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Astronomers have discovered two giant exoplanets, each about the size of Jupiter but with a density even lower than cotton candy, orbiting a star 1,110 light-years away in the constellation Volans. These rare "super-puff" planets, named TOI-791 b and TOI-791 c, are the largest known exoplanets of their kind and are now considered the lightest planets for their size, according to George Dransfield of the University of Oxford, who led the research.
As reported by Phys.org, the two planets have densities of just 0.038 and 0.047 grams per cubic centimeter, respectively, making them lighter than both cotton candy and shaving foam. In comparison, Jupiter’s density is about 1.33 grams per cubic centimeter, or up to 35 times greater than these newly identified worlds.
The discovery was made possible through data from NASA’s Transiting Exoplanet Survey Satellite (TESS) and nearly eight years of ground-based observations, including uninterrupted sessions in Antarctica that captured the planets’ unusually long transits of more than 11 hours each. By measuring subtle changes in the timing of these transits, astronomers were able to estimate the planets’ masses and calculate their extremely low densities. The findings were published in the journal Monthly Notices of the Royal Astronomical Society.
Fewer than 40 super-puff planets have been confirmed among the nearly 6,300 known exoplanets. It is extremely rare to find two such planets in the same system. The planets are believed to be made mostly of hydrogen and helium, but NASA’s James Webb Space Telescope will conduct follow-up observations to confirm their chemical makeup.
Dransfield described them as resembling "a nice blob of shaving foam, fresh from the can," and explained that their color would likely be white or blue, depending on whether their skies are cloudy. The pair are locked in a 5:3 mean-motion resonance, meaning that for every five orbits of the inner planet, the outer completes three. This unusual relationship provides a unique laboratory for studying the origins and evolution of super-puff planets.
Scientists hope that deeper studies of these exotic worlds will shed light on how planets form and evolve, and help answer questions about the diversity of planetary systems beyond our own. Additional observations using advanced telescopes are planned to further investigate their mysterious, ultra-light atmospheres.