Planet Nine’s existence is not the first time astronomers have predicted a planet they couldn’t see. Clyde Tombaugh was looking for Planet X when he accidentally found Pluto. After Voyager 2 flew by Neptune in 1989, new calculations determined there was no Planet X. But three years ago, a disturbance was detected at the far reaches of the Solar System.
The 170-Year Search for a Planet Beyond Neptune
Published: June 10, 2019
Onscreen: There may be a 9th planet on the edge of our solar system. It’s not Pluto and it’s not “Planet X.”
Konstantin Batygin: The edge of the solar system and the Kuiper Belt continues to be a new frontier in astronomy. The search for Planet 9 however has taking that New Frontier and kind of kicked it into a higher gear.
Onscreen: A Planet Beyond Neptune. Funded in part by Draper.
It’s not the first time astronomers have predicted the existence of a planet they could not see. Take the case of Planet X.
Mike Brown: Many people these days think Planet X just generically refers to anything out there. Planet X is a very specific hypothesis from Percival Lowell.
He thought the orbits of Uranus and Neptune weren't quite going around exactly how they were supposed to be going. So, Percival Lowell did calculations and predicted where Planet X was and how big it should be. Clyde Tombaugh was looking for Planet X when he accidentally found Pluto.
If you look at the New York Times headline from the day of the announcement of the discovery of Pluto it says that it is possibly as large as Jupiter certainly larger than the Earth. Everybody thought it was big because everybody thought it was the thing that Percival Lowell had predicted. And you know this is wrong by a factor of 250,000 or so.
Onscreen: Pluto turned out to be less than 15,000 miles in diameter — smaller than Earth’s moon. In 2006, Pluto was reclassified as a dwarf planet, bringing the solar system’s planetary count back down to eight.
As for Planet X, after Voyager 2 flew by Neptune in 1989, we knew Neptune’s precise mass.
New calculations determined there were no irregularities in the orbits of Neptune and Uranus, so there was no Planet X.
Brown: I remember being asked are there any other planets and I was like no this is it. We now know that there are eight planets in the solar system and and nothing else. And I was pretty convinced that this is true until about three years ago.
Onscreen: Three years ago, a disturbance was detected at the far reaches of the solar system. The leading suspect? Planet 9.
Next: The Evidence for Planet 9
Produced and Directed by: Caitlin Saks and Terri Randall
Camera: Dave Arabia
Digital Producer: Ana Aceves
Additional Footage: Adler Planetarium, IAU, Joe Tucciarone, Ed Bell Media, Alden Tombaugh, NASA/Public Domain, Konstantin Batygin
© WGBH Educational Foundation 2019