Planets are observed forming into the most common types, super-Earths and sub-Neptunes.

Astronomers tracked four young planets around the 20-million-year-old star V1298 Tau, measured unusually low densities and modest masses using transits and transit-timing variations, and report the planets are losing atmospheres and are likely to contract into super-Earths and sub-Neptunes.

An international team of astronomers has observed four young planets orbiting the 20-million-year-old star V1298 Tau and published the results in Nature. The planets appear very large in radius but were measured to have relatively low masses, producing very low bulk densities. Researchers used nearly a decade of ground- and space-based transit observations and transit-timing variations to determine the planets' orbits and to estimate their masses. These observations address a long-standing question about how the galaxy’s most common planets, those between Earth and Neptune in size, form and evolve. Key findings: - The host star V1298 Tau is about 20 million years old and carries four young, rapidly evolving planets. - The planets have radii roughly 5–10 times Earth’s radius but masses of only about 5–15 times Earth’s mass, yielding very low densities. - Measurements relied on transits and transit-timing variations gathered from a network of ground- and space-based telescopes over nearly ten years. - The planets have already lost substantial portions of their upper gaseous envelopes and show signs of continued atmospheric loss and contraction. - The team reports these objects are likely the early-stage progenitors of the common mature super-Earth and sub-Neptune population. Summary: The reported measurements provide observational proof that some young, large-radius planets are extremely low-density and are actively losing atmospheres. The researchers say these planets are expected to shrink over billions of years into super-Earths and sub-Neptunes that are common around other stars. Undetermined at this time.