Astronomers have confirmed the existence of two gas giants with record-low densities and extremely expanded shells. Their orbits are in perfect gravitational synchrony, and the planets themselves are among the least dense of all known planets of this size.
Two Worlds of TOI-791
The TOI-791 system is located approximately 230 light-years from Earth in the constellation Volans. Two planets, TOI-791 b and TOI-791 c, were confirmed using ground-based telescopes after being detected by the TESS space telescope, according to Universe Today. Both are similar in size to Jupiter but are dozens of times less massive.
The density of TOI-791 b is 0.038 gr/cm³, while that of TOI-791 c is 0.047 gr/cm³. For comparison, cotton candy has a density of about 0.05 grams per cubic centimeter. Both worlds would literally float on the surface of water, which has a density of 1 gram per cubic centimeter.
Planets like these are rare, but they have been discovered before, notably in the Kepler-51 system. The TOI-791 c planets turned out to be even less dense. The study’s findings were published in the peer-reviewed journal MNRAS.
Orbital rhythm without disruptions
Both planets are in a 5:3 orbital resonance. While TOI-791 c completes three full orbits around its star, TOI-791 b completes exactly five. The precision of this ratio is 0.07%. Because of this resonance, astronomers were able to measure the masses of both planets based on variations in transit time. The gravitational influence of the planets on each other periodically accelerates or slows their motion, causing the transit time to shift by up to 50 minutes.
Such a stable configuration indicates that the system has had a tranquil history. The authors of the study describe them as related worlds that could not have formed near the star; instead, they formed much farther away, in a colder part of the protoplanetary disk, and then gradually migrated to their current orbits.
What holds the atmosphere in place?
The biggest mystery surrounding TOI-791 b and TOI-791 c is how they retain their gaseous atmospheres. The star TOI-791 is a red dwarf, and such stars are significantly more active than the Sun, so the planets are exposed not only to heat but also to powerful bursts of ultraviolet and X-ray radiation, which accelerate the evaporation of their atmospheres. Scientists do not yet have precise estimates of the rate of this process.
They hope to find the answer using the James Webb Space Telescope. Atmospheric spectroscopy should detect hydrogen, carbon, nitrogen, and oxygen—that is, the main elements that make up the gas envelopes of the giant planets—and their proportions will reveal where these worlds formed and how much longer their atmospheres will last. Observations are already scheduled for the upcoming cycle.
