Starlight Blurs the View of Exoplanet Atmospheres express
A quiet flute melody gets lost in the roar of drums. That's what astronomers face when trying to catch the faint light that has passed through a distant planet's atmosphere while its star blazes nearby.
A team of scientists observed WASP-31 b — a hot Jupiter, a gas giant orbiting very close to its star. They used the VLT in Chile and the ESPRESSO instrument, which splits light into colors like a prism. When the planet passes in front of its star, some starlight filters through its atmosphere, and gases leave their "fingerprints" in the spectrum — dark lines at specific wavelengths. But the star also has such lines, and they blend together, obscuring the planet's.
The scientists applied sophisticated methods to remove the interference, but even then they couldn't detect any gas in WASP-31 b's atmosphere. Earlier observations hinted at potassium and the molecule CrH, but those traces were not confirmed now. The reason isn't that the gases are absent, but that the contamination from the star is too strong.
It's like trying to make out a pattern on a glass pane with a bright lamp shining behind it. Until we figure out how to switch off the lamp or block it, the pattern stays invisible. Astronomers need new techniques to separate the planet's light from the star's, or even future giant telescopes won't help.
Original abstract
as written by the authors arXiv:2602.15501
Context. Hot Jupiters are ideal natural laboratories to investigate atmospheric composition and dynamics. However, high-resolution transmission spectroscopy is currently limited by our capability of removing planet-occulted line-distortion (POLD) contamination from the signal. Aims. In this paper, we aim to characterize the transmission spectrum of WASP-31 b from two and a half transits observed with the ESPRESSO spectrograph at the VLT. Methods. The Rossiter-McLaughlin (RM) signature was analyzed using the RM "revolutions" method. Before extracting the transmission spectrum of the planet, we corrected the dataset for telluric lines using molecfit and further modeled the POLD deformations using EvE. Results. We confirm the planet low sky-projected spin-orbit angle from previous studies and further refine its value to $$\lambda = -0.09^{+0.31}_{-0.32}$$ deg. We do not detect any species (including previously detected species such as K or CrH) in the planetary atmosphere. In most cases the non-detections are due to the strong POLDs contamination or lack of observable lines in the ESPRESSO wavelength range, and so previous detections cannot be ruled out. Conclusions. Planet-occulted line-distortion contamination continues to be the main limitation of high-resolution transmission spectroscopy for species present in both the star and the planet, hindering atmospheric detections even with state-of-the-art models, in particular for planets with a low sky-projected spin-orbit angle. Developing advanced techniques to isolate planetary signatures is of utmost importance in the advent of ELT-like observations.
Hot Jupiters are gas giants that orbit so close to their stars that a year on them lasts just a few Earth days.