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Astronomers using NASA’s Hubble Space Telescope
observed the smallest exoplanet to have water

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vapor detected in its atmosphere.

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At only around twice the Earth’s diameter,
the planet GJ 9827d lies 97 light-years away

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and could be an example of potential planets
with water-rich atmospheres elsewhere in our

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galaxy.

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However, it remains too early to tell whether
Hubble measured a small amount of water vapor

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in a puffy hydrogen-rich atmosphere, or if
the planet’s atmosphere is mostly made of

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water.

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Currently the scientific team leading the
study has two possibilities.

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The planet is still clinging to a hydrogen-rich
atmosphere laced with water, making it a mini-Neptune.

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Or, it could be a warmer version of Jupiter’s
moon Europa, which has twice as much water

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as Earth beneath its crust.

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Over a period of three years, Hubble observed
the planet during 11 transits — events in

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which the planet crossed in front of its star.

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During transits, starlight is filtered through
the planet’s atmosphere and displays the

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spectral fingerprint of water molecules.

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Because the planet is as hot as Venus, at
800 degrees Fahrenheit, it definitely would

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be an inhospitable, steamy world if the atmosphere
were predominantly water vapor.

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Hubble’s discovery opens the door to studying
the planet in more detail.

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It’s a good future target for NASA’s James
Webb Space Telescope to do infrared spectroscopy

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to look for other atmospheric molecules.

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Regardless of GJ 9827d’s uninhabitable nature,
water on a planet this small is a landmark

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discovery, pushing us closer than ever to
characterizing truly Earth-like worlds, and

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thanks to the Hubble Space Telescope we’ll
be able to continue our search for more planets

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like our own.

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