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Astronomers using the NASA/ESA Hubble Space Telescope have achieved a significant breakthrough by detecting water vapor in the atmosphere of the smallest exoplanet observed to date. GJ 9827d, with a size approximately twice that of Earth, stands as a potential example of planets with water-rich atmospheres beyond our solar system. According to team member Björn Benneke from the Université de Montréal, this marks a pivotal moment in directly demonstrating the existence of planets with water-rich atmospheres around distant stars.
Despite this achievement, it remains challenging to conclusively determine whether the observed water vapor indicates a small amount in a hydrogen-rich atmosphere or if the planet’s atmosphere is predominantly composed of water, a remnant from an early hydrogen/helium atmosphere evaporating under stellar radiation.
The lead author of the study, Pierre-Alexis Roy from the Université de Montréal, emphasized the significance of their observing program designed to detect molecules in the atmosphere of the small exoplanet GJ 9827d, specifically targeting water vapor. Whether water vapor proves dominant or is merely a minor component in a hydrogen-rich atmosphere, both outcomes are considered exciting. According to Björn Benneke, another team member from the Université de Montréal, this marks a notable advancement as, until now, directly detecting the atmosphere of such a diminutive planet had not been achievable.
The study suggests that, given its Venus-like temperature of approximately 425 degrees Celsius, GJ 9827d would present as an inhospitable, steamy world if its atmosphere were predominantly composed of water vapor. Furthermore, as astronomers delve into studying smaller planets, a transition is expected where hydrogen diminishes, and atmospheres become more akin to Venus, dominated by carbon dioxide.
The research team is currently considering two plausible scenarios for the nature of the exoplanet GJ 9827d. One possibility is that the planet retains a hydrogen-rich envelope interwoven with water, resembling a mini-Neptune. Alternatively, it could resemble a warmer version of Jupiter’s moon Europa, harboring a composition of approximately half water and half rock. Björn Benneke suggests that the planet may possess a residual water-rich atmosphere layered on a smaller rocky body.
If the planet indeed has a lingering water-rich atmosphere, it likely formed farther from its host star, in a colder region where water existed in the form of ice. Subsequently, the planet migrated closer to the star, subjected to increased radiation, causing the hydrogen to heat up and escape, or continue to escape due to the planet’s weak gravity. The alternative theory suggests that the planet originated near the hot star, possessing a trace of water in its atmosphere.
The Hubble Space Telescope program conducted observations of the exoplanet GJ 9827d during 11 transits over three years, as the planet crossed in front of its star. These transits allowed scientists to analyze starlight filtered through the planet’s atmosphere, revealing the distinctive spectral signature of water molecules. The study also indicated the potential presence of clouds in the planet’s atmosphere, positioned low enough not to obstruct Hubble’s view, enabling the probe of water vapor above the clouds.
This discovery paves the way for more in-depth investigations, with the upcoming NASA/ESA/CSA James Webb Space Telescope set to perform infrared spectroscopy to identify additional atmospheric molecules. Discovered by NASA’s Kepler Space Telescope in 2017, GJ 9827d completes an orbit around its red dwarf star every 6.2 days and is located 97 light-years away in the constellation Pisces.
