Discovery of Elias 2-24 b Confirms Core Accretion in the Youngest Known Exoplanet

The international astronomical teams operating the Keck Observatory's NIRC2 vortex coronagraph now hold the definitive evidence to settle a decades-old debate in planetary physics, and their upcoming observation cycles will determine exactly how fast a gas giant can form. By peering into the narrow gap of a circumstellar disk 450 light-years from Earth, researchers have confirmed the existence of Elias 2-24 b, an exoplanet less than one million years old. This observation proves that core accretion—the slow buildup of rocky material followed by a rapid gas sweep—can occur much faster than previously thought, forcing a rewrite of the timeline for how solar systems take shape.[1][2][3]

The discovery, published on September 16 in The Astrophysical Journal Letters, shatters the previous age floor for confirmed exoplanets. Elias 2-24 b is roughly two to four times the mass of Jupiter and orbits its host star at a distance of 55 astronomical units, or about 10 times farther out than Jupiter sits from the Sun. "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 co-author of the study.[1][3]

Two primary mechanisms dominate the theoretical debate over how gas giants emerge from the swirling debris of a newborn star. The disk instability model suggests that a massive cloud of gas rapidly collapses under its own gravity, forming a planet almost instantly. The core accretion model requires dust grains to stick together into pebbles, which gradually accumulate into a solid rocky core before generating enough gravity to pull in a massive atmospheric envelope.[2]

Because core accretion is a multi-step, sequential process, standard physics models dictated that building a Jupiter-sized world would take at least 5 million years. That timeline becomes even more stretched at 55 astronomical units, where the raw material of the protoplanetary disk is spread thinly across a vast orbital circumference. Yet Elias 2-24 b sits exactly in a cleared gap within its disk, actively pulling in gas and dust at an age of less than one million years.[1][2]

"This is the youngest planet detected so far, and because it is still actively accreting material from its surroundings, we're able to observe a stage of planet formation that is rarely seen directly," said Andrea Bernardi, a doctoral candidate at the Universidad Diego Portales and the study's lead author. The planet's existence demonstrates that the pebble-accretion phase can operate at extreme efficiencies, rapidly building a core even in the sparse outer reaches of a stellar system.[1][4]

The confirmation was not the result of a single, sudden telescope exposure, but a meticulous archival hunt spanning multiple years and observatories. The Atacama Large Millimeter/submillimeter Array (ALMA) had previously mapped the Elias 2-24 system, revealing distinct dark rings in the dust that strongly suggested a planet was carving a path. However, directly imaging the planet required blocking the overwhelming glare of the host star.[1][2][5]

In 2018, Alice Zurlo and her research team used the Keck Observatory's coronagraph in Hawaii to observe the star, detecting a faint signal at the edge of the instrument's limits. Because a single 3.85-sigma detection was insufficient to definitively claim a discovery, the signal remained unconfirmed. Bernardi's team subsequently searched the NASA-funded Keck Observatory Archive, analyzing seven young disk-hosting stars, and recovered the same faint point of light in Elias 2-24's disk from a separate June 2020 observation.[2][6]

By stitching the 2018 and 2020 observations together, the astronomers tracked the object's precise proper motion. The data confirmed the point of light was moving in lockstep with the star rather than drifting as a stationary background artifact. When combined with supporting data from the European Southern Observatory’s Very Large Telescope, the multi-instrument synthesis pushed the detection past the required confidence threshold.[1][2][4]

The research team is now preparing proposals to obtain spectroscopy measurements of Elias 2-24 b, which will constrain its dynamical mass and atmospheric temperature. As next-generation instruments like NASA's Nancy Grace Roman Space Telescope prepare for launch, the astronomical community will soon have the resolution to determine whether this rapid, sub-million-year accretion is an extreme outlier or the standard blueprint for planetary birth.[1][3]