One of the planets with low density compared to Earth
NASA
Four planets orbiting a young star in our galaxy are so light they have the density of polystyrene and could be a key missing link in helping 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 similar planetary systems, but almost all of them are formed around stars billions of years old, making it difficult to explain how they take shape.
Now the team is under the leadership John Livingston at the Astrobiology Center in Tokyo, Japan, and Eric Petigura UCLA has identified four tightly clustered 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 the planetary system that we see throughout the galaxy,” Petigura says.
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 differences in the time it takes each planet to complete its orbit and pass in front of the star due to the gravitational forces of attraction between the four worlds. By measuring these small differences, they were able to more accurately calculate the radius and mass of each planet.
However, for this method to work, they needed to know in advance 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 outermost planet, so they had to use educated guesses – and if their guess was wrong, then all their calculations would fail.
“I thought it was honestly kind of a stupid idea,” Petigura says. “There were so many ways we could have been wrong… when we first recovered [the outermost planet’s] during the transit I almost fell out of my chair; it was like someone had punched a golf hole.”
Once they accurately measured the orbital periods of all the planets and calculated their radii and masses, they were able to estimate the density of each planet. They found that these are among the lowest exoplanets known, with radii five to ten times that of Earth but masses only a few times greater.
“These planets have the density of foam, but extremely low density,” Petigura says.
This is because the planets are in the process of being compressed due to gravitational forces, forming planets whose radius is only one to three times the radius of the Earth, so-called super-Earths or sub-Neptunes. The researchers modeled how the planets would evolve and found that they would eventually become this type of planet.
The planets of V1298 Tau are in what is called an orbital resonance, which means that the planets' orbital times are multiples of each other. This fits with astronomers' understanding of how most planetary systems, including our solar system, form, says Sean Raymond of the University of Bordeaux in France. They start out as crowded systems with clear orbital resonances, but then become unstable in terms of the ratio of their periods.
“This discovered system of nearby, lower-mass planets orbiting a very young star represents a potential precursor to a typical sub-Neptune system,” says Raymond. “This discovery is surprising because it is very difficult to characterize such young systems.”
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