Cotton-Candy Worlds Orbiting TOI 791
Astronomers have identified two “superpuff” exoplanets orbiting the sunlike star TOI 791, located approximately 1,113 light-years from Earth. Roughly the size of Jupiter but possessing less than 6% of its mass, these worlds exhibit densities akin to cotton candy or shaving foam. The discovery, detailed in the Monthly Notices of the Royal Astronomical Society, offers a new window into the evolution of low-density planets.
The Mechanics of a Planetary Dance
The TESS space telescope first flagged the pair as they crossed their host star, dimming its light. This transit method allowed researchers to calculate their radii at 0.993 and 1.155 times that of Jupiter.
To pin down their elusive density, a team including University of Oxford astrophysicist George Dransfield turned to the Antarctic Search for Transiting ExoPlanets (ASTEP) telescope. Because the ASTEP site in Antarctica remains in continuous darkness for three months, it provided the unique ability to monitor the entire 12-hour duration of the planetary transits. By observing the gravitational “tugs” the planets exerted on one another—a phenomenon Dransfield describes as a “fun little dance”—the team calculated their masses to be 9.5 and 18.6 times that of Earth.
Defying Conventional Planetary Physics
These findings cement the pair’s status as rare “superpuffs.” Their densities are measured at 0.038 and 0.047 grams per cubic centimeter, a stark contrast to Jupiter, which holds 317 times Earth’s mass.
Why are they so oversized? One theory suggests the planets formed further from their host star and migrated inward, where intense heat caused their atmospheres to balloon. Another possibility involves the age of the system. Theoretical astrophysicist Anthony Piro of the Carnegie Observatories notes that the host star appears to be spinning quickly, suggesting it is relatively young. These planets may simply be in an intermediate stage of cooling and could contract as they age.
While some researchers have previously proposed that giant ring systems could create the illusion of a larger planet, Piro suggests this is unlikely for the TOI 791 system, as the coincidence of two such worlds around a single star makes that explanation improbable.
Targeting Atmospheres with James Webb
The scientific community views these planets as vital subjects for understanding planetary formation. Dransfield indicated that the next phase of research will involve using the James Webb Space Telescope to analyze the chemical composition of the planets’ atmospheres. By probing these rare worlds, astronomers hope to contextualize Earth in the wider context of the cosmos and determine the range of possible outcomes for planetary systems throughout the cosmos.
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