Astronomers find 18 planet-sized objects in distant star field

Posted: Thursday, October 05, 2000

WASHINGTON - Scientists are rethinking some basic theories about planets after astronomers found 18 planet-sized gas balls drifting free in a star field some 1,200 light years from Earth.

Experts wonder whether faint objects of this size, which have never before been detected, are failed stars or planets without a sun, said Maria Rosa Zapatero Osorio of California Institute of Technology in Pasadena, Calif.

"Everybody agrees with our mass determination of these objects, but what we should call them is not decided yet," said Osorio. "There is a nomenclature problem."

Osorio is lead author of a study appearing Friday in the journal Science.

The objects don't meet the classic definition of a planet because they are free-floating and nomadic instead of being locked into the orbit of a star, like the Earth and its sister planets in the solar system.

The smallest of the 18 objects has mass equal to 5 to 8 Jupiters, the largest is 13 to 15 Jupiter masses. This puts the largest on the edge of being a brown dwarf, an object between 15 and 75 times the mass of Jupiter.

"We have observed them in the very early stages of evolution," said Osorio.

Jupiter, the largest planet in the solar system, has a mass 318 times greater than the Earth. The sun, which is a star, has a mass about 333,000 times greater than the Earth.

The planet-like bodies are in a star cluster within the constellation Orion, about 1,200 light years away. A light year is the distance light travels in a year in a vacuum, about 6 trillion miles.

Osorio said the planet-like bodies she found could be "failed stars," objects that formed independently but never got big enough to become a star or even a brown dwarf.

Stars are thought to be created when clouds of matter are gravitationally attracted to each other and form a growing ball of gas. When enough mass is collected, the pressure inside is great enough to fuse hydrogen atoms and start the nuclear fires that give stars their light and heat.

A brown dwarf forms in the same way, but never collects quite enough mass to ignite the hydrogen fusion furnace of a star. Instead it has a dull glow from burning an isotope of hydrogen that takes less energy to fuse.

Osorio said the objects she found may be smaller versions of the brown dwarf and their faint light comes only from the heat of formation.

"They have formed very recently (in astronomical terms) and are still under their own gravitational collapse," she said. This collapse creates the heat and light that Osorio and her team detected using powerful telescopes in Spain, the Canary Islands and Hawaii.

Eventually, the heat from the gravitational collapse will cool and the objects will no longer be detectable from Earth, she said.

Osorio said it is assumed the objects are about the age of the Sigma Orionis star cluster in which they were found, about five million years.

Spectrographic readings, which analyze the light from celestial sources, indicated the objects have the chemistry and temperature of young giant planets, and not that of brown dwarfs, she said.

Joan R. Najita, an astronomer at the National Optical Astronomy Observatory in Tucson, Ariz., said the discoveries by Osorio and her team are significant and are apt to kick off an intensified search for such objects by other astronomers.

Najita said a brown dwarf study by her team at the NOAO had suggested there could be small, drifting objects in distant star fields. The Osorio paper confirms that, she said.

"We think the idea of there being free-floating planetary-mass objects out there is very likely to be true," Najita said. "There is a nomenclature problem. These objects are not planets in the classic sense because they do not orbit a star like the sun."

Najita, Osorio and others said the existence of the small, nomadic planets challenges the traditional ideas for how bodies form in space. One possibility, they said, is the objects originally formed around some star and were then kicked aimlessly wandering by some gravitational disruption, such as the tug of another star or of another planet.

About 50 planets outside the solar system have been detected by astronomers, but all of these objects are in orbit of distant stars. In fact, these planets have not been seen directly, but detected through the gravitational effect they have on the host stars.

The objects reported by Osorio and her team were viewed directly.

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