Astronomers have made an exciting discovery using the Hubble Space Telescope: they have detected water molecules in the atmosphere of a small exoplanet located 97 light-years away from Earth. This exoplanet, named GJ 9827d, is about twice the diameter of Earth and is the smallest exoplanet to have water vapor in its atmosphere. While water is essential for life as we know it, this planet is unlikely to host any form of life due to its searing temperatures that would turn a water-rich atmosphere into scorching steam.
The true nature of GJ 9827d’s atmosphere is still unknown, but this discovery opens up possibilities for further investigation into the origins of planets beyond our solar system. The findings of this study were published in The Astrophysical Journal Letters.
According to Laura Kreidberg, the study’s coauthor and managing director of the atmospheric physics of exoplanets department at the Max Planck Institute for Astronomy in Heidelberg, Germany, “Water on a planet this small is a landmark discovery. It pushes closer than ever to characterizing truly Earth-like worlds.”
Despite its potential for harboring water vapor, GJ 9827d is an inhospitable world with temperatures reaching a scorching 800 degrees Fahrenheit (427 degrees Celsius), making it as hot as Venus. However, this discovery is significant because it provides direct evidence that planets with water-rich atmospheres can exist around other stars.
Björn Benneke, another coauthor of the study and a professor at the University of Montreal’s Trottier Institute for Research on Exoplanets, explains, “This is an important step toward determining the prevalence and diversity of atmospheres on rocky planets.”
The research team is still uncertain whether Hubble detected water vapor traces within a hydrogen-rich atmosphere or if the planet has a water-rich atmosphere due to the evaporation of its original hydrogen and helium atmosphere by its host star. Pierre-Alexis Roy, the lead study author and a doctoral student at the University of Montreal’s Trottier Institute, states, “Either result would be exciting, whether water vapor is dominant or just a tiny species in a hydrogen-dominant atmosphere.”
GJ 9827d was initially discovered by NASA’s Kepler mission in 2017. It orbits a red dwarf star in the Pisces constellation and completes a single orbit every 6.2 days. Astronomers observed the exoplanet during 11 transits over three years, allowing them to measure the signature of water molecules by analyzing starlight filtering through its atmosphere.
Benneke explains that as astronomers study smaller planets, there must be a transition where these worlds no longer have hydrogen in their atmospheres and instead have atmospheres more similar to Venus, which is dominated by carbon dioxide. Understanding more about GJ 9827d’s atmosphere could help astronomers classify the type of world it is.
Currently, the research team has two possible theories about GJ 9827d’s nature. It could be a mini-Neptune with a hydrogen-rich atmosphere that contains water vapor. If this is the case, the planet likely formed at a greater distance from its host star, where it was colder and water existed in the form of ice, similar to Neptune and Uranus in our solar system. As the planet migrated closer to its star and experienced more stellar radiation, the hydrogen heated up and escaped.
Alternatively, GJ 9827d could be a warmer version of Jupiter’s icy moon Europa, with an ocean beneath a thick, icy crust. Benneke suggests that the planet might be composed of half water and half rock.
Water is one of the most common molecules found across the universe, and its detection plays a crucial role in the search for life beyond Earth. Thomas Greene, an astrophysicist at NASA’s Ames Research Center, explains, “Observing water is a gateway to finding other things.” This discovery by Hubble opens the door for future study of similar planets using the James Webb Space Telescope, which can provide additional infrared observations and detect carbon-bearing molecules like carbon monoxide, carbon dioxide, and methane. By comparing a planet’s elemental inventory to that of its host star, astronomers can gain insights into how it was formed.
The research team has already observed GJ 9827d with the Webb telescope to search for water and other molecules, and the data will be shared in the future. Kreidberg expresses anticipation, stating, “We can hardly wait to see what those data reveal. Hopefully, we can now settle the question of water worlds once and for all.”