How the most common types of planets are formed.

A team measured masses for four close-in planets around the young star V1298 Tau and found they are very low-density "super-puff" worlds; the researchers report that stellar stripping over time can shrink such planets into the hot super-Earths that are commonly observed.

The study examines how common close-in planets known as hot super-Earths may form. Mercury is unusually small compared with many other planetary systems, where larger worlds between Earth and Neptune often orbit much closer to their star. Astronomers have debated whether those planets migrate inward after forming farther out or form close to their star. The new work studies the 20-million-year-old star V1298 Tau and its four close-in planets to test these ideas. By modeling transit timing variations the team derived the planets' masses and densities. Key findings: - V1298 Tau is about 20 million years old and hosts four planets that orbit closer to their star than Mercury orbits the Sun. - Transit measurements indicate the planets have diameters about 5 to 10 times that of Earth. - Transit-timing analysis yielded masses of roughly 5 to 15 Earth masses for the four planets. - Density calculations show all four are extremely low-density "super-puff" worlds with large, diffuse hydrogen–helium atmospheres. - The study reports that stellar flares and winds from the young star will strip those atmospheres over time. - The team estimates these planets will be whittled down to masses and densities similar to the common hot super-Earths as the system matures. Summary: The findings suggest an evolutionary path in which young, gas-rich close-in planets become denser as stellar activity removes their extended atmospheres. This process offers a possible explanation for the prevalence of hot super-Earths. Undetermined at this time.