Astronomers Discover Two Exceptionally Low-Density Gas Giants 1,100 Light-Years Away
Astronomers have identified two rare gas giant exoplanets, TOI-791 b and TOI-791 c, orbiting an F7-type dwarf star approximately 1,110 light-years from Earth in the southern constellation Volans. Although similar in size to Jupiter, these planets exhibit extraordinarily low densities of 0.038 and 0.047 grams per cubic centimeter respectively, far less dense than Jupiter's 1.33 g/cm³ and even less dense than cotton candy. By comparison, Earth's density is 5.5 g/cm³.
These "sibling" planets likely formed together from the same protoplanetary disk and are locked in a rare 5:3 orbital resonance, meaning the inner planet completes five orbits for every nearly three orbits of the outer planet. This gravitational interaction causes measurable variations in their transit timings across their host star, enabling researchers to determine their masses and densities.
The planets were first detected by citizen scientists analyzing data from NASA's TESS (Transiting Exoplanet Survey Satellite) in 2019 and 2023. Subsequent global telescope observations, including eight years of data from the Antarctic ASTEP telescope at Concordia Station, allowed uninterrupted monitoring of their unusually long transits lasting over 11 hours, the longest continuous ground-based observations of planetary transits recorded.
Scientists are investigating how such low-density planets form, hypothesizing that they possess massive hydrogen and helium atmospheres accumulated in the cold outer regions of their star's protoplanetary disk. The system offers a unique laboratory to study the formation and evolution of super-puff planets. Future observations with the James Webb Space Telescope may reveal atmospheric compositions and further insights.
Researchers emphasize the importance of international collaboration combining space- and ground-based telescopes across continents and Antarctica to uncover the true nature of these extraordinary exoplanets. The findings were published in the Monthly Notices of the Royal Astronomical Society.
