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Pluto has a new moon

Artist's impression of Pluto and Charon

Artist's impression of Pluto and its large moon Charon (diameter 1,043km). Hubble telescope observations have uncovered a previously unseen Plutonian moon, probably only 13 to 34km across.

ASTRONOMERS USING THE Hubble Space Telescope have discovered a fourth moon orbiting the icy dwarf planet Pluto. The tiny, new satellite, temporarily designated P4, was uncovered in a Hubble survey searching for rings around the dwarf planet.

The new moon is the smallest discovered circling Pluto. It has an estimated diameter of 13 to 34 km. By comparison, Charon, Pluto’s largest moon, is 1,043 km across, and the other moons, Nix and Hydra, are in the range of 32 to 113 km in diameter.

“I find it remarkable that Hubble’s cameras enabled us to see such a tiny object so clearly from a distance of more than 5 billion km,” said Mark Showalter of the SETI Institute, who led this observing programme with Hubble.

Hubble image showing motion of Pluto's four moons

This composite of two Hubble images—taken on June 28, 2011 and July 3, 2011—shows Pluto's four satellites in motion. P4 is the as-yet-unnamed new moon.

Mission to Pluto

The finding is a result of ongoing work to support NASA’s New Horizons mission, scheduled to fly through the Pluto system in 2015. The mission is designed to provide new insights about worlds at the edge of our Solar System.

Hubble’s mapping of Pluto’s surface and discovery of its satellites have been invaluable to planning for New Horizons’ close encounter.

“This is a fantastic discovery,” said New Horizons’ principal investigator Alan Stern of the Southwest Research Institute. “Now that we know there’s another moon in the Pluto system, we can plan close-up observations of it during our flyby.”

Moons formed in a smash-up

The new moon is located between the orbits of Nix and Hydra, which Hubble discovered in 2005. Charon was discovered in 1978 at the US Naval Observatory and first resolved using Hubble in 1990 as a separate body from Pluto.

The dwarf planet’s entire moon system is believed to have formed by a collision between Pluto and another planet-sized body early in the history of the Solar System. The smash-up flung material that coalesced into the family of satellites observed around Pluto.

Artist's concept of Pluto's satellite system

An artist's concept of Pluto's satellite system with newly discovered moon P4 highlighted.

Lunar rocks returned to Earth from the Apollo missions led to the theory that our moon was the result of a similar collision between Earth and a Mars-sized body 4.4 billion years ago.

Scientists believe material blasted off Pluto’s moons by micrometeoroid impacts may form rings around the dwarf planet, but the Hubble photographs have not detected any so far.

No sign of rings yet

“This surprising observation is a powerful reminder of Hubble’s ability as a general purpose astronomical observatory to make astounding, unintended discoveries,” said Jon Morse, astrophysics division director at NASA Headquarters in Washington.

P4 was first seen in a photo taken with Hubble’s Wide Field Camera 3 on June 28. It was confirmed in subsequent Hubble pictures taken on July 3 and July 18. The moon was not seen in earlier Hubble images because the exposure times were shorter.

There is a chance it appeared as a very faint smudge in 2006 images, but was overlooked because it was obscured.

Adapted from information issued by NASA. Images and graphics courtesy NASA / A. Feild (STScI) / ESA.

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Pluto out in the cold again

2003 UB313 and its tiny moon

Artist's impression of the distant icy body 2003 UB313 and its tiny moon.

New research from The Australian National University has further reduced the status of Pluto by suggesting there are many more dwarf planets in the Solar System than previously thought.
ANU astronomers have just published results that would reclassify what it is to be a dwarf planet, increasing the number of Pluto’s fellow travellers by a factor of ten.
The International Astronomical Union classifies objects in the Solar System into three groups: planets, dwarf planets, and small Solar System bodies. In 2006, Pluto was demoted from planet to dwarf planet, leaving eight planets, five dwarf planets and thousands of small Solar System bodies orbiting the Sun.
Dr Charley Lineweaver and Dr Marc Norman from the ANU Planetary Science Institute looked at how spherical the icy moons in our Solar System are, and recalculated the size of objects at the boundary between dwarf planets and small Solar System bodies.
Previous estimates have classified icy objects with radii larger than 400 km as dwarf planets. The new research suggests that this radius should be closer to 200 km, which would increase the number of objects classified as dwarf planets to roughly 50.

2003 UB313 and its tiny moon

Artist's impression of the distant icy body 2003 UB313 and its tiny moon.

What is a dwarf planet?

The boundary between dwarf planets and small Solar System bodies is based on whether the object is round or not.
“Small Solar System objects are irregularly shaped, like potatoes,” Dr Lineweaver said. “If an object is large enough that its self-gravity has made it round, then it should be classified as a dwarf planet.”
“We calculated how big rocky objects (like asteroids) have to be, and how big icy objects (like the moons of the outer planets and objects further out than Neptune) have to be, for their self-gravity to make them round,” he said.
“For icy objects we found a ‘potato radius’ of roughly 200km – about half as large as the roughly 400km radius now used to classify dwarf planets,” he added.
“The boundary between dwarf planets and small Solar System bodies is somewhat arbitrary, but is based on the concept of hydrostatic equilibrium, or how round an object is. Whether the self-gravity of an object is strong enough to make the object round depends on the strength of its material.”
“That is why strong rocky objects need to have a radius of roughly 300km before they turn from lumpy, potato-shaped bodies into spheres, while weaker icy objects can be spheres with a radius of only roughly 200km.”
Dr Lineweaver and Dr Norman’s paper The Potato Radius: a Lower Minimum Size for Dwarf Planets will be published in the Proceedings of the 9th Australian Space Science Conference.
Adapted from information issued by ANU. Images: Lineweaver, Norman and Chopra/ A.Schaller for STScI.