Astronomers using NASA’s Hubble Space Telescope observed the smallest exoplanet to have water
vapor detected in its atmosphere.
At only around twice the Earth’s diameter, the planet GJ 9827d lies 97 light-years away
and could be an example of potential planets with water-rich atmospheres elsewhere in our
galaxy.
However, it remains too early to tell whether Hubble measured a small amount of water vapor
in a puffy hydrogen-rich atmosphere, or if the planet’s atmosphere is mostly made of
water.
Currently the scientific team leading the study has two possibilities.
The planet is still clinging to a hydrogen-rich atmosphere laced with water, making it a mini-Neptune.
Or, it could be a warmer version of Jupiter’s moon Europa, which has twice as much water
as Earth beneath its crust.
Over a period of three years, Hubble observed the planet during 11 transits — events in
which the planet crossed in front of its star.
During transits, starlight is filtered through the planet’s atmosphere and displays the
spectral fingerprint of water molecules.
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 vapor.
Hubble’s discovery opens the door to studying the planet in more detail.
It’s a good future target for NASA’s James Webb Space Telescope to do infrared spectroscopy
to look for other atmospheric molecules.
Regardless of GJ 9827d’s uninhabitable nature, water on a planet this small is a landmark
discovery, pushing us closer than ever to characterizing truly Earth-like worlds, and
thanks to the Hubble Space Telescope we’ll be able to continue our search for more planets
like our own.
"Follow us on social media @NASAHubble"