This image shows Pluto (lower left) and its large moon Charon (upper right). Notice how large Charon is compared to Pluto, and how close the moon is to the planet. This image was taken by the Hubble Space Telescope on February 21, 1994.
Click on image for full size
Image courtesy Dr. R. Albrecht (ESA/ESO Space Telescope European Coordinating Facility) and NASA.

The Moons of Pluto

Pluto has known moons. Charon, the largest by far, was discovered in 1978 by the American astronomer James Christy. The other two moons, which are much smaller, were spotted in 2005 by a team led by Alan Stern and Harold Weaver.

Charon is so large compared to Pluto, and orbits so close to the planet, that many astronomers call the pair a "binary planet" (double planet). Like Earth's moon, Charon always keeps one face towards Pluto, for the moon's orbital period and rotational period (the length of its day) are identical.

The smaller moons bear the temporary names S/2005 P1 and S/2005 P2 until their discoveries can be confirmed. S/2005 P1 is about 150 km (93 miles) across, and orbits Pluto once every 38 days at a distance of 64,500 km (40,100 miles). S/2005 P2 is about 100 km (62 miles) across, and orbits the planet every 25.5 days at a distance of 49,500 km (30,800 miles). Stern, Weaver, and their colleagues found the moons in May 2005 using the Hubble Space Telescope. They were intentionally searching for new moons of the distant planet in preparation for the New Horizons space mission to Pluto. Their survey led them to believe that if Pluto has any more moons, they must be very small; less than about 16 km (10 miles) in diameter.

S/2005 P1 and S/2005 P2 will be given permanent names after their discoveries have been confirmed by further observations. Confirmation seems almost certain, since other astronomers have already identified the moons in older Hubble photos from June 2002 that they took a second look at after hearing of the discoveries.

If Pluto has any rings, we haven't yet been able to see them.

Last modified January 23, 2006 by Randy Russell.

You might also be interested in:

Cool It! Game

Check out our online store - minerals, fossils, books, activities, jewelry, and household items!...more

Pluto

Pluto is a frigid ball of ice and rock that orbits far from the Sun on the frozen fringes of our Solar System. Considered a planet, though a rather odd one, from its discovery in 1930 until 2006, it was...more

Charon - largest moon of Pluto

Charon is by far the largest of Pluto's // Call the moon count function defined in the document head print_moon_count('pluto'); known moons. Charon was discovered by the American astronomer James Christy...more

Pluto & Charon, a binary planet

Of all the planets and moons in the solar system, Pluto and Charon are the two which resemble each other the most closely. They are almost the same size, and they are very close together. They are so...more

Hubble Space Telescope

The Hubble Space Telescope (HST) was one of the most important exploration tools of the past two decades, and will continue to serve as a great resource well into the new millennium. The HST is credited...more

Searching for more moons of Pluto

Astronomers are beginning to organize a search for more moons of Pluto. Pluto has only one known moon, Charon, but astronomers think there might be other moons orbiting Pluto that haven't yet been discovered....more

Makemake: a Dwarf Planet

Makemake is a dwarf planet in our Solar System. Makemake was discovered on March 31, 2005 by a team of astronomers led by Mike Brown of the California Institute of Technology. The International Astronomical...more

Questions to answer about Pluto

Pluto is so far away, and has never been explored. Questions to answer about Pluto include the following: What are the geologic features of the surface. (pictures of the surface) If there are bare spots,...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA