Planets beyond Neptune Facts for Kids

This page is about hypothetical planet(s). For known objects beyond Neptune, see trans-Neptunian object.

Pluto, with its five known moons. Pluto was a planet, from 1930 to 2006. Its orbit is farther out than that of Neptune.

When the planet Neptune was found in 1846, many people wondered if there were even more planets farther away from the Sun. In the early 1900s, an astronomer named Percival Lowell thought there must be another large object beyond Neptune. He believed this "Planet X" could explain why the orbits of Uranus and Neptune seemed a bit wobbly. He thought the gravity of a big, unseen ninth planet was pulling on them.

Pluto's Discovery and Reclassification
Is There Still a Planet X?
Objects Beyond Neptune's Orbit
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Pluto's Discovery and Reclassification

In 1930, Clyde Tombaugh discovered Pluto. This seemed to prove Lowell's idea was right! Pluto was officially called the ninth planet until 2006.

However, there was a problem. In 1978, scientists found that Pluto was much smaller than they thought. It was too small for its gravity to affect the giant planets like Uranus and Neptune. This led to a new search for a tenth planet. Later, in the early 1990s, the Voyager 2 spacecraft sent back even more accurate measurements. These showed Pluto was even tinier. Scientists realized that the earlier "wobbles" in Uranus's orbit were due to measurement errors, not an unseen planet. After this, the search for a "Planet X" was mostly stopped.

After 1992, many small, icy objects were found beyond Neptune. Some of these had orbits similar to or even wider than Pluto's. This made astronomers wonder if Pluto should still be called a planet. In 2006, the International Astronomical Union (IAU) decided to create a new group called dwarf planets. Pluto and its largest neighbors were reclassified as dwarf planets. This left only eight main planets in our Solar System. Pluto became the best example of a dwarf planet.

Is There Still a Planet X?

Today, most astronomers agree that the original "Planet X" that Lowell imagined does not exist. However, the idea of "Planet X" has become a general term. It now refers to any undiscovered planet that might be far out in our Solar System. This is true even if it's not related to Lowell's original idea. Scientists have suggested other possible planets beyond Neptune, based on different clues.

Objects Beyond Neptune's Orbit

Beyond Neptune's path, there's a huge area called the Kuiper belt. This belt is home to many icy objects, including ones called centaurs and Plutinos. Many of these were found between 1980 and 1992.

In 2004, the media got excited again when an object called Sedna was discovered. It was twice as far from the Sun as Pluto! Sedna is not a planet. In 2005, the discovery of Eris was announced. Eris was first called a planet, but it was also later reclassified. Today, it is the biggest of the known dwarf planets. Many other objects have been found, but none of them are solid rock planets. They are more like large asteroids or comets.

As of 2013, it's still possible that a planet exists far beyond Neptune's orbit. If such a planet exists, it would need to meet certain conditions:

If it's about the size of Earth, it would have to be around 100 AUs away. That's about 2.5 times farther than Pluto is from the Sun.
Another possibility is that its orbit is very stretched out (called elliptic) or tilted far away from the main plane of the Solar System (the ecliptic).

Scientists have also suggested other very distant, hypothetical objects. One idea was a hypothetical star called Nemesis, located very far away. Another idea was a hypothetical planet called Tyche in the Oort cloud, which could be as large as Jupiter. However, as of 2013, neither of these ideas could be proven.