Using the Hubble Space Telescope, astronomers have discovered that the atmosphere of a relatively small planet outside the solar system is rich with water vapor. Don't plan a vacation to this destination just yet, however. The planet's surface is hot enough to melt lead, meaning it's a steamy world inhospitable to life as we know it.
More specifically, the team behind this finding says the extra-solar planet, or exoplanet, named GJ 9872d exhibits Venus-like temperatures of 752 degrees Fahrenheit (400 degrees Celsius). But that doesn't make this discovery any less exciting.
Though scientists have found water vapor in the atmospheres of many extra-solar planets before, the Hubble Telescope's observations of this hot and steamy world, designated GJ 9827d, represent the smallest exoplanet around which this vital element for life has been found thus far.
"The discovery of water on GJ 9827d is exciting because it’s the smallest planet yet where we've detected an atmosphere," Laura Kreidberg, team member and director of the Max Planck Institute for Astronomy's Atmospheric Physics of Exoplanets department, told Space.com. "It pushes closer than ever to characterizing truly Earth-like worlds."
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GJ 9827d is around twice as wide as Earth and orbits a star called GJ 987, which is located around 97 light-years away from us toward the constellation of Pisces. The planet is just one of three Earth-like worlds orbiting this star, which appears to be around 6 billion years old.
"This would be the first time that we can directly show through atmospheric detection, that these planets with water-rich atmospheres can actually exist around other stars," Björn Benneke, team member and a scientist at the Trottier Institute for Research on Exoplanets at Université de Montréal, said in a statement. "This is an important step toward determining the prevalence and diversity of atmospheres on rocky planets."
A major question remains, however: What type of planet is GJ 9872d?
"The nature of these small-ish planets, between two and three times the size of Earth, is really uncertain," Kreidberg said. "They could be true super-Earths, with a large rocky core and a light atmosphere on top, or they could be something completely different, like a water world made predominantly from water ice that has no analog in our own solar system."
Water world or hydrogen-rich mini-Neptune?
Hubble observed GJ 9827d for three years and watched as the world crossed the face of its star, or "transited" it, 11 times. Because chemical elements and compounds absorb light at characteristic wavelengths, as light from a parent star passes through a planet's atmosphere, it carries fingerprints of the elements that comprise the planet itself.
Currently, the astronomers behind this discovery aren't certain whether Hubble detected a small amount of water in a puffy ,hydrogen-rich atmosphere when it examined GJ 9872d — or, if the planet's atmosphere is predominantly made of water.
"Either result would be exciting, whether water vapor is dominant or just a tiny species in a hydrogen-dominant atmosphere," Pierre-Alexis Roy, research lead author and a scientist at the Trottier Institute for Research on Exoplanets at Université de Montréal, said in the statement.
If GJ 9872d has spent its 6 billion-year lifetime close to its parent star, intense radiation should have boiled away any primordial hydrogen present, leaving the tiny planet with an atmosphere dominated by water vapor. This seems to be supported by the fact that attempts to detect hydrogen around GJ 9872d have thus far failed.
Alternatively, if GJ 9872d is still clinging to a hydrogen-rich envelope laced with water, it would be classified as a mini-Neptune, a type of planet less massive than Neptune but that still resembles the solar system ice giant in possessing a thick atmosphere of hydrogen and helium.
On the other hand, the exoplanet could resemble a larger and hotter version of Jupiter's moon Europa, which is believed to host twice as much water as Earth sealed beneath a thick icy crust. "The planet GJ 9827d could be half water, half rock. And there would be a lot of water vapor on top of some smaller rocky body," Benneke said.
Should GJ 9827d still possess a thick atmosphere of water vapor, this would imply that it was born further out from its star — where temperatures would've been lower — before migrating to the position we see today.
This migration would have resulted in the exoplanet being blasted with more radiation from its host star, transforming potential ice on GJ 9827d into liquid water and water vapor. Any present hydrogen would've gotten heated, eventually beginning to leak from the planet's atmosphere due to the world's relatively low gravity; this leaking could still be occurring while astronomers observe the exoplanet today.
"Until now, we had not been able to detect the atmosphere of such a small planet directly. And we're slowly getting in this regime now," added Benneke. "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."
The study of GJ 9827d with Hubble has marked the planet as a prime target for a follow-up investigation conducted with the James Webb Space Telescope (JWST). This work is already underway, with the $10 billion telescope capable of delivering more details about this potential water world.
"GJ 9827d is being observed with JWST to learn more about its atmospheric composition and search for additional molecules like carbon dioxide," Kreidberg concluded. "Observations are ongoing, and we’ll have more answers soon!
"Hopefully, we can now settle the question of water worlds once and for all."
The team's research was published last year in The Astrophysical Journal Letters.
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