Rogue planet find makes astronomers ponder theory

WASHINGTON (Reuters) -- Eighteen rogue planets that seem to have broken all the rules about being born from a central, controlling sun may force a rethink about how planets form, astronomers said Thursday.

They said they found a planet-rich region -- near a star in the constellation Orion -- where stars, brown dwarfs and large, gassy planet-sized objects all exist without the discipline of a solar system.

Instead of orbiting neatly around a central star, they drift along in a loose collaboration, the team of Spanish, American and German researchers report.

"They look like giant gas balls," Maria Rosa Zapatero-Osorio of the California Institute of Technology in Pasadena (Caltech) and of the Instituto de Astrofisica de Canarias in Tenerife, Spain, who worked on the study, said in a telephone interview.

The stars nearby are relatively young -- just one million to five million years old, as opposed to the sun which is more than five billion years old and the Earth, which is 4.5 billion years old.

The planets are also young, Zapatero-Osorio said. "They are still contracting, collapsing because of their own gravity. With time, they will look like Jupiter and Saturn."

With one exception. Jupiter and Saturn obediently orbit the sun, and are believed to have formed from the same swirling disk of gas and dust that formed the entire solar system.

The gas giants in Orion do not seem to have formed that way. "The formation of young, free-floating, planetary-mass objects like these is difficult to explain by our current models of how planets form," Zapatero-Osorio said.

"We think they originated in a similar way to stars and brown dwarfs -- a big cloud broke down into small pieces. Some were large enough to produce stars, while other fragments were very small and they yielded the objects we discovered in the cluster."

But, she added, "Perhaps these objects were ejected from their orbits from their original birthplaces around the stars."

The cluster looked at by the international team lies in the constellation Orion -- one of the best-known constellations to amateur star-watchers -- near a star to the southeast of Orion's "belt."

"Only on very clear nights we can see this star. There are many other stars in cluster but because they are at 1,200 light-years they are not so visible to our eye," Zapatero-Osorio said.

Light travels 186,000 miles (300,000 km) per second, and a light-year is the distance it travels in a year.

"It's an area that has a high concentration of stars, and they are homogenously distributed within the cluster -- one star, one brown dwarf, one planetary mass body, one star, one brown dwarf, one planetary mass body and so on," Zapatero-Osorio said.

They are not linked to one another in an orbit, but do move together as a cluster, she said.

She said there are hundreds more planet-like objects in the cluster but her team concentrated on a single, small area for their report, to be published in Friday's issue of the journal Science.

Zapatero-Osorio said she is applying to use Hawaii's giant Keck telescope to look for even smaller objects in the cluster.

Usually, researchers predict that a planet exists by looking for its gravitational effects on a nearby sun. Most planets outside our solar system are far too dim to see using visible light. But Zapatero-Osorios' team actually saw the objects they describe.

They used spectrometers, which measure both visible and non-visible energy, and found they seem to be made of matter that resembles the stuff found in known planets.

Many stars in our own galaxy, the Milky Way, may have formed in a similar manner to the Orion stars, she said. So there could be similar, hard-to-see planets floating around free near the solar system.

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California Institute of Technology

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