Astronomers using the NASA/ESA Hubble Space Telescope observed the smallest exoplanet with water vapor detected in its atmosphere. The planet GJ 9827d, which is roughly twice the diameter of Earth, may be an example of potential planets with water-rich atmospheres elsewhere in our galaxy.

“This will be the first time we can directly demonstrate through atmospheric detection that these planets with water-rich atmospheres may actually exist around other stars.” said team member Bjorn Benneke from the University of Montreal. “This is an important step towards determining the prevalence and diversity of atmospheres on rocky planets.”

However, it is too early to tell whether Hubble has spectroscopically measured small amounts of water vapor in a loose, hydrogen-rich atmosphere, or whether the planet’s atmosphere consists mainly of water left over from the evaporation of the primitive hydrogen-helium atmosphere. effect of stellar radiation. .

“Our observation program is specifically designed not only to detect molecules in the planet’s atmosphere, but also to look for water vapor. Any results will be fascinating, whether dominated by water vapor or a small species in an atmosphere dominated by hydrogen.” said Pierre-Alexis Roy, lead author of the scientific paper from the University of Montreal.

“So far we have not been able to directly detect the atmosphere of such a small planet. And now we are gradually entering this mode” – added Benneke. “At some point, when studying smaller planets, there has to be a transition where these smaller worlds no longer have hydrogen and have atmospheres that are more Venus-like (dominated by carbon dioxide).”

Since the planet is as hot as Venus, about 425 degrees Celsius, it would be an absolutely uninhabitable, steamy world if the atmosphere consisted mostly of water vapor.

Currently the team has two options. The planet still clings to a hydrogen-rich, water-permeated crust, making it a mini-Neptune. It could also be a hotter version of Jupiter’s moon Europa, which has twice as much water under its crust as Earth. “The planet GJ 9827d may be half water and half rock. And a smaller rocky body would have a lot of water vapor on it.” said Benneke.

If a planet has a water-rich atmosphere, it must have formed farther from its star than its current location, where temperatures are lower and water exists in the form of ice. In this scenario, the planet will move closer to the star and receive more radiation. The hydrogen was then heated and escaped, or was still in the process of escaping, from the planet’s weak gravity. An alternative theory is that the planet formed near a hot star with traces of water in its atmosphere.

The Hubble program observed the planet during 11 transits (events in which a planet passes in front of its star) over three years. During transit, starlight is filtered through the planet’s atmosphere and carries the spectral signature of water molecules. If a planet has clouds, they are low enough that they do not completely block the atmosphere that Hubble can see, and Hubble can examine the water vapor above the clouds.

Hubble’s discovery paves the way for more detailed examination of the planet. A good goal is for the NASA/ESA/CSA James Webb Space Telescope to perform infrared spectroscopy to look for other atmospheric molecules. GJ 9827d was discovered by NASA’s Kepler space telescope in 2017. It orbits the red dwarf every 6.2 days. The star GJ 9827 is located 97 light-years from Earth in the constellation Pisces.