In June 1983, newspaper headlines declared that NASA’s Pioneer 10 spacecraft had left the solar system, crossing beyond the orbit of Neptune.
It was the common view of the time: All of the solar system’s big, interesting things — the sun and the nine planets — were behind Pioneer 10. (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 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.
On Tuesday, New Horizons, the NASA spacecraft that snapped spectacular photographs of Pluto in 2015, will provide humanity with a close-up of one of these mysterious, distant and tiny icy worlds.
Its target of exploration is believed to be just 12 to 22 miles wide, known as 2014 MU69 — its designation in the International Astronomical Union’s catalog of worlds — or Ultima Thule, the nickname bestowed upon it by the New Horizons team.
This will be the farthest object ever visited by a spacecraft.
New Horizons will speed past Ultima Thule at 31,500 miles per hour and pass within 2,200 miles of the surface. What the probe finds could reveal much about the earliest days of the solar system and 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, Colo. “It’s going to be really cool.”
For decades after its discovery in 1930, Pluto remained a small and icy oddity with a tilted, elongated orbit. Then, in 1992, David C. Jewitt and Jane Luu discovered Albion, a much smaller object than Pluto, in this region beyond Neptune.
As more of these tiny, icy worlds were found, Pluto no longer seemed strange. Instead it was just another inhabitant of what became known as the Kuiper belt, named after Gerard Kuiper, an astronomer who had speculated about the existence of a ring of debris beyond Neptune in 1951.
Today, more than 2,000 worlds have been discovered in the outer parts of the solar system, and there are most likely millions more.
In the main Kuiper belt, these objects fall into two groups. The first consists of objects that look as if they were pushed outward by Neptune. Many of those, including Pluto, are tilted at an angle to the rest of the solar system.
This distribution has contributed to the now-accepted idea that the giant planets — Jupiter, Saturn, Uranus and Neptune — were not always where they are now, but migrated to their current orbits.
A project called the Outer Solar System Origins Survey systematically tracked about 1,000 Kuiper belt objects for four years. That information could lead to hints of Earth- or Mars-size planets that formed in the solar system’s youth and were then cast out into interstellar space.
The scientists working on the survey do not have any firm conclusions yet. “We’re just starting to tease apart stuff,” said Brett J. Gladman, a professor of astronomy at the University of British Columbia in Canada and principal investigator for the survey.
The second group of Kuiper belt objects, known as the cold classicals, have nearly circular orbits and lie almost on the same plane as the planets. That suggests they have been largely undisturbed since the birth of the solar system. Ultima Thule, the target of New Horizons, is a cold classical Kuiper belt object.
“This thing has always been cold,” said S. Alan Stern, the principal investigator of the mission, “and it’s not large enough to have a geological engine like Pluto. It should be a real window into the earliest days of the solar system.”
Objects in the outer solar system tend to be red, caused by the chemical reactions induced by radiation, but the cold classicals are even redder — ultrared, astronomers say.
A surprising number of the cold classicals, about a third, are binaries — two objects about the same size orbiting each other.
For Dr. Levison, this abundance of pairs could say something about the gentle nature of how the building blocks of planets came together. It’s possible that collisions out there are rare enough that such pairs have survived over the eons.
Dr. Malhotra is among the scientists who assert that the outer part of the Kuiper belt appears slightly warped, and that this could be a sign of gravity being exerted by something larger. Thus, not only might planets comparable in size to Mars and Earth have existed in this region, but they might also still be there today.
Dr. Gladman, for one, agrees. “I would bet my career there’s something of the scale of Mars- to Earth-size objects in the outer solar system,” he said.
Even farther out, beyond the Kuiper belt, are another group of objects that are even more puzzling. The first, Sedna, was discovered in 2003. At its closest approach to the inner solar system, it is seven billion miles from the sun, or more than twice as far out as Neptune, and too far away to ever have been gravitationally kicked by any of the giant planets. At its farthest orbit, Sedna is some 87 billion miles from the sun.
“It really changed our view of what might happen in the outer part of the solar system,” said Scott J. Kenyon, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics.
Other bodies in Sedna-like orbits have since been discovered. Some astronomers say that aspects of the orbits appear to be similar, pointing to the influence of a mini-Neptune that has been called Planet Nine, traveling on a slow elliptical orbit more than 20 billion miles from the sun. (Neptune is about 2.8 billion miles from the sun.)
“Given what we know, that’s the odds-on favorite,” Dr. Kenyon said.
Others think Planet Nine is a figment. “I’ve never thought it is real,” said Dr. Levison, who cannot think of how a planet that large could end up in such a distant orbit.
Dr. Gladman said analysis of data from his survey showed no signs of influence from a Planet Nine.
Astronomers are also curious about smaller Kuiper belt objects — too small to be seen by telescopes — and that is where the New Horizons flyby of Ultima Thule could help.
On Pluto and Charon, the largest moon of Pluto, New Horizons saw fewer small craters than had been expected. That suggests there might not be as many small Kuiper belt objects as expected out there to collide with the larger ones. (An alternate idea is that some process on Pluto and Charon is erasing the smaller craters.)
This could turn into a crucial clue in how planets formed. Instead of the dust grains gradually coalescing into larger and larger clumps, it could point to a faster process in which a cloud of them collapsed at once into a sizable chunk of rock and ice.
Dr. Gladman and his colleagues recently posted a paper that predicts the distribution of crater sizes that will be seen on Ultima Thule’s surface.
The data from the New Horizons flyby will trickle back slowly until late 2020, so they will not get quick proof if they are correct.
On the other hand, “it’s possible we’ll know we’re completely wrong with the first image,” Dr. Gladman said. “It’s a prediction. It’s fun to make predictions.”