Scientists discover asteroid with moons / Tiny planetary system orbiting between Mars and Jupiter

Asteroids triple - This is the image to go with Asteroids11. Credit: European Southern Observatory Asteroids triple - This is the image to go with Asteroids11. Credit: European Southern Observatory Photo: European Southern Observatory Photo: European Southern Observatory Image 1 of / 1 Caption Close Scientists discover asteroid with moons / Tiny planetary system orbiting between Mars and Jupiter 1 / 1 Back to Gallery

A UC Berkeley astronomer and his French colleagues have discovered what must be the smallest planetary system of them all, an asteroid with two tiny moons existing amid all the countless billions of asteroids circling the sun between the orbits of Mars and Jupiter -- and the delighted scientists are guessing there may be as many as 10 more moons as yet undiscovered around the same asteroid.

The group, scanning the sky with a huge telescope in Chile and the latest in high-tech optics, have calculated and clearly observed two of the asteroid's moons whirling around it in widely separated orbits.

A report on their triple asteroid discovery is being published today in the journal Nature.

The scientists' story is a neat one, harking back to the origins of our own solar system and the formation of planets; it's also an example of how star-gazing these days can be accomplished by computer links to distant telescopes without ever leaving home base.

Berkeley research astronomer Frank Marchis, leading a group at the Paris Observatory, spent six months last year scanning a group of asteroids large enough to be considered minor planets. Marchis ultimately focused in on one object known as 87 Sylvia with a telescope run by the European Southern Observatory high on a mountain top in Chile's Atacama dessert.

He didn't have to go to Chile to keep watch: Data from the telescope's observations flowed into the computers of his Paris colleagues, and e-mail relayed the data to his own computer in Berkeley.

Sylvia, with a diameter of about 175 miles, is one of the largest objects among the immense swarms in the asteroid belt. Four years ago, planet hunter Michael E. Brown of Caltech and his colleagues used the Keck telescope in Hawaii to discover that Sylvia has one satellite barely 11 miles in diameter that circles its parent from about 860 miles away.

Brown's discovery made Sylvia and its moon the 60th known "binary" asteroid in the sky, but Marchis and his colleagues examined Sylvia more closely, exploiting the high-resolution capabilities of a technology called adaptive optics. To their astonishment, they found another tiny moonlet only 4. 4 miles in diameter swinging around Sylvia at a distance of 450 miles.

"We first saw this triple system last March, and we couldn't believe it," Marchis said in a telephone interview from Rio de Janeiro, where he is attending an international conference on asteroids, comets and meteors. "We were really skeptical, but all of us did our own analysis of the data independently, and it took us a month. Now, we think there might be as many as 10 more tiny moonlets in orbit there, but it will take a much more powerful telescope to find them.

"This is a big step in the study of asteroids -- it's really enjoyable and more than exciting."

The 87 in the name 87 Sylvia refers to Sylvia's being the 87th minor planet ever discovered. It was named for Rhea Sylvia, the vestal virgin in Roman mythology who was raped by Mars and bore the twins who founded Rome. So Marchis and his colleagues named the two new-found asteroid satellites Romulus and Remus -- names that the all-powerful International Astronomical Union has officially accepted. After calculating the mass and density of Sylvia and its two moonlets, Marchis has concluded that the mother asteroid is probably nothing more than a loose pile of rocky rubble -- more than half of it empty space with a little frozen water inside and all of the rocky bits held together by its own weak gravity -- weak, perhaps, but strong enough to keep the two small satellites in well-behaved orbits running in the same prograde direction as Earth's moon circles us high overhead.

"It's really a neat result," said David Morrison, a longtime expert on asteroids who is the senior scientist of the NASA Astrobiology Institute at the Ames Research Center in Mountain View. "And it's wonderful what astronomers can accomplish using powerful ground-based telescopes equipped with adaptive optics."

The origin of Sylvia and its satellites is a story in itself: It's one of many so-called "rubble pile asteroids," and according to Marchis, it must have been formed hundreds of millions of years ago. Like other asteroids with single satellites, it apparently formed when two larger asteroids smashed into each other with such force that they burst into showers of rocky fragments, each fragment a mini-planet all its own.

Some of those small disrupted fragments began attracting each other under their own gravity until they clumped together by accretion and became massive enough to attract a moon -- or in this first-ever discovery, to attract two moons and become a full-scale mini-planetary system.

The story of Sylvia and its accretion, Marchis and his colleagues said in their Nature report, can yield "unique information" about the formation of our own Solar System -- a process that began in a vast swirling ring of dust and gas surrounding the sun more than 4.5 billion years ago and that continued until all the planets, their satellites and the asteroids had formed by similar accretion.