NASA TESS Telescope Satellite Puffiest Exoplanets
Photo credit: NASA / Daniel Rutter
Astronomers poring over years of data from NASA’s planet-hunting satellite have confirmed a pair of worlds that rank among the largest and least dense ever detected. A sun-like star called TOI-791 hosts them both, sitting roughly 1,113 light years away in the southern constellation Volans.


NASA TESS Telescope Satellite Puffiest Exoplanets
One planet spans roughly Jupiter’s width yet contains barely 3% of its mass. The other is larger than Jupiter but weighs only 5.9 percent as much. Densities this low put these objects in unique company, with material scattered so thin that it resembles cotton candy rather than rock or normal gas giant innards. Repeated dips in the star’s brightness alerted researchers to the possibility of planets crossing its face. TESS gathered the necessary data throughout the course of a seven-year effort spanning more than 1,100 days. These extended orbital periods, 139 days for the inner planet and 232 days for its outer companion, need continuous monitoring from high Earth orbit.

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Ground-based follow-up work then pinpointed the masses using a smart indirect path. The gravitational force between the two planets causes the exact timing of each transit to fluctuate by minor but significant amounts. These time adjustments revealed the very low weights without requiring exact speed estimates from the star itself.

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NASA TESS Telescope Satellite Puffiest Exoplanets
The investigation was led by George Dransfield of Oxford University and published in the Royal Astronomical Society’s Monthly Notices. He pointed out that only a few of these super-puffy planets are known anywhere, so finding two in the same system is even more remarkable. Their extraordinary features make them ideal targets for studying how planets form and evolve. Jon Jenkins, who directs TESS science processing at NASA’s Ames Research Center, provided a clear picture of the larger conundrum. These planets stand out because existing models of big planet creation did not account for things this huge with such little mass. They pose a direct challenge to conventional assumptions about how such worlds come together.

Steve Howell, also at Ames, observed that the largest planets frequently direct the long-term evolution of their entire systems via gravity and orbital motion. Studying these lighter equivalents provides new insights into that influence, despite the fact that the planets themselves are significantly lighter than Jupiter. Further observations with the James Webb Space Telescope could reveal the gasses that fill their bloated envelopes. Such information could reveal which substances help keep the worlds inflated against their weak surface gravity and whether they formed further out before traveling to their current broad pathways.