Super-low-density planets offer a glimpse into the formation of common planetary systems.
Four planets orbiting a young star in our galaxy have densities comparable to polystyrene, offering a crucial missing piece in understanding how the most common planetary systems form. These planets are remarkably light, with densities lower than any known exoplanet, and their radii are between five to ten times Earth's, but their masses are only a few times greater.
This solar system stands out from the typical Milky Way planetary systems, which usually contain planets larger than Earth but smaller than Neptune. Astronomers have discovered hundreds of such systems, but most of these are found around stars that are billions of years old, making their formation challenging to explain. The newly identified planets, orbiting a 20-million-year-old star named V1298 Tau, are a recent formation, as they are tightly clustered and appear to have formed recently.
The research team, led by John Livingston at the Astrobiology Center in Tokyo, Japan, and Erik Petigura at the University of California, Los Angeles, used telescopes in space and on Earth to observe these planets for five years. They measured the time it took for each planet to complete an orbit and pass in front of the star, allowing them to calculate the planets' radii and masses more accurately. However, they needed to make an educated guess about the orbital periods of the outermost planet in the absence of gravitational forces, which could have been incorrect.
Petigura admits the challenge, saying, 'I thought this was kind of a fool's errand. There were so many ways we could have gotten this wrong... The first time we recovered [the outermost planet's] transit, I almost fell out of my chair; it was like somebody getting a hole in one in golf.'
Once the orbital periods and masses were accurately determined, the researchers could estimate the planets' densities. These densities are among the lowest of any known exoplanet, and the planets are in the process of contracting due to gravitational forces, forming super-Earths or sub-Neptunes with radii around one to three times Earth's.
The planets around V1298 Tau are in orbital resonance, meaning their orbital times are multiples of each other, which aligns with the prevailing theory of planetary system formation, including our solar system. Sean Raymond at the University of Bordeaux in France notes that this system of close-in, lower-mass planets orbiting a very young star represents a potential precursor to a typical sub-Neptune system, and the discovery is remarkable due to the difficulty in characterizing such young systems.
