An international team of researchers has used the CARMENES spectrograph to study the atmosphere of HAT-P-67b, the largest but least dense transiting gas giant known to date. From the Calar Alto data, the puffy atmosphere of the exoplanet appears highly ionized and could be escaping at a rate of 10 million tons per second.
Planets in our solar system have vastly different atmospheres. The atmospheres of planets outside our solar system, or exoplanets, show an even richer diversity, and one of the goals of modern astronomy is to characterize this diversity.
However, this is no easy task. Exoplanet atmospheres are most often studied by measuring how different gases absorb different colors of starlight as the exoplanet transits in front of its star. The resulting spectral signals are typically over a thousand times fainter than the brightness from the host star, so detecting them requires exquisite precision.
For that reason, only the atmospheres of a small fraction of the 5000 known exoplanets are within reach of current telescopes. In particular, extremely low-density exoplanets are some of the best targets for atmospheric scrutiny. Their puffy atmospheres filter a larger fraction of the stellar light during the transit, producing larger (and therefore easier to detect) signals.
A team of astronomers led by Aaron Bello-Arufe, as part of his PhD work at the Technical University of Denmark (DTU), has studied the atmosphere of HAT-P-67b, the largest transiting exoplanet with a precisely measured radius. HAT-P-67b is also the lowest-density gas giant currently known. Its radius is twice that of Jupiter, but its mass is only that of Saturn, which results in a density lower than that of marshmallows. HAT-P-67b completes an orbit around its host star, bigger and hotter than the Sun, in less than 5 days. As a result, the planet is blasted by stellar radiation, which might contribute to inflating the radius of HAT-P-67b and drive atmospheric escape.
Using the CARMENES instrument on the 3.5 m telescope at the Observatory of Calar Alto, Bello-Arufe and collaborators studied, for the first time, the composition of the atmosphere of HAT-P-67b. HAT-P-67b is in a temperature regime where we expect atomic and molecular gases to coexist, so an instrument like CARMENES, which can access multiple colors of light with extremely high resolution, is particularly well suited to study the atmosphere of this exotic world.
“CARMENES has become a leading instrument in the study of exoplanet atmospheres.” emphasizes Aaron Bello-Arufe, now a postdoctoral fellow at NASA Jet Propulsion Laboratory. “A lot about what we want to know about the atmospheres of other planets, including the speeds of winds and escaping gas, is currently out of reach for even the largest space telescopes and is only measurable with powerful ground-based instruments like CARMENES.”
In only one night of CARMENES observations, the team detected sodium and ionized calcium in the atmosphere of HAT-P-67b. Ionized calcium has only been found in hotter planets, but in this case, it showed up very strongly in the spectrum of HAT-P-67b. The presence of such deep absorption was not predicted by the theoretical models, and it might be caused by an atmosphere that is highly ionized.
The data also revealed a particularly intriguing signal in the hydrogen and helium lines. Absorption in these lines usually indicates that part of the atmosphere is escaping to space. However, in the case of HAT-P-67b, the hydrogen and helium signals are also present before and after the transit of the planet observed with CARMENES. This could be the result of a giant cloud of escaping gas extending well beyond the planet. If so, HAT-P-67b would become an ideal target to study how atmospheres are lost to space and the role that the stellar environment plays. However, more observations are needed to confidently rule out stellar variability as the source of the signal.
The discovery was accepted for publication in the Astronomical Journal.
Bello-Arufe et al. (2023), The Astronomical Journal, DOI: 10.3847/1538-3881/acd935
Preprint available at https://arxiv.org/abs/2307.06356
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