Super-low density worlds reveal how common planetary systems form

Four planets orbiting a newly born star in our galaxy are so light they have the density of polystyrene and could provide a key missing link to help us understand how the most common planetary systems form.

This solar system is unusual compared to most other planetary systems in the Milky Way, which typically contain planets larger than Earth but smaller than Neptune. Astronomers have discovered hundreds of planetary systems like these, but almost all of them formed around stars that are billions of years old, making it difficult to explain how they take shape.

Now, a team led by John Livingston of the Tokyo Astrobiology Center, Japan, and Erik Petigura of the University of California, Los Angeles, has identified four closely grouped planets that appear to have formed recently, given that they orbit a young, 20-million-year-old star called V1298 Tau.

“We’re seeing a young version of a type of planetary system that we see all over the galaxy,” says Petigura.

V1298 Tau and its four planets were first discovered in 2017, but little was known about the planets themselves. The researchers used telescopes in space and on Earth to observe them for five years, looking for subtle variations in the time it takes for each planet to complete an orbit and pass in front of the star due to gravitational forces of attraction between the four worlds. By measuring these small differences, they could more precisely calculate the radius and mass of each planet.

However, for this method to work, it was necessary to know beforehand how long each of the four planets would take to orbit the star in the absence of these gravitational forces. They didn’t have this information about the most distant planet, so they had to resort to educated guesses – and if their guess was wrong, then all their calculations would have failed.

“Frankly, I thought it was a crazy mission,” Petigura says. “There were so many ways we could have gone wrong…the first time we recovered [the outermost planet’s] in transit, I almost fell off my chair; it was like someone making a hole-in-one in golf.

Once they had accurately measured the orbital periods of all the planets and calculated their radii and masses, they could then estimate the density of each planet. They found that they were among the lowest of all known exoplanets, with radii between five and ten times those of Earth, but masses only a few times larger.

“These planets have the density of polystyrene foam; their density is extremely low,” explains Petigura.

This is because the planets are contracting due to gravitational forces to form planets that are about one to three times the radius of Earth, called super-Earths or sub-Neptunes. The researchers simulated the evolution of the planets and found that they would eventually become these types of planets.

The planets of V1298 Tau are configured in what is called orbital resonance, meaning that the orbital times of the planets are multiples of each other. This matches astronomers’ picture of how most planetary systems, including our solar system, form, says Sean Raymond of the University of Bordeaux in France. They start as crowded systems with sharp orbital resonances, but then become unstable in terms of the ratio of their periods.

“This discovered system of nearby, low-mass planets orbiting a very young star represents a potential precursor to a typical sub-Neptune system,” says Raymond. “This discovery is surprising, as it is very difficult to characterize such young systems.”