Astronomers Confirmed a Jupiter-Mass Planet Less Than a Million Years Old, Sitting 55 Times Farther From Its Star Than Earth Is From the Sun. Every Planet-Formation Model Says It Shouldn't Exist Yet.

The youngest planet ever confirmed is less than a million years old, roughly the mass of Jupiter, and orbits a star 450 light-years away at 55 times the Earth-sun distance, a position and an age that, according to the best current models of how planets form, should not be possible together. The confirmation of Elias 2-24 b, published Wednesday, September 16, in The Astrophysical Journal Letters, came not from a new telescope but from a doctoral student combing through archived images at the W. M. Keck Observatory in Hawaii.

"Our planet-formation models already struggled to explain the previous record holders for the youngest known planet, a four-way tie between two planets orbiting the star PDS 70 and two planets orbiting the star WISPIT 2, which are all more than 5 million years old," said Lucas Cieza, a professor at the Instituto de Estudios Astrofísicos in Chile and a co-author of the paper. "Elias 2-24 b shows us that even our best planet-formation models are still missing some important processes." The standard picture, called core accretion, has a rocky core slowly gather gas from the disk around a young star; at Jupiter's distance from the sun, that is supposed to take about 5 million years, and it takes longer the farther out you go. Elias 2-24 b is ten times farther out than Jupiter and appears to have finished the job in a fifth of the time.

The planet had been hiding in plain sight for a decade. About a decade ago, the ALMA radio array in Chile imaged the dusty disk around the young star Elias 2-24 and found a clean gap in it, the kind of ring that a forming planet carves as it sweeps up material. The European Southern Observatory's Very Large Telescope then picked up a faint point of light sitting inside that gap. Astronomers argued about it. A gap that early and that far out contradicted the models, and a single dot could be a background star or an artifact. "The planets should be found within the gaps, since they are carving them," said Andrea Bernardi, a doctoral candidate at the Universidad Diego Portales in Chile who led the new study. "And that's exactly where we found Elias 2-24 b."

Bernardi's team went to the Keck Observatory Archive, a partnership between Keck and the NASA Exoplanet Science Institute at Caltech, and searched observations of seven young stars with disks taken with the NIRC2 vortex coronagraph, an instrument that blocks the star's glare so fainter companions can be seen. They found the same point of light in Keck images from 2018 and 2020. Stitching the epochs together let them track its motion, and it moved with the star, in orbit, the way a planet does and a background object does not. "We usually hear about telescopes working separately, but this confirmation was possible only by using multiple telescopes together," Bernardi said.

The discovery underscores how little astronomers can actually see of the process they are trying to model. Of the roughly 6,000 confirmed exoplanets, the overwhelming majority were found by the transit method, which requires a planet to pass in front of its star from Earth's point of view and works best for worlds that are old, close in and no longer buried in dust. "The galaxy churns out new stars and planets continuously, so there are many in every stage of evolution," Cieza said. "That means we can see the entire process in theory, but there is a large gap in what most telescopes can detect. We are mostly blind to these baby planets right now."

That may be about to change. NASA's Nancy Grace Roman Space Telescope, launched on August 30, carries a far more capable coronagraph than Keck's and should be able to image planets much closer to their stars, including true Jupiter analogs that are currently lost in the glare. Elias 2-24 b, Bernardi said, "is at the limit of what current telescopes can detect, but with new instruments like Roman, such detections should become easier." Cieza was blunter: "It's incredible that with modern technology, we are actually able to see planet formation in action, and Roman will take planet hunting to the next level."