The origin story of the most common planets in our Galaxy has been uncovered, solving a decades-old puzzle about why the Universe is filled with worlds larger than Earth but smaller than Neptune.
An international team of astrophysicists has caught four ‘baby’ planets in the V1298 Tau system mid-transformation: shrinking from bloated, low-density giants into the compact super-Earths and sub-Neptunes that dominate the Milky Way.
“We’ve never had such a clear picture of these planets in their formative years,” says lead author John Livingston.
The four youthful planets are extremely low in density despite their large radii: five to 10 times Earth’s size, but only five to 15 times its mass. That makes them as light as Styrofoam.
As they age, stellar radiation strips away their thick primordial atmospheres, causing them to contract and increase in density.
“The unusually large radii of young planets led to the hypothesis that they have very low densities, but this had never been measured,” adds co-author Trevor David.
The team used tiny variations in the timing of the planets’ transits to weigh them for the first time, providing the crucial proof.
“This fundamentally changes how we think about planetary systems,” says Livingston.
“V1298 Tau shows us that today’s super-Earths and sub-Neptunes start out as giant, puffy worlds that contract over time.
“We’re essentially watching the Universe’s most successful planetary architecture in the making.”