A Journey Into the Solar System's Outer Reaches, Seeking New Worlds to Explore

New Horizons will speed past an object nicknamed Ultima Thule at 31,500 mph and pass within 2,200 miles of the surface, seeking clues to the earliest days of the solar system. Ultima Thule is 4 billion miles from the sun, in an area where many astronomers within recent memory believed there wouldn’t be much that was worthy of study.

It was once a common view that all of the solar system’s big, interesting things — the sun and the nine planets — had been found. When NASA’s Pioneer 10 spacecraft crossed the orbit of Neptune in June 1983, some newspaper headlines declared that it had left the solar system. (Pluto was still a planet then, but it was at the innermost part of its orbit and closer to the sun than Neptune.)

Thirty-five years later, the Kuiper belt — the region Pioneer 10 was just entering and that New Horizons continues to explore — and the spaces beyond are perhaps the most fascinating parts of the solar system. In their vast, icy reaches are clues about how the sun and planets, including ours, coalesced out of gas and dust 4.5 billion years ago.

“The Kuiper belt object studies are revolutionizing all of solar system studies,” said Renu Malhotra, a professor of planetary sciences at the University of Arizona.

Even farther out might be bodies the size of Mars or Earth, or even a larger one some astronomers call Planet Nine, and technological advances could usher in a new age of planetary discovery.

But first astronomers will get their closeup of Ultima Thule, believed to be just 12 to 22 miles wide. It is also known as 2014 MU69 — its designation in the International Astronomical Union’s catalog of worlds — and studying it could help reveal what else lies in the Kuiper belt.

“I’m more excited to see it than I was of Pluto,” said Harold F. Levison, a planetary scientist at the Southwest Research Institute in Boulder, Colorado. “It’s going to be really cool.”

This article originally appeared in The New York Times.