The following questions were answered by astronomer Dr. Cathy Imhoff of the Space Telescope Science Institute.

I have heard that there is another planet called X. What do you know about it?
Planet X is the nickname of the possible tenth planet of our solar system. (By the way, Pluto was called Planet X before it was discovered!) You may know that both Neptune and Pluto were discovered because astronomers carefully mapped the orbits of the other planets and found small deviations in their expected orbits. They then tried to calculate if the pull of another planet could cause those differences. It was a great victory for those astronomers when Neptune was discovered — it was the first planet discovered in modern times, the first planet that was not known to people for thousands of years. Pluto was especially hard to find because it is small, so its gravity causes only small changes in the other planets'orbits, and also because it is faint and hard to find in a sky full of stars. Ever since then people have wondered if there is yet another planet further out than Pluto. Astronomers have searched the tiny deviations in the planets'orbits for clues. At one point, someone claimed that his calculations showed that Planet X had to be a big planet, way far out in the solar system, in orbit at a big angle to the rest of the solar system. But this was shown to be impossible — if it had existed it would have pulled apart the whole solar system!

When a new NASA satellite called IRAS was launched, we hoped that it could be used to find Planet X. IRAS looks at the sky in the infrared, which would make finding another planet much easier, and it could also find very faint things. People looked very hard (everyone wanted to be the one to find the new planet!), but there is no sign of Planet X. So right now, we believe that there is no Planet X.

What is this thing I have heard of called Planet X?
For a while we thought there might be a tenth planet. Since no one had found it yet, we called it Planet X (X means unknown). Some very careful searches have been made for another planet outside the orbit of Pluto. In recent years, there was a detailed map of the sky made in infrared wavelengths. A planet very far from the sun would not be very bright in ordinary, visible light, but should be pretty bright in the infrared. No planet was found. So we now believe that there is no tenth planet. If a new planet were found, it would be given a new name. For a long time, there were only six known planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, and Saturn. These are the ones that the ancient Greeks knew. Even Newton and Galileo didn't know about any other planets.

Uranus was discovered accidentally in 1781 by William Herschel, a German-English astronomer. This caused a great deal of excitement. People were amazed to realize that there were other planets out there. During this time people were learning to calculate very precise orbits for the planets based on observations of their motions through the solar system. Within a few years of the discovery of Uranus, these astronomers realized that its orbit wasn't quite right. In 1843, English astronomer John Couch Adams guessed that this was due to another, more distant planet and calculated where that planet might be to cause the irregularities in the orbit of Uranus. Meanwhile Leverrier, a French astronomer, did a similar prediction, not knowing about Adams's work. Based on these predictions, Galle, an astronomer at Berlin Observatory, looked for the new planet — and found it the first night that he looked! This was Neptune. Well, after Neptune was discovered, people watched its orbit to see if there was evidence of yet another planet. Sure enough, there was. But this was getting hard to compute. There were several predictions, including one by Percival Lowell, the American astronomer who is famous for his preoccupation with the canals of Mars. Lowell really wanted to find this new planet, which he called Planet X. He searched for it for 10 years, until his death in 1916. After his death, his brother helped to continue the search. He set up a special telescope and equipment and hired an observer just to look for the planet. Finally, in 1930 (24 years later!) the observer, Clyde Tombaugh, found Pluto. The name Pluto was chosen in part to honor Lowell, whose initials are PL.

So the original Planet X was Pluto. (There is a book by this name — Planet X — that describes the search for Pluto.) Is there a planet beyond Pluto? Many people have tried to compute its orbit and look for it, unsuccessfully. Then the IRAS satellite was launched and carefully surveyed the sky in the infrared. Any other planet in our solar system should be visible in the infrared to this satellite. None was found. So most of us believe that there is no new Planet X out there.

Do you think that there could be a "Planet X" outside of the orbit of Pluto, or perhaps a black hole there, which causes odd weather and gravitational effects here on Earth?
There have been various searches for a "Planet X" in our solar system. We now think that there is none. Some predictions were made on the basis of tiny deviations in the planets'orbits, but they have now been found to be just small errors. There was a big search for other planets using the IRAS satellite. IRAS looks at the sky in the infrared, and any decent-sized planet out there should be visible in the infrared. But nothing was found. I suppose that a small black hole could be out there, but we have no indication of one.