How Pluto Got Its Moons


With the announcement last week that astronomers have found a fifth moon orbiting around Pluto, the dwarf planet’s system got a little more crowded. Of course, Pluto hasn’t gained a new moon; we’re just getting better at finding the satellites already out there.

Finding moons of a planet that far away isn’t easy. It’s taken astronomers a long time to find Pluto’s moons, and there’s likely more to find.


ANALYSIS: Pluto Now Has Five (Yes, Five) Moons

In 1929, Illinois-born astronomer Clyde Tombaugh arrived at the Lowell Observatory in Flagstaff, Arizona. At the time our solar system stopped at Neptune but astronomers were on the hunt for “Planet X,” a theoretical planet outside the orbit of Neptune that would explain the observed oddities of Uranus’ orbit. Percival Lowell, the Observatory’s founder, had predicted Planet X’s rough position in the sky but never found it.

Tombaugh picked up the task. He imaged sections of the sky where Planet X was expected to be in orbit. Then, he looked at those images in a blink comparator, a machine that alternates quickly between two images of the night sky to show objects moving against the stars.

On Feb. 18, 1930, he found a tiny dot that moved between two of his images. He’d discovered Pluto. Instruments of the day registered Pluto as a very bright planet, leading astronomers to suppose it was an Earth-sized planet orbiting far beyond Neptune.

ANALYSIS: Pluto: Not a Planet; Still Very Interesting

It wasn’t until 1978 that our understanding of Pluto began to change. Better technology yielded clearer pictures that revealed new details about the dwarf planet’s size and composition. Most importantly, astronomers found that Pluto wasn’t as large as Earth at all. On June 22, James Christy discovered — also while working at the Lowell Observatory — that Pluto had a moon, Charon.

Charon is a massive satellite. It’s so big that it and Pluto actually orbit a barycenter, a point in space between the two bodies. To the less sensitive instruments of the early 20th century, pair looked like one bigger body.

It was another 27 years before our picture of Pluto’s system was refined further. In May 2005, the Hubble Space Telescope’s Pluto Companion Search Team, which included New Horizons’ Principle Investigator Alan Stern, found two more moons in images taken on the 15th and 18th of the month. Hubble images taken in June 2002 first suggested the existence of these moons, which were named Nix and Hydra after their orbits were confirmed.

HOWSTUFFWORKS: Why is Pluto not a planet?

Pluto’s fourth moon, still nicknamed “P4,” was discovered in July 2011 by the Hubble Space Telescope’s Pluto Companion Search Team. Using the telescope’s Wide Field Camera 3 to look for any rings the dwarf planet might have, observations made on June 28, July 3 and July 18 revealed the moon; P4 was later identified in archived images from Hubble from February 15, 2006 and June 25, 2010. The smallest moon discovered to that point, it was only visible because the team took longer shots of the Pluto system than they had before.

“P5″ was found the same way. Observations made using Hubble’s Wide Field Camera 3 in late June and early July of this year revealed the small body. Thought to be smaller than P4, this new moon is the most recent in what will likely be the ongoing discovery of Pluto’s system. When New Horizons reaches Pluto in 2015, it will give us the first in situ data from the dwarf planet, when more mysteries will no doubt be revealed.

Image: Artist’s impression of the New Horizon’s encounter with Pluto and Charon in 2015. Credit: Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute (JHUAPL/SwRI)

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