Galaxy's most common planets begin life as puffy, low-density giants

Astronomers used transit-timing variations to weigh four planets around the 20‑million‑year‑old star V1298 Tau and found they are very low density despite large radii; the researchers report these planets will lose atmosphere and contract over billions of years into the compact super‑Earths and sub‑Neptunes common in the Milky Way.

An international team reports a young planetary system that helps explain how the Galaxy's most common planets form. The system orbits V1298 Tau, a star about 20 million years old. Four planets there have radii several times Earth's size but unusually low masses and densities. The researchers measured tiny timing shifts in repeated transits to determine the planets' masses over about a decade. Key findings: - V1298 Tau is reported as a roughly 20‑million‑year‑old star hosting four transiting planets. - The team tracked transits for roughly ten years and used transit‑timing variations to estimate the planets' masses. - Each planet has a radius around 5–10 times Earth's radius but a mass only about 5–15 Earth masses, implying very low bulk density. - The planets are described as highly inflated now and are reported to be losing atmosphere after the protoplanetary disk dispersed. - The authors present this system as a missing link showing how these inflated young worlds can evolve into the compact super‑Earths and sub‑Neptunes that are widespread in the Milky Way. Summary: This system provides direct evidence of an early, inflated stage in planet evolution, with very large radii and low densities. The researchers report the planets will continue to lose atmosphere and contract over billions of years, which could explain how many compact super‑Earths and sub‑Neptunes form. Undetermined at this time.