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

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

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

Earth’s mass and size suggest that it is either a large rocky planet, known as super-Earth, or a small gas giant, called sub-Neptune, currently forming.

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

The system is also notable for still having its protoplanetary disk of dust and gas, meaning the star and planets are still taking shape, although this disk is strangely misaligned relative to at the system level for reasons that are not clear. . “We’re not really sure what’s causing the misalignment,” says Barber. “It is possible that a stellar flyby occurred during the formation of the system.”

The planet is extremely close to its star, completing one orbit every nine days, which is also confusing because it is unclear whether planets can form in such close proximity. They can move inward over time, as is thought to have happened in our solar system when some giant planets jostled for position. “This suggests that rapid migration of planets is a reality,” says Barber.

Although we know of other young planets, they tend to be much larger worlds. This could give us a more accurate picture of how the worlds in our own solar system came to be. “We are trying to extrapolate from these other worlds how quickly planet formation could have taken place in the early solar system,” explains Melinda Soares Furtado at the University of Wisconsin-Madison.

Some young stars have even been observed with holes in their protoplanetary disks after just half a million years, suggesting the existence of planets forming “in tandem with their host stars,” she says.

“It seems like things are happening early,” says Soares-Furtado, “so it’s really cool to take snapshots of systems like this.”