Astronomers using NASA’s Hubble Space Telescope have observed the smallest exoplanet where water vapour has been detected in the atmosphere.
At only approximately twice Earth’s diameter, the planet GJ 9827d could be an example of potential planets with water-rich atmospheres elsewhere in our galaxy.
“This would be the first time that we can directly show through an atmospheric detection, that these planets with water-rich atmospheres can actually exist around other stars,” says team member Björn Benneke of the Trottier Institute for Research on Exoplanets at Université de Montréal. “This is an important step toward determining the prevalence and diversity of atmospheres on rocky planets.”
Co-principal investigator Laura Kreidberg of Max Planck Institute for Astronomy adds: “Water on a planet this small is a landmark discovery. It pushes closer than ever to characterising truly Earth-like worlds.”
However, it remains too early to tell whether Hubble spectroscopically measured a small amount of water vapour in a puffy hydrogen-rich atmosphere, or if the planet’s atmosphere is mostly made of water, left behind after a primeval hydrogen/helium atmosphere evaporated under stellar radiation.
“Our observing program, led by principal investigator Ian Crossfield of Kansas University, was designed specifically with the goal to not only detect the molecules in the planet’s atmosphere, but to actually look specifically for water vapour,” says the science paper’s lead author, Pierre-Alexis Roy of the Trottier Institute for Research on Exoplanets at Université de Montréal.
“Either result would be exciting, whether water vapour is dominant or just a tiny species in a hydrogen-dominant atmosphere.”
“Until now, we had not been able to directly detect the atmosphere of such a small planet,” adds Benneke. “And we’re slowly getting in this regime now.
“At some point, as we study smaller planets, there must be a transition where there’s no more hydrogen on these small worlds, and they have atmospheres more like Venus (which is dominated by carbon dioxide).”
Astronomers using NASA’s Hubble Space Telescope have observed water vapour in the atmosphere of the smallest exoplanet ever detected. Located 97 light-years away, planet GJ 9827d is approximately twice the size of Earth.
Credit: NASA’s Goddard Space Flight Centre/Lead Producer: Paul Morris
Because the planet is as hot as Venus, at 800 degrees Fahrenheit, it definitely would be an inhospitable, steamy world if the atmosphere were predominantly water vapour.
At present the team is left with two possibilities. One scenario is that the planet is still clinging to a hydrogen-rich atmosphere laced with water, making it a mini-Neptune.
Alternatively, it could be a warmer version of Jupiter’s moon Europa, which has twice as much water as Earth beneath its crust.
“The planet GJ 9827d could be half water, half rock. And there would be a lot of water vapour on top of some smaller rocky body,” says Benneke.
If the planet has a residual water-rich atmosphere, then it must have formed farther away from its host star, where the temperature is cold and water is available in the form of ice, than its present location.
In this scenario, the planet would have then migrated closer to the star and received more radiation. The hydrogen was heated and escaped, or is still in the process of escaping the planet’s weak gravity.
The alternative theory is that the planet formed close to the hot star, with a trace of water in its atmosphere.
The Hubble program observed the planet during 11 transits – events in which the planet crossed in front of its star – that were spaced out over three years. During transits, starlight is filtered through the planet’s atmosphere and has the spectral fingerprint of water molecules.
If there are clouds on the planet, they are low enough in the atmosphere so that they don’t completely hide Hubble’s view of the atmosphere, and Hubble is able to probe water vapour above the clouds.
“Observing water is a gateway to finding other things,” says Thomas Greene, astrophysicist at NASA’s Ames Research Centre. “This Hubble discovery opens the door to future study of these types of planets by NASA’s James Webb Space Telescope.
“JWST can see much more with additional infrared observations, including carbon-bearing molecules like carbon monoxide, carbon dioxide, and methane. Once we get a total inventory of a planet’s elements, we can compare those to the star it orbits and understand how it was formed.”
GJ 9827d was discovered by NASA’s Kepler Space Telescope in 2017. It completes an orbit around a red dwarf star every 6,2 days. The star, GJ 9827, lies 97 light-years from Earth in the constellation Pisces.
Featured picture: This is an artist’s concept of the exoplanet GJ 9827d, the smallest exoplanet where water vapour has been detected in the atmosphere. The planet could be an example of potential planets with water-rich atmospheres elsewhere in our galaxy. With only about twice Earth’s diameter, the planet orbits the red dwarf star GJ 9827. Two inner planets in the system are on the left. The background stars are plotted as they would be seen to the unaided eye looking back toward our Sun. The Sun is too faint to be seen. The blue star at upper right is Regulus; the yellow star at center bottom is Denebola; and the blue star at bottom right is Spica. The constellation Leo is on the left, and Virgo is on the right. Both constellations are distorted from our Earth-bound view from 97 light-years away.
NASA/ESA/Leah Hustak (STScI)/Ralf Crawford (STScI)