Water and carbon dioxide detected in the atmosphere of a hot super-Neptune exoplanet

Using the James Webb Space Telescope (JWST), astronomers have characterized the atmosphere of a hot super-Neptune exoplanet designated WASP-166 b. As a result, they found that the atmosphere of this alien world contains water and carbon dioxide. Their findings were reported Dec. 31 on the arXiv preprint server.

WASP-166 b is about seven times larger and 32 times more massive than the Earth. It orbits its host star every 5.44 days, at a distance of approximately 0.067 AU from it. The planet is relatively hot as its equilibrium temperature is estimated to be 1,270 K. The parameters of WASP-166 make it a representative of the so-called hot Neptune desert—a region of parameter space at high insolation fluxes and intermediate planet radii that is very sparsely populated.

The parent star WASP-166, which is located some 368 light years away, has a spectral type of F9V, and is about 20% larger and more massive than the sun. The star has an effective temperature of 6,050 K, metallicity at a level of 0.19 dex, and its age is estimated to be 2.1 billion years.

A team of astronomers, led by Andrew W. Mayo of San Francisco State University, decided to conduct a detailed atmospheric characterization of WASP-166 b. For this purpose, they employed JWST’s Near Infrared Imager and Slitless Spectrograph (NIRISS) and Near Infrared Camera (NIRCam).

“We collected two transit observations of the planet with JWST, one transit with NIRSpec BOTS G395M/F290LP (2.80 − 5.17 µm) and one transit with NIRISS SOSS Order-1 (0.85−2.81 µm),” the researchers wrote in the paper.

The observations found that the atmosphere of WASP-166 b contains water and carbon dioxide, as they dominate the obtained spectrum. Moreover, JWST allowed the team to make a weak detection of ammonia and a weak detection of the cloud deck pressure.

The remainder of WASP-166 b’s atmosphere is assumed to be composed of the fill gases—helium and hydrogen, in a primordial solar ratio. The researchers also searched for other compounds like carbon monoxide or ammonia, but found none in the JWST data.

According to the paper, WASP-166 b has a planetary carbon to oxygen ratio of approximately 0.282, which is lower than that of the host star (0.41) and significantly lower than that of the sun (0.55). It turned out that the exoplanet has a high atmospheric metallicity—at a level of about 1.57.

The authors of the paper concluded that the obtained results can be explained in the case of WASP-166 b through planetesimal accretion followed by core erosion or photoevaporation.

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