Using the Hubble Space Telescope, an astronomical research team has discovered signs of water vapor in the atmosphere of the exoplanet GJ 9827d. “It would be the first time that we can directly show, by studying atmospheres, that these water-rich planets can actually exist around other stars.”explains Björn Benneke (Trottier Institute for Research on Exoplanets at the Université de Montréal) according to a press release from the Max Planck Society.
GJ 9827d is the smallest exoplanet on which water vapor has been discovered to date. However, it is not yet clear whether Hubble measured a small amount of water vapor in a bloated, hydrogen-rich atmosphere or whether the atmosphere consists mainly of water that remained after a primordial hydrogen/helium atmosphere evaporated under the star’s radiation.
The exoplanet GJ 9827d and its star
The Exoplanet GJ 9827d is located 97 light-years away from us and its diameter is about twice that of Earth. Its mass is 3.53 Earth masses.
The Neptune-like exoplanet takes around 6.2 days to orbit its star. It is only 0.0559 astronomical units (AU) away from its star. One AU corresponds to the average Sun-Earth distance.
Due to its proximity to its star, the planet is as hot as Venus, with a temperature of around 400 degrees Celsius. The planet was discovered in 2017 using NASA’s Kepler telescope.
Its star, GJ 9827, is a K-type main sequence star. This type of star is less massive than our Sun and therefore cooler. That’s why they glow in an orange light. Its apparent magnitude is 10,250 mag. This means its magnitude is between our Neptune (7.8 mag) and the dwarf planet Pluto (13.9 mag). The system is located in the constellation Pisces.
Groundbreaking discovery
According to the research team, if the planet does indeed have a residual water-rich atmosphere, it must have formed at a greater distance from its host star, where temperatures were lower and water was present in the form of ice. GJ 9827d may have migrated to its star over time. The existing ice then turned into water and finally into water vapor.
In this scenario, the hydrogen escaped or is still escaping its planet’s weak gravity. Alternatively, GJ 9827d could have formed close to its star. Due to Hubble’s discovery, this exoplanet was also recently studied by the James Webb Space Telescope.
“Water on such a small planet is a groundbreaking discovery,” said Laura Kreidberg, director of the APEx (Atmospheric Physics of Exoplanets) department at the Max Planck Institute for Astronomy. She can hardly wait to evaluate the Hubble data: “Hopefully we can now settle the question of water worlds once and for all.”