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New Moons for Saturn

by David Jewitt

Astronomer David Jewitt (photo by Jing Li).

When I grew up in the 1960s and 1970s, the number of moons for each planet was small, fixed, and memorable. One could look them up in textbooks. Earth had 1 moon, Mars 2, Jupiter 12, and Saturn had 9, with the newest member of its family, Phoebe, having been discovered in 1898. Little did I guess that by 2005, the number of known Jovian and Saturnian moons would have jumped to 63 and 46, respectively, or that my collaborators and I would be responsible for a great many of these new discoveries.

New irregular satellite S/2004 S11 is located in the center of this image taken with the Subaru Telescope.

The latest batch of 12 moons of Saturn was discovered on December 12, 2004, by a team consisting of IfA alumnus Scott Sheppard (PhD, 2004), Parrent postdoctoral fellow Jan Kleyna, and me. We used the unparalleled survey power of the 8-meter (26-foot) Subaru Telescope on Mauna Kea to identify the new moons, and in the next three months, we obtained follow-up measurements to refine their orbits at the Gemini and Keck telescopes. Our ongoing work is a systematic exploration of the planetary satellite systems that capitalizes on the large telescopes on Mauna Kea. A measure of the power of these telescopes is demonstrated by the amazing fact that the new moons are too faint to be seen by NASA's Cassini spacecraft now orbiting Saturn, even though Cassini is much closer to the moons. This is because the telescopes on the summit of Mauna Kea are simply so much bigger than the one on the spacecraft.

All these new objects belong to the class of "irregular" moons, meaning that their orbits are large, highly inclined with respect to the planetary equator, and elliptical in shape. It is thought that such objects were initially in independent orbits around the Sun and were then captured by the planets. The strongest clue that they were captured is that most irregulars are retrograde, that is, they orbit the planet in the direction opposite to the spin of the planet. Only capture can explain this.

The biggest puzzle presented by the new moons, 11 of which are retrograde, is that there is no known way for a planet to capture a moon in the modern solar system. Capture must have occurred in the early solar system, when physical conditions were different from those that now prevail. And therein lies the scientific interest in the irregular moons: As relics from the ancient past, they promise to tell us about early times and about the way in which the planets themselves formed.

The mechanism of capture that has gained the most favor in the planetary science community involves the bloated atmospheres of the young giant planets slowing down Sun-orbiting bodies so that they can be retained by the gravity of these planets. This "gas drag" mechanism might explain the irregular moons of Jupiter and Saturn, including the 12 new moons, because both planets are known to have high gas content--they consist primarily of hydrogen and helium, just like the Sun and other stars. However, Uranus and Neptune also hold substantial populations of irregular satellites. The gas drag model has never been shown to work around these planets, since they possess 50 to 100 times less gas than Jupiter or Saturn.

What's next? We are not finished with our program of discovery, and we expect to find many new moons of the outer planets in the years to come. IfA postdoctoral fellow Tommy Grav is taking physical measurements of these moons to study trends in their colors and compositions. Already he has found that the moons belong to groups, distinguished both dynamically and by their colors. This suggests that many are splinters from smashed parent bodies. Yanga Fernández, IfA's Spitzer fellow, is conducting a separate program to measure the reflectivity of the moons as an independent measure of their composition and size. This vibrant research program, of central importance to understanding the formation of the solar system, is enabled by our access to the full suite of telescopes on Mauna Kea.

Related Links
12 New Moons for Saturn
Hawaii Irregular Satellites Page

Moon or Satellite?

In "astro-speak," they are satellites. There is only one Moon, and it is Earth's satellite.

However, in everyday usage, the word "satellite" has come to be associated with man-made orbiting objects, while "moon" refers to any natural satellite of a planet. Is this a big deal? Not at all. As long as we know what we're talking about, both names are fine. Increasingly, planetary astronomers use them interchangeably.