Scientists detect water vapor on an odd Neptune-like planet

TOI-674 b is an ideal candidate for observations with the James Webb Space Telescope.


When exoplanet TOI-674 b was first discovered, it caught the attention of Jonathan Brande for several reasons.

“It's a good-sized planet orbiting a small star — it has a very large transit signal,” Brande, a doctoral student in physics and astronomy at the University of Kansas, said during a press conference for the virtual 239th meeting of the American Astronomical Society on Thursday. “This makes it relatively easy to study compared to many other planets.”

After probing at the recently discovered planet with different space telescopes, Brande and a team of international astronomers detected water vapor in its atmosphere.

The discovery could help scientists better understand how the Solar System's gas giants formed and prepare for future observations of water vapor in exoplanets’ atmospheres with the recently launched James Webb Space Telescope (JWST).

WHAT’S NEW — Exoplanet TOI-674 b is a Neptune-like gas giant that orbits around a red dwarf star located about 150 light years away from Earth. It’s about 1.3 times the size of Neptune and about 23 times the mass of Earth. The planet has a tight orbit around its star, completing one full orbit in less than two days.

“Planets the size of Neptune and a bit bigger are very uncommon at these orbits, what astronomers call the Neptune desert,” Brande says. “Planets in the desert are very rare, possibly due to losing their atmospheres from stellar radiation or because they migrate away from these close quarters.”

Therefore, scientists try to gather as much data as possible on these planets to find out how they got there. TOI-674 b was initially discovered by NASA’s Transiting Exoplanet Survey Satellite (TESS), which surveys stars to look for exoplanets that orbit them. It does this through the transit method, in which a planet comes between its star and Earth, causing a slight dip in brightness. The team of scientists then conducted further observations of the exoplanet using the Hubble Space Telescope and data from the now-retired Spitzer Space Telescope.

After observing its atmosphere, the scientists detected traces of water vapor.

NASA illustration of the TESS craft, which looks for planets around distant stars.

NASA's Goddard Space Flight Center

WHAT’S NEXT — This isn’t the first time scientists have found water vapor in an exoplanet’s atmosphere. In 2019, scientists found water vapor in the atmosphere of a super-Earth exoplanet located 110 light years away.

However, this discovery is unusual because Neptune-like planets rarely have an easily detectable atmosphere since they do not orbit closely to their stars. The team of scientists is still not sure exactly how much water vapor there is in the planet’s atmosphere.

“In order to measure these abundances more precisely and determine the metal content, we will need to use the recently launched James Webb Space Telescope,” Brande says.

JWST launched in December 2021 and will likely begin its mission in the summer. The telescope will study the atmosphere of exoplanets in greater detail than ever before, with a high enough resolution to witness planets passing in front of their home star to draw out atmospheric properties when they transit.

By measuring the amount of water vapor in TOI-674 b, scientists will better understand where the exoplanet formed in its star system and how it migrated to its current position.

“There’s a location that we call the frost line, interior to which volatile species like water tend to be irradiated by their host star and stay in their evaporated states,” Brande says. “But further out, these can actually condense and freeze into solid materials more available for making planets.”

So if you have a planet that formed outside of the frost line and accretes quite a bit of water and then through some other evolutionary process migrates in towards the star, that would look different than a planet that was formed interior to the frost line and couldn’t accrete as much water, according to Brande.

Understanding the origin story of the exoplanet TOI-674 b will help scientists better understand the gas giant planets of our own Solar System, and the entire star system as a whole so that we can also learn about where Earth fits into the whole picture.

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