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James Webb Space Telescope will help assess atmospheres of strange 'sub-Neptunes'

Artist's impression of sub-Neptune GJ 1214 b.
Artist's impression of sub-Neptune GJ 1214 b. (Image credit: NASA, ESA, CSA, and D. Player (STScI))

The sharp mirrors of the James Webb Space Telescope will finally be able to probe into the atmospheres of sub-Neptunes, which are mysterious planets that aren't found anywhere near Earth.

The observatory is in the middle of a commissioning period that will last until about June. But when it is ready, the observatory will seek out sub-Neptunes that are close to their parent stars to assess more about the "fundamental nature" of these large planets, NASA said.

Although classified as planets that are bigger than Earth but smaller than Neptune, sub-Neptunes remain mysterious by many metrics despite the discovery of hundreds of these worlds, NASA said in a statement (opens in new tab) late in 2021.

"Are they dense, Earth-like balls of rock and iron, blanketed in thick layers of hydrogen and helium gas? Or less dense mixtures of rock and ice, surrounded by steamy, water-rich atmospheres?" NASA asked. "With limited data and no planets of similar size and orbit in our own solar system to use for comparison, it has been difficult to answer these questions."

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Answering these questions will require a deep dive into the sub-Neptunes' atmospheres, using a technique called transmission spectroscopy. Webb will watch each planet as the world passes across the face of its respective parent star. Some of the wavelengths or colors of starlight will be filtered out due to gases in the planet's atmosphere, allowing scientists to look for a distinctive "signature" for different elements.

Unfortunately, aerosols (tiny particles or droplets) that tend to congregate in sub-Neptunian atmospheres do obscure the view. Such aerosols will scatter starlight and make the signatures almost impossible to interpret, with most observatories. Webb, however, has an advantage as it is located in a deep spot in space far from interfering light, and comes equipped with a large mirror to render sharp images of distant objects.

An observation program aims to look at two sub-Neptune-size planets in Webb's first year of observations: GJ 1214 b, which you can think of as the original sub-Neptune due to its early discovery announcement back in 2009, and the much more recently found TOI-421 b, announced in 2020.

Artist's impression of HD 21749b, a sub-Neptune three times bigger and 23 times more massive than Earth. HD 21749b lies about 53 light-years from Earth and orbits a star about the the size of the sun. (Image credit: NASA/MIT/TESS)

GJ 1214b orbits a red dwarf star relatively nearly Earth and has already been featured in dozens of studies. Other advantages of looking at this planet include its short orbital period (making many transits possible in a short time) and the planet's large size relative to its star (which allows great observations of its atmosphere).

The team plans to use Webb’s Mid-Infrared Instrument (MIRI) to look at the planet for 50 hours straight, capturing a little more than one orbit of the exoplanet. Scientists will use transmission spectroscopy to look for water, methane or ammonia. Thermal emissions, visible in mid-infrared light in which Webb is sensitive, should provide information about the planet's temperature and reflectivity.

Along with these observations, the research team plans to use Webb's ability to sense very small changes in how much light the system emits while the planet orbits the star. The "curve" or brightness graph of these changes will allow an approximate map of the average planetary temperature, by longitude. This map will in turn allow astrophysicists to better model the planet's atmosphere composition and circulation.

This NASA chart depicts the number alien planet candidates identified by NASA's Kepler spacecraft as of January 2014. We have found many more since this point. (Image credit: NASA Ames Research Center)

TOI-421 b will be examined to see how similar it might be to Titan, a Saturnian moon famous for its smog-like, organic-filled haze. Webb will look at the TOI-421 b during two occasions, observing transits using its Near-Infrared Imager and Slitless Spectrograph (NIRISS) and its Near-Infrared Spectrograph (NIRSpec).

The two instruments working together should provide an excellent transmission spectrum of the planet. The suspicion right now is TOI-421 b has much clearer skies than Titan. If that is indeed the case, the spectrum should easily show the components of water, methane and carbon dioxide. If aerosols are scattering light, Webb is likely to still gather enough data to show off what the aerosols are made of.

The studies will be co-led by Eliza Kempton of the University of Maryland–College Park, who specializes in theoretical modeling of exoplanet atmospheres, and Jacob Bean, an astronomer at the University of Chicago who has led many other exoplanet studies.

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Elizabeth Howell
Staff Writer, Spaceflight

Elizabeth Howell, Ph.D., is a staff writer in the spaceflight channel since 2022. She was contributing writer for Space.com (opens in new tab) for 10 years before that, since 2012. As a proud Trekkie and Canadian, she also tackles topics like diversity, science fiction, astronomy and gaming to help others explore the universe. Elizabeth's on-site reporting includes two human spaceflight launches from Kazakhstan, three space shuttle missions in Florida, and embedded reporting from a simulated Mars mission in Utah. She holds a Ph.D. and M.Sc. in Space Studies from the University of North Dakota, and a Bachelor of Journalism from Canada's Carleton University. Elizabeth is also a post-secondary instructor in communications and science since 2015. Her latest book, Leadership Moments from NASA, is co-written with astronaut Dave Williams. Elizabeth first got interested in space after watching the movie Apollo 13 in 1996, and still wants to be an astronaut someday.