Planets beyond Neptune

Planets beyond Neptune are mysterious worlds that scientists think might exist far out in our Solar System. After Neptune was discovered in 1846, many people wondered if there could be another planet even farther away. A man named Percival Lowell spent years looking for a planet he called Planet X, which he believed could explain small changes in the orbits of Uranus and Neptune.

In 1930, astronomer Clyde Tombaugh discovered Pluto, which was excited as the ninth planet. But later studies showed Pluto was too small to affect the orbits of the bigger planets. This led some to wonder if there might be a tenth planet instead. As we learned more about space, we found many small icy objects beyond Neptune, which caused scientists to rethink what should count as a planet. In 2006, Pluto was reclassified as a dwarf planet, meaning Neptune became the farthest known planet from the Sun.

Even though the old idea of Planet X didn’t pan out, some astronomers still believe there might be a big, unseen planet out there. They call it Planet Nine. This possible planet could be between 2 and 15 times the size of Earth and might be traveling on a strange, tilted path around the Sun, far beyond where we’ve seen anything else. While we haven’t found it yet, this idea helps explain why some distant objects in space move the way they do.

Early speculation

See also: Discovery of Neptune

In the 1840s, mathematicians and astronomers wondered if another planet might exist beyond Neptune. They noticed small strange movements in the orbits of the planets Uranus and Neptune and thought the gravity of an unknown planet might be the reason.

Many astronomers, including Urbain Le Verrier and Alexis Bouvard, suggested that one or more planets could be hiding far from the Sun. Some even calculated where these planets might be, though no one found them at the time. Years later, in 1930, the planet Pluto was discovered, which made some think they had finally found the missing planet astronomers had been looking for.

Planet X

Main article: Pluto

In 1894, Percival Lowell started looking for a planet beyond Neptune. He thought this planet, which he called Planet X, might explain why Uranus moved in a way that didn't match what scientists expected. Lowell spent years taking pictures of the sky, hoping to find this hidden planet.

In 1930, Clyde Tombaugh discovered Pluto. At first, many people thought Pluto might be the Planet X that Lowell had predicted. However, further studies showed that Pluto was too small to affect the orbits of other planets. Scientists realized that the idea of a large Planet X causing changes in Uranus's orbit was not correct. Today, most experts believe that Planet X, as Lowell imagined it, does not exist.

Discovery of further trans-Neptunian objects

See also: History of the Kuiper belt

After Pluto was discovered, no more objects beyond Neptune were found until 1992, when 15760 Albion was spotted. Since then, thousands of these objects, called trans-Neptunian objects, have been discovered. Most are part of the Kuiper belt, a group of icy bodies that orbit the Sun just beyond Neptune.

In 2005, astronomers found an object named 2003 UB₃₁₃, later called Eris. At first, some thought it might be the tenth planet because it was slightly larger than Pluto. However, in 2006, a new definition of what counts as a planet was created, and both Pluto and Eris were classified as dwarf planets instead. Some scientists still believe these objects should be called planets.

Subsequently proposed trans-Neptunian planets

Although most astronomers think Lowell's Planet X does not exist, some believe a large unseen planet might create effects in the outer Solar System. These objects are often called "Planet X," though they may be different from Lowell's idea.

The discovery of Sedna raised questions about its orbit. Sedna's path is so unusual that scientists wonder what could have caused it. Some ideas include a passing star, capture from another planetary system, or the pull of an unseen planet beyond Neptune.

In recent years, astronomers have studied the orbits of distant objects beyond Neptune. They noticed that some objects have orbits that seem to be grouped together in a way that suggests the pull of a hidden planet. This idea led to the hypothesis of a planet called Planet Nine, which could be about ten times the size of Earth and located far beyond Neptune.

Constraints on additional planets

Observations from space telescopes like the WISE telescope have shown that there are no large planets, such as ones the size of Saturn or Jupiter, hiding far from the Sun. These telescopes help scientists look for heat signals that big planets would give off.

Studies of how planets like Saturn, Earth, and Mars move in their orbits suggest that any unknown planet would have to be very far away from the Sun. For example, a planet even a little bigger than Earth would need to be hundreds of times farther from the Sun than Earth is.

Scientists also study the paths of objects that orbit far beyond Neptune. Some of these objects seem to be grouped in ways that suggest the pull of a hidden planet, possibly hundreds of times farther from the Sun than Earth is.