Newly discovered planets orbiting V1298 Tau are unusually lightweight, possessing a density comparable to polystyrene. This discovery may bridge critical gaps in our understanding of planetary system formation.

Unlike most planets in our Milky Way galaxy, which are often larger than Earth and smaller than Neptune, this solar system showcases an uncommon configuration. Astronomers have cataloged numerous planetary systems that formed billions of years ago, complicating our understanding of their genesis.

The research team, led by John Livingstone from the Astrobiology Center in Tokyo and Eric Pettigura from UCLA, has identified four dense planets that likely formed recently around a young star, V1298 Tau, which is around 20 million years old.

“We are examining younger models of the types of planetary systems commonly found across our galaxy,” Pettigura remarked.

Initially discovered in 2017, V1298 Tau and its accompanying planets remained largely unstudied until now. Over five years, researchers utilized both terrestrial and space telescopes to observe tiny variances in orbital durations, revealing intricate gravitational interactions among the four planets. These measurements enable more precise calculations of each planet’s radius and mass.

To effectively employ this observational method, researchers required initial estimates of each planet’s orbital duration without gravitational interference. Lacking that data for the outermost planet, they relied on educated conjectures, risking inaccuracies in their calculations.

“I initially had my doubts,” Petitgras admitted. “There were numerous potential pitfalls… When we first acquired data from the outermost planet, it felt as exhilarating as making a hole-in-one in golf.”

By accurately measuring the orbital durations and subsequently estimating the radii and masses, the team determined the densities of the planets. They discovered these are the lowest-density exoplanets known, with radii spanning five to ten times that of Earth, yet only a few times its mass.

“These planets exhibit a density akin to Styrofoam, which is remarkably low,” Pettigura explained.

This low density can be attributed to the planets’ ongoing gravitational contraction, potentially classifying them as super-Earths or sub-Neptunes—types of planets typically formed during the evolutionary stages.

The planets of V1298 Tau operate in a so-called orbital resonance, indicating their orbital periods are harmonically related. This observation aligns with astronomers’ theories on the formation of most planetary systems, including our own solar system, which initially have tightly packed configurations that eventually evolve into less stable arrangements, according to Sean Raymond from the University of Bordeaux in France.

“This newly identified system of close, low-mass planets revolving around a relatively young star could provide insights into typical sub-Neptunian systems,” Raymond pointed out. “This discovery is remarkable due to the inherent challenges in characterizing such youthful systems.”