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.