The youngest planet we know of is under a million years old

Astronomers have confirmed a planet that is less than a million years old. It's called Elias 2-24 b, and NASA announced it on September 16. That makes it by far the youngest planet we know about. The old record was held by four planets at once, and all of them were over five million years old.

It sits about 450 light-years away in the Ophiuchus star-forming region, and it's still inside the disk of dust and gas it formed from. The usual methods don't work on a planet like that, because the disk is too thick and murky to watch the planet cross in front of its star. So the team, led by Andrea Bernardi, used two telescopes together. ALMA in Chile had already mapped a clean circular gap in that disk about a decade ago. Then the Very Large Telescope picked up a faint point of infrared light sitting right inside the gap.

The record isn't the part I care about. What gets me is that the planet weighs roughly as much as Jupiter and orbits at 55 AU, more than ten times farther out than Jupiter is from the Sun. Current models need something like five million years to build a Jupiter at Jupiter's distance. Building something that heavy, that far out, in under a million years is not what those models predict.

So either giant planets form much faster than we thought, or they form some other way that nobody has modelled properly yet. Either way the standard picture needs work.

Source: https://science.nasa.gov/universe/newfound-baby-planet-smashes-record-for-youngest-known-world/