Two Jupiter-sized worlds so light they resemble “candy floss or shaving foam” circle a distant star.
“We liken it in density to something like candy floss or shaving foam. They’re the lightest planets that are Jupiter-sized,” astrophysicist George Dransfield of the University of Oxford told Lisa Grossman at Science News.
Researchers report the discovery and confirmation of the pair, named TOI-791 b and c, in the Monthly Notices of the Royal Astronomical Society. The inner planet measures roughly the radius of Jupiter yet packs a density of only 0.038 grams per cubic centimetre; its slightly larger sibling reaches 0.047 grams per cubic centimetre. Both rank among the lowest-density giant planets ever measured.
“A transiting warm Jupiter is a rare beast,” the researchers write in the paper.
Only about 1.5 per cent of known planets fall into this category — gas giants with orbital periods between 20 and 300 days that also cross in front of their stars. Systems containing two such transiting giants are rarer still; TOI-791 is just the ninth known. The planets sit within 0.07 per cent of a 5:3 orbital resonance, producing measurable transit-timing variations of up to 50 minutes.
The host is an F7 star 341 light-years away in TESS’s southern continuous viewing zone. Space-based photometry first flagged the signals, but the decisive confirmation came from the Antarctic Search for Transiting ExoPlanets telescope. ASTEP recorded multiple complete transits lasting more than 11 hours — the longest-duration events ever observed in their entirety from the ground.
“They’re in this fun little dance,” Dransfield says. “We were able to measure the masses of the planets by how much they kick each other’s orbits.”
High-resolution imaging and spectroscopy ruled out false positives, while dynamical modelling of the timing variations yielded planetary masses of roughly 9.5 and 18.6 Earth masses. These figures translate into densities lower than 0.05 grams per cubic centimetre, making the worlds the largest planets yet found in that regime.
“Indicating that TOI-791 b and c are two of the lowest density giant planets ever detected,” the team writes. The only comparable objects are the super-puff planets of the Kepler-51 system. Whether TOI-791’s pair formed with unusually thick hydrogen-helium envelopes or host face-on rings remains open; an age estimate for the star will be crucial.
“Finding things like superpuffs, which are incredibly rare, means we can learn more about how planets form, how planets evolve and what outcomes are possible. That also helps us contextualize Earth in the wider context of the cosmos,” Dransfield concludes.
Journal Reference: Monthly Notices of the Royal Astronomical Society DOI: 10.1093/mnras/stag864
