There is a new record holder for “most distant known object orbiting the sun”—an icy world nicknamed FarFarOut. The finding is preliminary, but researchers are now performing follow-up observations to nail down this object’s exact distance and the details of its orbit. Like so many of its far-flung siblings in the sun’s dark hinterlands, eventually FarFarOut could provide astronomers with vital new insights about our solar system’s outer frontier.
For the last six years, astronomers Scott Sheppard, of the Carnegie Institution for Science and Chad Trujillo, of Northern Arizona University, have been probing the heavens in the deepest all-sky survey ever performed for solar system bodies. They are on the hunt for Planet X, a small dwarf planet far beyond Pluto whose existence they proposed in 2014. So far, that search has yielded 62 distant objects, which make up about 80 percent of all those known beyond 60 astronomical units (AU). (One AU is equal to the Earth–sun distance, and Pluto’s average distance is just shy of 40 AU.) Just last year, the pair made headlines with not one but two major discoveries—the dwarf planet 2015 TG387, nicknamed the Goblin, and 2018 VG18, nicknamed FarOut. The pair’s dominance in the race to find ever-more distant denizens of the solar system is largely due to their dedication—they spend lots of time at telescopes, making observations at least every other month.
FarFarOut is the newest result from their single-minded search. While snowed-in during a blizzard late last month, Sheppard decided to review some of the two terabytes of data he and Trujillo had captured last year during observations on the Subaru telescope in Hawaii. In two images of the sky taken one day apart during January 2018 he spied a dim, drifting object—its change in position a sign it was not a background star, but rather a newfound object in a distant orbit around the sun. “It’s just barely detectable,” Sheppard notes.
The pair’s preliminary measurements, Sheppard says, suggest FarFarOut is between 130 and 150 AU from the sun, most likely around 140 AU. He estimates this newest-known member of the solar system is about 400 kilometers across, about half as wide as the dwarf planet Ceres. But that estimate is based on FarFarOut’s brightness, which depends on its reflectivity—a property of the body that is as of yet unknown. “It could be as dark as coal or as bright as snow,” he says, with either case generating substantially different size estimates.
Sheppard was in Chile last week, combing the sky as part of his scheduled survey. It will take some time to review the new data for signs of FarFarOut, but a new detection would provide one more data point for firming up the object’s still-hazy route around the sun. Ideally, one last follow-up measurement next month could yield a final determination of its orbit.
Once FarFarOut’s orbit is found, the fun can really begin. Astronomers’ catch-all term for icy bodies beyond Neptune is “trans-Neptunian objects,” which come in various flavors. Some circle the sun just beyond the giant planet’s gravitational reach. Others are scattered farther out in orbits that never take them closer to the sun than the cusp of Neptune’s influence, making them seem “detached” from the rest of the solar system that lies closer-in. Knowing an orbit “ties into whether we can say anything interesting about what population of trans-Neptunian objects this belongs to,” says Michele Bannister, a Queen’s University Belfast astronomer who was not involved with Sheppard and Trujillo’s work but also hunts for new objects in the solar backwaters.
If, at 140 AUs, FarFarOut is at perihelion—its closest approach to the sun—that would be “amazingly interesting,” says Konstantin Batygin, an astronomer at the California Institute of Technology who was not involved in the survey. In 2016, based in part on Sheppard and Trujillo’s 2014 speculations about “Planet X,” Batygin and his Caltech colleague, astronomer Mike Brown, announced their own search for a similar object, which they prefer to call “Planet Nine.” Whatever one might call it, both teams—as well as many other astronomers—have been hunting for this elusive world ever since.
A perihelion of circa 140 AU would demand something weighty—something perhaps like Planet X or Planet Nine—had somehow wrested FarFarOut from the grip of Neptune’s gravity. “If that’s the case, they tell an extremely important story about Planet Nine,” Batygin says. And there is, a priori, good reason to suspect FarFarOut’s perihelion is just so: Most objects in the far reaches of the outer solar system are discovered near their closest approach, simply because that is when they reflect the most sunlight and are easiest to spot. But the case of FarFarOut may not be so simple, Batygin says, because Sheppard and Trujillo’s survey is changing the rules of the game. “The types of observations Scott and Chad are doing push the boundary,” he says. By staring at the sky on a regular basis for an extensive amount of time with state-of-the-art instruments on the world’s largest telescopes, the pair is sensitive to dimmer, more distant objects than most other surveys could ever see. That means there is a chance FarFarOut is at a more distant point in its orbit, which, paradoxically, would make it less interesting as far as the search for Planet X or Planet Nine goes. If Sheppard and Trujillo did not catch FarFarOut close to perihelion, “it could be an object in a perfectly normal, highly interacting with Neptune orbit” that never crossed paths with any putative undiscovered planet lurking past Pluto, Bannister says.
Much like Sheppard and Batygin, Bannister also stresses more time and more data are unavoidably needed to know just how significant FarFarOut will be. “It’s far too soon to be discussing any implications,” she says. “There are no implications for the outer solar system until there’s an orbit.”