JWST spots methane on a giant exoplanet, but its star may be distorting the signal

Using the James Webb Space Telescope (JWST), astronomers from Johns Hopkins University (JHU) and elsewhere have observed a giant exoplanet known as HATS-75 b. Results of the new observations, published April 8 on the arXiv pre-print server, yield important information on the atmosphere of this planet.

What do we know about the HATS-75 system?

Discovered in 2021 with the Hungarian-made Automated Telescope Network-South (HATSouth) and NASA’s Transiting Exoplanet Survey Satellite (TESS), HATS-75 b is a giant planet orbiting an M-dwarf star at a distance of some 637 light-years. It has a radius of approximately 0.88 Jupiter radii and is about half the mass of Jupiter. The planet orbits its host every 2.79 days at a distance of 0.03 AU from it, and its equilibrium temperature is estimated to be 772 K.

The parent star, HATS-75, is an M dwarf about 40% smaller and less massive than the sun. It has an effective temperature of 3,790 K and metallicity at a level of 0.52 dex. The age of HATS-75 is estimated to be 14.9 billion years.

Therefore, HATS-75 b is one of the latest additions to a small but growing group of about 30 so-called GEMS—giant exoplanets around M-dwarf stars. In general, GEMS have radii ranging from 8 to 15 Earth radii, and have masses of at least 0.25 Jupiter masses.

Revisiting HATS-75 b with JWST

A team of astronomers led by JHU’s Reza Ashtari recently revisited the HATS-75 planetary system with JWST to investigate the chemical composition of HATS-75 b’s atmosphere.

“Our observations offer an opportunity not only to characterize the atmospheric composition of HATS-75 b, but also to explore how stellar heterogeneity influences the interpretation of transmission spectra in low-mass star systems,” the scientists write in the paper.

Haze or stellar contamination

By analyzing the transmission spectrum of HATS-75 b acquired with JWST’s Near-Infrared Spectrograph (NIRSpec), the astronomers found a slightly larger transit depth at shorter wavelengths. This points to hazes or stellar contamination from cool spots due to stellar heterogeneities outside the transit chord—the transit light source (TLS) effect.

Assuming that the TLS scenario is true when it comes to explaining the transmission spectrum of the investigated exoplanet, the researchers found strong evidence for methane, moderate evidence for carbon dioxide, and weak evidence for carbon monoxide. However, they did not detect water due to spot-induced degeneracy.

These findings suggest that the atmosphere of HATS-75 b has a sub-solar metallicity and an elevated carbon-to-oxygen ratio.

The authors of the study note that an atmosphere with a haze can also explain the observed transmission spectrum of HATS-75 b. However, while a haze-dominated model can fit the data well, Ashtari’s team disfavors this hazy hypothesis based on independent circumstantial evidence provided by star spot crossings. They explain that the collected evidence unveils characteristics similar to those required to produce the observed TLS contamination.

“HATS-75 b thus joins the rare population of giant planets around M-dwarfs observed with JWST, underscoring both the challenges of stellar contamination in transmission spectroscopy and the importance of careful host-star treatment in revealing the true chemistry of these uncommon worlds,” the scientists conclude.

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