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UH Astronomers Find 11 New Jupiter Moons

by Scott Sheppard and David Jewitt

Three images of a newly found satellite (circled) show
its motion relative to the background

Discovering new objects has always been a thrill for astronomers. Galileo began the modern era of astronomical discovery in 1609, when he used his homemade telescopes to discover the 4 largest moons of Jupiter. In late 2000, four astronomers from the University of Hawai‘i Institute for Astronomy discovered 11 new outer satellites of Jupiter using the UH 2.2-meter (88-inch) telescope atop Mauna Kea. This raises the number of known moons of Jupiter to 28, and more than doubles the number of known outer satellites.

Aided by Dr. Yanga Fernandez and Dr. Eugene Magnier, we found the satellites during a systematic search of space near Jupiter. The discoveries resulted from a combination of the fine seeing over Mauna Kea, a large telescope, and one of the world's largest CCD imaging cameras, which was developed at the IfA in 1996.

The discoveries are the largest number of satellites ever found at one time, and mean that there could be hundreds of satellites larger than about 1 mile in diameter around Jupiter.

The new satellites are far smaller than Jupiter's largest moons. They are about 2 to 4 miles in diameter, which puts them among the smallest known satellites in the solar system. By comparison, Jupiter's largest satellite, Ganymede, is larger than the planets Mercury and Pluto. The 11 newly discovered satellites join 9 previously known, all of which follow "irregular orbits," meaning their orbits are large, highly elliptical, and inclined to Jupiter's equator. They fall into three distinct groups: (1) Nine have orbital radii of about 13 million miles and orbit the planet in the direction opposite to its rotation (retrograde orbits). They join 5 previously known moons with similar dynamical characteristics. (2) One has an orbital radius of about 7.5 million miles and orbits the planet in the same direction as its rotation (prograde orbit). It joins 4 previously known moons with similar characteristics. (3) The other new satellite, actually discovered in 1975 but lost until now, has a prograde orbit with an orbital radius of about 4 million miles, and is dynamically dissimilar to any other known outer satellite of Jupiter.

Early in the history of the solar system, these irregular satellites were probably asteroids orbiting the Sun when the young planet Jupiter captured them. The best evidence for this hypothesis is the retrograde orbits of some these satellites. However, it was not easy for Jupiter to directly capture a passing asteroid. Some of the asteroid's kinetic energy had to be dissipated to slow it enough for Jupiter to capture it. Scientists theorize that the young Jupiter had a bloated atmosphere that extended much beyond its current cloud tops. Friction with this atmosphere could have slowed the asteroids enough to allow them to be captured. The fact that some of the satellites belong to "families" with similar orbital characteristics supports this hypothesis. Perhaps these families formed when the original bodies broke up on capture, because of the pressure exerted by impact with the extended atmosphere.

Congratulations to graduate student Scott Sheppard, who won the 2001 Helen Jones Farrar/ARCS Scholarship worth $5,000 for his work on Jupiter satellites.