atmosphere on GJ 9827 d suggests that other small, rocky exoplanets could also possess similar atmospheres, paving the way for future explorations.
Scientists have long theorized the existence of 'steam worlds,' planets with thick, water-vapor-rich atmospheres. These planets orbit close enough to their stars that water exists as steam rather than beneath an icy layer. Ongoing JWST observations of GJ 9827 d are planned to further investigate the components of its intriguing atmosphere.