Astronomers Detect Water Floating in The Atmosphere of a ‘Super Neptune’

The recently discovered planet TOI-674 b, a bit bigger than Neptune and orbiting a red-dwarf star about 150 light-years away, is a member of an exclusive club: Exoplanets, or planets around other stars, known to have water vapor in their atmospheres.

The planet’s distance, size and relationship to its star make it especially accessible to spaceborne telescopes. At 150 light-years, it’s considered “nearby” in astronomical terms. The star itself, relatively cool and less than half as big around as our Sun, can’t be seen from Earth with the naked eye, but this too translates into an advantage for astronomers.

As the comparatively large planet—in a size-class known as “super Neptune”—crosses the face of its smallish star, starlight shining through its atmosphere can be more easily analyzed by our telescopes. Those equipped with special instruments called spectrographs—including the just-launched James Webb Space Telescope—can spread this light into a spectrum, revealing which gases are present in the planet’s atmosphere.

Fortunately, astronomers have now noticed something unusual about this recently discovered middle-range giant, a finding that could help tell us why it’s so special: there happen to be hints of water floating in its atmosphere.

Observing the signature of water on planets far from our own oceanic world is exciting for a number of reasons, not least in telling us just how unique our own planet might be.

But having a breakdown of the kinds of gases in an exoplanet’s atmosphere – particularly water vapor – is like having the details of its cosmic birth certificate, giving planetary scientists a clearer understanding of how and where it formed in its solar system.

Planets that form beyond a point where the star’s radiation can easily sublimate ice into gas will have a much better chance of holding onto water, for example, raising the chances that a short-orbit Neptune with plenty of water is far from its place of birth.

The next step will be to collect more data on the precise amount of water in TOI 674b’s atmosphere, as well as other features, such as its metallicity.

The planet has only been on our radar for around a year, following its discovery using data from NASA’s Transiting Exoplanet Survey Satellite. But it’s already proved interesting enough for researchers to turn other instruments, such as the Hubble Space Telescope, towards the planet to probe its secrets. (The team also looked at older data from the retired Spitzer Space Telescope.)

Knowing how TOI 674b fell into such a hot embrace with its star will help us fill in the bigger picture of how other solar systems evolve, and whether our own is boringly normal or a unique gem in an ocean of chaos.