Astronomers Find 2 Jupiter-Size Super-Puff Planets 1,100 Light-Years Away

Q: How can Jupiter-sized planets exist with densities less than cotton candy and no solid core?

They can exist because size and density are not the same thing: a planet can be as wide as Jupiter yet contain only a small fraction of Jupiter’s mass if most of its volume is an enormous, very light atmosphere. That appears to be the case for TOI-791 b and TOI-791 c. Both are roughly Jupiter-sized, but they hold only about 3% and 5.9% of Jupiter’s mass, respectively. Their bulk is thought to be dominated by hydrogen and helium, the lightest common gases. Those gases can form a huge, extended envelope that makes the planet look large in transit data while keeping its average density extraordinarily low—below water, shaving foam, and even cotton candy. The “no solid core” part is the bigger scientific puzzle. One leading idea is that these planets formed far from their star, in a cold, gas-rich part of the protoplanetary disk where they could gather thick atmospheres without building a large rocky core. Another emerging explanation is that under extreme pressure and heat, hydrogen may mix with molten silicates and iron instead of separating into neat layers. In that case, the planet would not have a distinct solid core at all, but a deep, mixed fluid interior. That same internal mixing may also help keep such planets inflated for longer, slowing their contraction and preserving their “super-puff” appearance. Researchers say follow-up observations, including with the James Webb Space Telescope, could test these ideas by probing the planets’ atmospheres more directly.

3 articles · Updated · Metro.co.uk · Jul 9

TOI-791 b and TOI-791 c, two exoplanets about Jupiter-sized, were identified as ultra-low-density “super-puffs” roughly 1,100 light-years from Earth.
Jupiter is 35 times denser than TOI-791 b and 28 times denser than TOI-791 c, putting both worlds below the density of water and in a class with fewer than 37 known examples.
The Oxford-led team used transit observations from Antarctica’s ASTEP telescope and the planets’ gravitational tugs on each other to estimate their densities and probe atmospheres likely rich in hydrogen and helium.
The pair is especially unusual because only four other systems are known to host multiple super-puffs, and these two also sit in a near 5:3 orbital resonance with transits lasting up to 11 hours.
Researchers say the system could help explain how some gas giants form with relatively small cores in gas-rich regions around young stars.