Planet 10 times the size of Earth is one of the youngest ever found

A world seen orbiting a 3-million-year-old star about 520 light years from Earth is one of the youngest known planets, offering a window into early planet formation.

The star is an early-stage dwarf star, one much dimmer and less massive than our sun. Its age has been estimated by comparing the intensity and wavelengths of the light it emits with other stars.

Madyson Barber at the University of North Carolina at Chapel Hill and her colleagues studied the star using NASA’s Transiting Exoplanet Survey Satellite (TESS). They found a planet about a third of the mass of Jupiter and 10 times the diameter of Earth by noticing the dip in the star’s light as the planet passed in front.

The world’s mass and size suggest it is either a large rocky planet, known as a super-Earth, or a small gas giant, called a sub-Neptune, in the process of formation.

We think Earth took between 10 million and 20 million years to form, about 4.5 billion years ago, says Barber. “So it was kind of surprising to see anything at 3 million years.”

The system is also notable for still having its protoplanetary disc of dust and gas, meaning the star and planets are still in the process of taking shape, although that disc is oddly misaligned out of the plane of the system for reasons that aren’t clear. “We’re not super sure what caused the misalignment,” says Barber. “It’s possible a stellar flyby happened as the system was forming.”

The planet is extremely close to its star, completing an orbit every nine days, which is also puzzling as it is unclear whether planets can form in such proximity. They can move inwards over time, as is thought to have taken place in our solar system when some of the giant planets jostled for position. “It hints at fast migration of planets being a thing,” says Barber.

While we know of other young planets, they have tended to be much larger worlds. This one could give us a closer representation of how the worlds in our own solar system came into being. “We try to extrapolate from these other worlds how quickly planet formation might have taken hold in the early solar system,” says Melinda Soares-Furtado at the University of Wisconsin-Madison.

Some young stars have even been seen with gaps in their protoplanetary disc after just half a million years, hinting at the existence of planets forming “in tandem with their host stars”, she says.

“It looks like things happen early,” says Soares-Furtado, “so it’s really cool to grab snapshots of systems like this one.”