Astronomers have discovered four exceptionally low-density planets orbiting a 20-million-year-old star named V1298 Tau, offering insights into the formation of common planetary systems. These worlds, with densities akin to polystyrene, are seen as precursors to super-Earths and sub-Neptunes. The findings, based on five years of observations, highlight a young version of systems prevalent across the galaxy.
In a significant advancement for exoplanet research, a team led by John Livingston at the Astrobiology Center in Tokyo, Japan, and Erik Petigura at the University of California, Los Angeles, has characterized four planets around the young star V1298 Tau. First identified in 2017, the system was scrutinized over five years using space and ground-based telescopes to detect subtle orbital variations caused by gravitational interactions among the planets.
These variations allowed precise measurements of each planet's radius and mass. The planets exhibit radii between five and 10 times Earth's, but masses only a few times greater, resulting in densities comparable to Styrofoam. "These planets have the density of Styrofoam; they're extremely low-density," Petigura noted.
The star's youth—estimated at 20 million years—provides a rare snapshot of planetary evolution. The planets are currently contracting under gravity and are projected to evolve into super-Earths or sub-Neptunes, which are 1 to 3 times Earth's radius. They orbit in a tight cluster with periods in orbital resonance, where timings are multiples of each other—a configuration common in early planetary systems.
"We are seeing a young version of a type of planetary system we see all over the galaxy," Petigura explained. This setup contrasts with older systems typically observed, which are billions of years old and harder to study for formation processes.
Sean Raymond at the University of Bordeaux in France praised the discovery: "This discovered system of close-in, lower-mass planets orbiting a very young star represents a potential precursor to a typical sub-Neptune system. This discovery is amazing, in that it is very hard to characterise such young systems."
The research aligns with models of planetary formation, where initial crowded resonances often destabilize over time, mirroring aspects of our solar system's history. Published in Nature (DOI: 10.1038/s41586-025-09840-z), the study underscores the challenges in measuring the outermost planet's orbit, which required careful assumptions but ultimately succeeded.
