A Lonely Giant: Astronomers Track a Jupiter-Sized Planet Drifting Toward Us

 Could a vagabond giant from the outermost regions of interstellar space be poised on the cusp of sweeping by our cosmic doorstep? Astronomers say yes and they have the data to support it. Using the Pan-STARRS survey telescope on Hawaii and the European Southern Observatory’s Very Large Telescope (VLT) in Chile, researchers have found a giant rogue planet, possibly as massive as Jupiter, speeding toward the solar system. It is a transient discovery made possible by the combination of wide-sky monitoring and the capability of high-resolution spectroscopy.

Pan-STARRS first notified astronomers to the object with weak, creeping light signals near quiet stars. The adaptive optics and spectrographs of the VLT then allowed researchers to unravel its light into a spectrum, which indicated absorption lines typical of a gas giant atmosphere. Spectroscopy the same technique researchers apply to study exoplanets can identify molecular constituents such as methane, ammonia, and water vapor and give clues to how the planet formed. As Núria Miret-Roig, who spearheaded similar rogue planet surveys, has described it, These measurements allow us to securely identify the faintest objects in this region, the rogue planets.

Early orbital simulations suggest that the body is not gravitationally bound to any star and is traveling through the solar system for the first time. Its mass and velocity suggest it will have slight gravitational pulls on other planets or minor bodies. Scientists are conducting three-body simulations in order to establish possible perturbations, a process that involves the restricted three-body problem and considers chaotic orbital resonances. Whereas current models indicate minimal disruption, even a Jupiter-mass object passing by tens of astronomical units can deflect cometary orbits in the Oort Cloud.

The find also reminds us of past interstellar visitors. ‘Oumuamua in 2017 and comet Borisov in 2019 were both smaller and faster, but they illustrated the diversity of interstellar objects. This size of the new arrival places it into a class by itself a rogue planet, maybe ejected from its home system during the early gravitational upheavals. Ejection from planet-planet, close stellar flybys, or even the implosion of a small gas cloud could have ejected it. Surveys like OGLE have also shown that Jupiter-mass rogues may number in the billions in the Milky Way.

In a close-up examination of such an object, a special laboratory for planetary science awaits. Spectroscopic measurements can establish whether its chemical makeup is analogous to gas planets in our home solar system or bears the imprint of an extraneous stellar nursery. If it still retains a circumplanetary disk as has been observed about some young free-floaters by the James Webb Space Telescope it might even carry the seeds of moons, a tiny planetary system drifting in the darkness.

Tracking it, however, is a challenge. Rogue planets are feeble, icy, and move against a dense stellar backdrop. The scientists are employing computer simulations that draw on nightly new astrometric observations, refining predictions for its closest approach. It will inform observing campaigns with professional gear as well as citizen science networks, following the lead of missions already detecting scores of close brown dwarfs and planetary-mass objects.

The Vera C. Rubin Observatory, set to begin full operation in the near future, promises to render such searches obsolete. Its Legacy Survey of Space and Time will take pictures of the southern sky a few nights out of every week, maybe catching many more red-handed interstellar interlopers. As Chris Lintott of Oxford once characterized this coming age, “Find the thing, point telescopes at it, argue about it. It’s going to be fun.”

For the moment, the gradual arrival of the rogue planet offers a window of opportunity. As it continues deeper into the solar system, its changing spectrum and motion will be scrutinized for proof of atmospheric activity, ring systems, or even the glint of distant satellites. With every visit, not only will the path of an unusual intruder be charted, but mysteries regarding how planetary systems form, grow, and sometimes launch their worlds out into the galaxy’s unknown frontier will be resolved.