KOI-351 g, also known as Kepler-90 g, is a super-puff: a planet about 7.7 times Earth's width but only about 15 times its mass, which gives it an unusually low density. It orbits Kepler-90, a Sun-like star about 2,800 light-years away that has eight known planets, every 211 days.
Its mass came from timing rather than from weighing. Kepler-90 g and its outer neighbour, the giant Kepler-90 h, pull on each other so strongly that their transits, the moments they cross the star, arrive many hours early or late. In 2013 Juan Cabrera's team reported that g's period changed by 25.7 hours between two consecutive transits, the largest such shift found at that point. In 2020 a team led by Yan Liang used those timing shifts to work out the planets' masses and found Kepler-90 g at about 15 Earth masses, with a density of only about 0.15 grams per cubic centimetre, a small fraction of water's.
Kepler saw Kepler-90 g cross its star only six times, which left room for doubt. In 2025 a team led by D. E. Shaw combined the timing with a decade of measurements of the star's wobble to predict when its next transit would be, and caught it with ground-based telescopes in May 2024. Their analysis confirmed a mass of about 15 Earth masses. Liang's team found that g and h orbit close to a 2 to 3 ratio but are not locked in resonance, and that the arrangement is stable over billions of years.