Astronomers have discovered a far-out world circling the sun.
How far out? It’s so far out that the discoverers nicknamed it “Farout.” All they can see is a pinkish dot of light in the night sky, but that is enough to infer that they are looking at a 300-mile ice ball orbiting more than 11 billion miles from the sun — more than three times as far out as Pluto, and the farthest object ever observed within the solar system.
It is the latest revelation in a distant region that was once expected to be empty, and studying its trajectory may help point to an as-yet-unseen ninth planet circling the sun far beyond Neptune.
On Monday, the International Astronomical Union’s Minor Planet Center announced the discovery and gave this object the designation 2018 VG18.
“Last month, we came across it moving very, very slow,” said Scott S. Sheppard of the Carnegie Institution for Science, one of the discoverers of VG18. “Immediately we knew it was an interesting object.”
The sun’s gravity decreases with distance. More distant worlds move slowly and take longer to complete an orbit than closer ones. A languid, dim speck of light showed up in images taken on Nov. 10 by the Japanese Subaru 8-meter telescope located atop Mauna Kea in Hawaii. Follow-up observations at Las Campanas Observatory in Chile this month confirmed the discovery.
Planetary scientists often use the distance from the sun to the Earth — defined as an astronomical unit, or 93 million miles — as a yardstick for measuring the solar system. Neptune is 30 astronomical units away, or 2.8 billion miles, and Pluto, currently on the outward leg of its orbit, is 34.5 astronomical units, or 3.2 billion miles from the sun.
Pluto was once regarded as the outer edge of the solar system. But starting in 1992, astronomers discovered a multitude of other icy worlds beyond Neptune, a region now known as the Kuiper belt. The Kuiper belt ends at a distance of about 50 astronomical units, and the space beyond that was thought to be largely empty.
But astronomers are now discovering objects like VG18 in this region, and they are yet sure how to explain how all of them got there.
VG18 is 120 to 130 astronomical units from the sun. It is the first solar system object ever spotted at a distance of more than 100 astronomical units. (Other objects are known to have orbits that swing much farther out than 100 astronomical units, but currently are closer.)
Astronomers do not yet have a good sense of VG18’s orbit — whether it is elliptical and zooms inward near Neptune, or if it is more circular and always stays far away. That information, which may require a few years of additional observations, will tell whether it fits with a prediction of a distant planet larger than Earth but smaller than Neptune.
So far, they can report that VG18 has a pinkish hue and, assuming it is moderately dark, guess that is about 300 miles wide. One trip around the sun likely takes at least 1,000 years. If VG18 is indeed that large, it would likely be massive enough for gravity to pull it into a round shape and fulfill the definition of a “dwarf planet,” the same category that includes the asteroid Ceres and the former planet Pluto.
Dr. Sheppard and his colleagues, as well as other astronomers, are surveying the sky for the hypothesized giant planet, often called Planet Nine. So far their searches have turned up only intriguing clues. In October, Dr. Sheppard and his colleagues reported the discovery of a world that was distant, albeit not as distant as VG18. They nicknamed it Goblin, because Halloween was approaching, and its orbit provided further evidence that Planet Nine may indeed exist.
VG18 lies close tot the limit of what current telescopes can detect. But it likely is not the last discovery, to be made in those nether regions Dr. Sheppard said: “If it's further out, we'll name it Way, Way Out or something.”