Water vapor detected in the atmosphere of a minuscule exoplanet by Hubble

Astronomers have made a groundbreaking discovery using the NASA/ESA Hubble Space Telescope – they have observed the smallest exoplanet where water vapor has been detected in its atmosphere. This planet, known as GJ 9827d, is just about twice the size of Earth and could potentially be a prime example of a water-rich atmosphere on other planets within our galaxy.

This significant finding marks the first time that astronomers have directly shown through atmospheric detection that planets with water-rich atmospheres can exist around stars other than our Sun. The team behind the discovery, including Björn Benneke from the Université de Montréal, explains that this is an important step toward understanding the prevalence and diversity of atmospheres on rocky planets throughout the universe.

At this stage, it is still difficult to determine whether Hubble has detected a small amount of water vapor in a hydrogen-rich atmosphere or if the planet’s atmosphere is entirely composed of water left over from its primordial hydrogen and helium atmosphere, which evaporated due to stellar radiation. Pierre-Alexis Roy, the lead author of the study, explains that their observing program was specifically designed not only to detect molecules in the planet’s atmosphere but also to search for water vapor. Regardless of the outcome, it’s an exciting development, as discovering dominant water vapor or even a trace species in a hydrogen-dominated atmosphere would be groundbreaking.

The team is thrilled with their ability to directly detect the atmosphere of such a small planet, as it marks a transition into studying smaller planets and their atmospheres. They believe that as they explore smaller worlds, eventually, they will reach a threshold where hydrogen is no longer present, and these planets will have atmospheres similar to that of Venus, which is predominantly composed of carbon dioxide.

However, GJ 9827d, with its scorching surface temperature of approximately 425 degrees Celcius, would be an inhospitable world if its atmosphere were primarily composed of water vapor. At present, the team is considering two possibilities for the planet’s composition. It may still be enveloped in a hydrogen-rich envelope mixed with water, making it a mini-Neptune. Alternatively, it could resemble a warmer version of Jupiter’s moon Europa, which has twice the amount of water beneath its surface compared to Earth.

If the planet does indeed possess a water-rich atmosphere, it must have formed farther away from its host star where temperatures are cold enough for water to exist in its icy state. Subsequently, the planet would have migrated closer to the star, exposing it to more radiation. The heat from the star would then cause the hydrogen to escape the weak gravitational pull of the planet, or it may still be in the process of escaping. The alternative theory suggests that GJ 9827d formed near the hot star and developed a trace amount of water in its atmosphere.

To gather this valuable information, the Hubble Space Telescope observed the planet during 11 transits – events in which the planet passes in front of its star. These observations were spaced out over three years. During these transits, starlight passes through the planet’s atmosphere, carrying the spectral fingerprint of water molecules. If there are clouds on the planet, they are located low in the atmosphere, allowing Hubble to explore water vapor levels above the cloud cover.

The discovery made by Hubble discovers new possibilities for studying GJ 9827d in greater detail. The next step is for the NASA/ESA/CSA James Webb Space Telescope to conduct infrared spectroscopy and search for other atmospheric molecules. This important milestone will further our understanding of exoplanets and their potential habitability.

GJ 9827d was initially discovered in 2017 by NASA’s Kepler Space Telescope. It completes an orbit around a red dwarf star every 6.2 days and is located 97 light-years away from Earth in the constellation Pisces. This exoplanet may hold the key to unraveling the mysteries of water-rich atmospheres on rocky planets, shedding light on the prevalence of life-sustaining conditions beyond our solar system.