Discovery of Planetoid Hints at Bigger Cousin in Shadows

Astronomers have discovered a second icy world orbiting in a slice of the solar system where, according to their best understanding, there should have been none.

“They’re in no man’s land,” Scott S. Sheppard, of the Carnegie Institution for Science in Washington, said of the objects, which orbit far beyond the planets and even the ring of icy debris beyond Neptune known as the Kuiper belt.

Intriguingly, the astronomers said that details of the orbits hint at perhaps an unseen planet several times the size of Earth at the solar system’s distant outskirts.

The new planetoid, an estimated 250 miles wide, is now 7.7 billion miles from the sun, about as close as it gets. At the other end of its orbit, the planetoid, which for now carries the unwieldy designation of 2012 VP113, loops out to a distance of 42 billion miles. Neptune, by contrast, is a mere 2.8 billion miles from the sun.

Much farther out, a trillion miles, the solar system is believed to be surrounded by a sphere of icy bodies known as the Oort cloud, where many comets are thought to originate. But between the Kuiper belt and the Oort cloud, astronomers had expected empty space.

The discovery, by Dr. Sheppard and Chadwick A. Trujillo of the Gemini Observatory in Hawaii, is reported in the journal Nature.

For convenience, the scientists shortened the 2012 VP113 designation to VP, which in turn inspired their nickname for the planetoid: Biden, after Vice President Joseph R. Biden. Dr. Trujillo said they had not decided what to propose for the official name.

The existence of 2012 VP113 could help explain why there is anything out there at all.

In the 2000s, when Michael E. Brown, an astronomer at the California Institute of Technology, scanned the outer solar system, his biggest discovery was Eris, a ball of ice in the Kuiper belt that was Pluto-size or slightly bigger, the impetus for the demotion of Pluto to dwarf planet.

Dr. Brown’s oddest discovery, however, came a couple of years earlier: Sedna, a 600-mile-wide planetoid also beyond the Kuiper belt, three times as far from the sun as Neptune. Its 11,400-year orbit stretches farther than that of 2012 VP113.

In the youth of the solar system, there would not have been enough matter out there to coalesce into something as large as Sedna. It was too far out to have been flung by the gravitational slings of big planets, but too close to have been nudged by the gravitational tides of the Milky Way.

Having found one such body, astronomers expected to quickly find more, and they came up with a name for them: Sednoids. But for years, no one found any.

For the latest search, Dr. Trujillo and Dr. Sheppard used a 13-foot telescope at the Cerro Tololo Inter-American Observatory in Chile. In November 2012, they spotted a moving point of light beyond the Kuiper belt — 2012 VP113. Follow-up observations last year confirmed it was a Sednoid. Scientists have come up with various ideas to explain such bodies. Dr. Brown, for one, thinks the Sednoids were pushed there when the sun was part of a dense cluster of stars — “a fossil record of the birth of the solar system,” he said.

Others suggest that a rogue planet, ejected from the inner solar system, dragged the Sednoids along as it flew through the Kuiper belt. Dr. Trujillo and Dr. Sheppard point out that the orbits of Sedna and 2012 VP113 have similarities to those of several other Kuiper belt bodies, which could be a sign of an unseen planet’s gravitational influence.

Computer simulations showed that the similarities could be explained by a planet with a mass five times that of Earth about 23 billion miles from the sun, too dim to be seen.

Harold F. Levison of the Southwest Research Institute in Boulder, Colo., who models the beginning of the solar system, agreed that this was a possibility. “I think they’ve convinced me there’s something going on,” he said of Dr. Trujillo and Dr. Sheppard. “But I think it’s too early to say that it’s a planet.”

The astronomers expect to find more Sednoids in the next few years, which could solve the mystery of their origin. “When we find 10 of them, I’ll tell you what the answer is,” Dr. Brown said.