A planet still forming about 450 light-years from Earth has been confirmed as the youngest known world. It is less than 1 million years old and already sits roughly 55 times farther from its star than Earth is from the Sun. That combination challenges how quickly astronomers think giant planets can grow.
Dubbed Elias 2-24 b, the planet lies inside a narrow gap in a disk of gas along withdust surrounding its young star. Astronomers had spotted clues there before, but a new analysis of observations from the W. M. Keck Observatory in Hawaii found the object in images taken nearly two years apart.
Its position and apparent motion strengthen the case that it belongs to the system, according to a study published in The Astrophysical Journal Letters.
“The planets should be found within the gaps, since they are carving them,” study lead author Dr. Andrea Bernardi said in a NASA release. “And that’s exactly where we found Elias 2-24 b.”
A Planet in a 30-AU-Wide Gap
The gap helps explain what makes this discovery unusual. Elias 2-24 b orbits about 55 astronomical units from the star, or roughly 5.1 billion miles away, while the gap spans about 30 astronomical units. At that distance, building a giant planet by the usual process of assembling a solid core and drawing in gas is difficult to explain within the system’s short lifetime.
Earlier observations with the Atacama Large Millimeter/submillimeter Array, or ALMA, mapped the gap in the disk. Other observations showed a faint source near it, raising the possibility of a planet. But a bright spot beside a young star can be difficult to identify. Disk debris, a distant background object, or image-processing artifacts may complicate the picture.
Dr. Bernardi and colleagues examined Keck observations from 2018 and reanalyzed another set from 2020. They recovered the source at both dates and found its position consistent with the earlier detections. They also tested whether it behaved like a fixed background object as the star moved across the sky. Their analysis found that the explanation was inconsistent with the measurements at a 3.2-sigma confidence level.
Those images have limits. Neither Keck detection individually met the commonly used five-sigma threshold, and the researchers could not yet measure the planet’s orbital motion. Their confirmation rests on combined evidence: repeated detections, agreement with observations from other instruments, the source’s motion relative to the star, and its location inside the disk gap.
The gap alone never proved a planet. Elias 2-24 b gives astronomers a source they can place directly alongside the structure it appears to be shaping.
Brightness Does Not Give a Simple Answer
Researchers compared the object’s infrared brightness with models of young giant planets, estimating a mass of roughly 1.9 to 4 times Jupiter’s. Yet the paper cautions against treating that range as a precise measurement.
A planet this young may still be pulling in material. Some of its light could come from that process rather than from heat retained inside the planet itself. Dust in the surrounding disk also affects what astronomers see. The researchers, therefore, describe their brightness-based mass assessments as upper limits. Models based on the gap’s shape allow a planet closer to Jupiter’s mass.
This is one of the most revealing parts of the result. The bright point in the image is evidence of a forming world, but its glow does not tell astronomers its mass without assumptions about its atmosphere and how it is growing. Future observations at different wavelengths could help separate those effects.
The system also puts the timing of planet development under pressure. Other directly imaged young planets cited in the study orbit stars around 5 million years old. Elias 2-24 is younger than 1 million years, while its planet orbits far beyond Jupiter’s distance from our Sun.
“Our planet-formation models already struggled to explain the previous record holders for the youngest known planet,” co-author Dr. Lucas Cieza explained. “Elias 2-24 b shows us that even our best planet-formation models are still missing some important processes.”
Researchers argue that the disk’s structure favors core accretion, in which smaller bodies grow into a core that gathers gas. A different proposed route involves a massive disk breaking into clumps under its own gravity.
The new observations do not capture Elias 2-24 b’s entire formation history. Still, they show a giant planet in a young, relatively narrow gap where the team’s models place a growing planet.
For now, astronomers have caught a brief stage of that growth rather than a finished planetary system. The team notes that additional observations could tighten the mass estimate and, eventually, show the planet’s motion around its star. Those measurements would help determine how a giant world formed so far out, so early.
“We are mostly blind to these baby planets right now,” Dr. Cieza said. “That means we can see the entire process in theory, but there is a large gap in what most telescopes can detect.”
“This is just the beginning of a new era of discovery,” Dr. Cieza added. “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.”
The recent study, “Searching for Embedded Protoplanets with the Keck/NIRC2 Vortex Coronagraph: Confirmation of a Core-accretion Planet in the Narrow Gap of the Elias 2-24 Disk,” appeared in The Astrophysical Journal Letters.
Tim McMillan is a retired law enforcement executive, investigative reporter and co-founder of The Debrief. His writing typically focuses on defense, national security, the Intelligence Community and topics related to psychology. You can follow Tim on Twitter: @LtTimMcMillan. Tim can be reached by email: tim@thedebrief.org or through encrypted email: LtTimMcMillan@protonmail.com