Scientists Puzzled by Distant Planet-Like Bodies

Oct. 5, 2000 -- Scientists are rethinking some basic theoriesabout planets after astronomers found 18 planet-sized gas ballsdrifting free in a star field some 1,200 light years from Earth.

Experts wonder whether faint objects of this size, which havenever before been detected, are failed stars or planets without asun, said Maria Rosa Zapatero Osorio of California Institute ofTechnology 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 journalScience.

Free-Wheeling Planets?

The objects don’t meet the classic definition of a planetbecause they are free-floating and nomadic instead of being lockedinto the orbit of a star, like the Earth and its sister planets inthe solar system.

The smallest of the 18 objects has mass equal to 5 to 8Jupiters, the largest is 13 to 15 Jupiter masses. This puts thelargest on the edge of being a brown dwarf, an object between 15and 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 318times greater than the Earth. The sun, which is a star, has a massabout 333,000 times greater than the Earth.

The planet-like bodies are in a star cluster within theconstellation Orion, about 1,200 light years away. A light year isthe distance light travels in a year in a vacuum, about 6 trillionmiles.

Stars That Flopped?

Osorio said the planet-like bodies she found could be “failedstars,” objects that formed independently but never got big enoughto become a star or even a brown dwarf.

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

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

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

“They have formed very recently [in astronomical terms] and arestill under their own gravitational collapse,” she said. Thiscollapse creates the heat and light that Osorio and her teamdetected using powerful telescopes in Spain, the Canary Islands andHawaii.

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

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

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

More to Find

Joan R. Najita, an astronomer at the National Optical AstronomyObservatory in Tucson, Ariz., said the discoveries by Osorio andher team are significant and are apt to kick off an intensifiedsearch for such objects by other astronomers.

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

“We think the idea of there being free-floating planetary-massobjects out there is very likely to be true,” Najita said. “Thereis a nomenclature problem. These objects are not planets in theclassic 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 bodiesform in space. One possibility, they said, is the objectsoriginally formed around some star and were then kicked aimlesslywandering by some gravitational disruption, such as the tug ofanother star or of another planet.

About 50 planets outside the solar system have been detected byastronomers, but all of these objects are in orbit of distantstars. In fact, these planets have not been seen directly, butdetected through the gravitational effect they have on the hoststars.