Posted on: Friday, May 17, 2002

Jupiter study adds 11 moons

By Jan TenBruggencate
Advertiser Science Writer

University of Hawai'i astronomers announced yesterday that they have newly identified 11 more moons of Jupiter, bringing that planet's total to 39 — more than any other planet in our solar system.

If this sounds like something you've heard before, that's because they found 11 last year as well.

The team led by planetary astronomer David Jewitt and graduate student Scott Sheppard expects to find still more when Hawai'i's weather clears.

All these newly identified moons, or satellites, are much smaller than the four big ones that the astronomer Galileo found in 1610. Those moons — Ganymede, Callisto, Io and Europa — range from 2,000 to 3,000 miles across.

The new ones are on the order of 2 miles across.

Amazing new technology in astronomy is allowing scientists to pick out objects in space that they have never before been able to see and track. In this case, Jewitt said, the largest digital imaging camera in the world was fitted on the Canada-France-Hawai'i Telescope atop Mauna Kea and aimed at Jupiter.

The camera takes digital images that measure 12,000 pixels by 8,000 pixels. That's nearly 100 million pixels, about 100 times the number taken by a standard consumer-model digital camera.

The images were processed and searched with a computer program written by Cambridge University scientist Jan Kleyna.

Jewitt said that once the possible satellites were identified, the astronomers used the UH's 2.2-meter telescope to confirm their findings and to weed out other objects such as asteroids that were in the region.

The major satellites of Jupiter orbit in a plane and in a direction the same as that of the planet's spin. But all the satellites found this year are orbiting on different planes and have retrograde orbits, meaning they spin around Jupiter in a direction opposite to that of the planet's orbit.

"We think they must have been captured by Jupiter. They were from someplace else and then Jupiter grabbed them," Jewitt said.

Since astronomers can identify no way for Jupiter to grab new moons now, it is assumed the satellites were captured long ago, perhaps in the first million years of the existence of our solar system, when conditions were different than they are today.

One theory — the gas drag hypothesis — suggests that when the solar system was younger, Jupiter was less dense but much bigger. The satellites may have entered the outer edge of the planet's atmosphere and been slowed enough by the drag of the planetary gas that they were trapped. They would have burned on entry into the atmosphere, but had enough size to survive once they slowed down. Smaller satellites would have been entirely burned up, like meteors in Earth's atmosphere.

Another theory is that as Jupiter was first expanding — attracting matter from the surrounding region — it grew so fast that its gravitational field snagged small objects that had been orbiting the sun alongside Jupiter. This is called the mass growth hypothesis.

Jewitt said there's still plenty to learn about the new jovian satellites, such as what they're made of, their shapes, how dense they are and so on. He said he hopes within the next year to use the giant Keck telescopes on Mauna Kea to get more details about them.