Free Astronomy Magazine March-April 2024

MARCH-APRIL 2024 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).” Because the planet is as hot as Venus at roughly 425 degrees Celcius, it definitely would be an inhospitable, steamy world if the atmosphere were predominantly water vapour. At present the team is left with two possibilities. The planet is still cling- ing to a hydrogen-rich envelope 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,” said 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 hydro- gen was then heated and escaped, or is still in the process of escaping, the planet’s weak gravity. The alter- native theory is that the planet formed close to the hot star, with a trace of water in its atmosphere. The Hubble programme 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 carries the spectral fingerprint of water molecules. If there are clouds on the planet, they are low enough in the atmosphere that they don’t com- pletely hide Hubble’s view of the at- mosphere, and Hubble is able to probe water vapour above the clouds. Hubble’s discovery opens the door to studying the planet in more de- tail. It’s a good target for the NASA/ESA/CSA James Webb Space Telescope to do infrared spec- troscopy to look for other atmos- pheric molecules. 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. T his is an artist’s conception of the exoplanet GJ 9827d, the smallest exo- planet where water vapour has been detected in its atmosphere. The planet could be an example of potential planets with water-rich atmospheres else- where in our galaxy. It is a rocky world, only about twice Earth’s diameter. It or- bits 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, which itself is too faint to be seen. The blue star at upper right is Regulus, the yellow star at bottom centre 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 and Ralf Crawford (STScI)] !