Astronomers Discover Two Super-Puff Exoplanets Larger Than Jupiter

Astronomers have announced the discovery of two “super‑puff” exoplanets orbiting a distant star, each about the size of Jupiter but with masses only a few times that of Earth. The planets, designated K2-18c and K2-18d, were identified through precise transit measurements by the Kepler Space Telescope and confirmed with follow‑up observations from the Hubble Space Telescope. Their extremely low densities—less than one‑tenth of Earth’s—suggest they are composed largely of hydrogen and helium, with a tenuous atmosphere that gives them a radius comparable to a gas giant while weighing only a fraction of it.

The discovery was made possible by combining transit photometry with radial‑velocity data, allowing astronomers to calculate both the planets’ sizes and masses. The host star, a red dwarf about 150 light‑years from Earth, shows the planets in a close, nearly circular orbit that keeps them warm enough for their atmospheres to remain inflated. The findings challenge current models of planet formation, which struggle to explain how such low‑mass bodies could retain massive gaseous envelopes without evaporating into space.

These super‑puffs add to a growing list of exoplanets that defy conventional classification, highlighting the diversity of planetary systems. Their unusual characteristics provide a natural laboratory for studying atmospheric escape, cloud formation, and the limits of planetary inflation. Future observations with the James Webb Space Telescope may reveal more about their composition and the processes that allow them to maintain such low densities.