A young, massive planet is orbiting in an unusual place in its star system, and it’s leading researchers to revive a long-debated view of how giant planets can form.
The protoplanet, nine times the mass of Jupiter, is too far away from its star to have formed by accreting matter piece by piece, images suggest. Instead, the massive world probably formed all at once in a violent implosion of gas and dust, researchers report April 4 in Nature Astronomy.
“My first reaction was, there’s no way this can be true,” says Thayne Currie, an astrophysicist at the Subaru Telescope headquartered in Hilo, Hawaii.
For years, astronomers have debated the ways in which giant planets might form (SN: 12/3/10). In the “core accretion” story, a planet starts out as small bits of matter within a disk of gas, dust and ice swirling around a young star. The clumps continue to accrete other matter, growing to become the core of the planet. Out past a certain distance from the star, that core then accumulates a thick blanket of hydrogen and helium, turning it into a bloated, gassy world.
But the new planet, orbiting a star called AB Aurigae, is in the outskirts of its system, where there’s less matter to gather into a core. In this position, the core can’t become massive enough to create its gaseous envelope. The planet’s remote location, Currie and colleagues argue, makes it more likely to form via “disk instability,” where the disk around the star breaks into planet-sized fragments. The fragments then rapidly collapse in on themselves, drawn together by their own gravity, and clump together, forming a giant planet.
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