New Images: They Might Be Planets

Astronomers have obtained images of a group of objects beyond our solar system that, based on their mass alone, could qualify as planets. The faint objects lie in a young star cluster, sigma Orionis, 1,200 light-years from Earth in the constellation Orion.

Three of the objects (reddish, marked by triangles) in the sigma Orionis star cluster that may be free-floating planets. Zapatero Osorio, et al./Science

Several recent studies suggest that these faint, cool bodies may be rampant throughout the galaxy. Previous evidence for extrasolar planets has been indirect, from the wobble they induce on their parent star (SN: 8/5/00, p. 84: Evidence grows for nearby planetary system).

If the newly found objects are indeed planets, they’re worlds apart from those in our solar system, report Maria Rosa Zapatero Osorio of the Instituto de Astrofisica de Canarias in Tenerife, Spain, and her collaborators in the Oct. 6 Science. The objects are much heavier and younger than the sun’s nine planets. Moreover, they roam freely through the star cluster rather than orbiting a parent star.

According to the standard theory, brown dwarfs arise as stars do–from the collapse of vast clumps of cold gas and dust. Planets, in contrast, are thought to condense later, from the disks of material that swaddle newborn stars.

“I think the detections themselves are quite nice,” says Alan P. Boss of the Carnegie Institution of Washington (D.C.). “The main problem is that the authors have chosen to use the p word,” he adds. “I feel strongly that this use [of planet] is misleading and will only confuse people.”

Some astronomers, however, rely on mass—rather than location—to distinguish brown dwarfs from planets. Although brown dwarfs aren’t as heavy as stars and can’t sustain nuclear fusion at their cores, they’re hefty enough at least 13 times Jupiter’s mass, to have burned nuclear fuel briefly. Planets are too lightweight to shine this way.

“I would regard objects with less than 13 Jupiter masses as planets,” says Gibor S. Basri of the University of California, Berkeley. Zapatero Osorio’s team has “made as good a case as I have seen for ‘free-floating planets’ having been found.” Some of the bodies may have once orbited stars but were expelled by their parent.

Zapatero Osorio says her team chose sigma Orionis for its planet hunt because the star cluster is nearby, contains little dust, and is no more than 5 million years old. Planets are easier to detect when they’re young, and knowing the age and distance of the cluster made it easier to calculate the true brightness and mass of objects that lie within it.

In near-infrared and visible-light images, the team spied 18 reddish objects that seemed cool and lightweight enough to be planets. The researchers then obtained visible-light spectra for two of the objects and infrared spectra for a third. The spectra indicate that the bodies aren’t distant quasars, far-off galaxies, or reddened stars masquerading as planetlike objects.

Models suggest that if the cluster is 5 million years old, then the 18 objects are between 8 and 15 times the mass of Jupiter. Most of the bodies would weigh less than 13 Jupiters. If the cluster is as young as 1 million years, then all 18 of the objects would weigh less than this cutoff, the team finds.

Boss says he prefers to call these low-mass objects “sub-brown dwarf stars.” His calculations show that a cloud of gas and dust could spawn brown dwarfs only three times the mass of Jupiter.

Over the next 10 million years, an astronomically brief interval, the sigma Orionis cluster could easily expel these free-floating objects, says Zapatero Osorio. If the objects are as abundant in other clusters, the number of free-floaters dispersed through the Milky Way could rival the number of stars. “We may be surrounded by these objects,” she says, but because they would cool as they grow older, they would be difficult to see.

Other studies support that intriguing notion. When English astronomers Philip W. Lucas of the University of Hertfordshire in Hatfield and Patrick F. Roche of the University of Oxford examined another star cluster, Orion’s Trapezium, they found hints of 13 free-floaters with planetlike masses. They reported their observations in the June 1 Monthly Notices of the Royal Astronomical Society.

Recently, Joan R. Najita of the National Optical Astronomy Observatories in Tucson and her colleagues took a census of brown dwarfs in the star cluster IC 348 and found that the lower-weight dwarfs greatly outnumber the heavier ones, they report in the Oct. 1 Astrophysical Journal. She says that although the census didn’t include objects weighing less than 13 times Jupiter’s mass, her team’s study provides “a strong hint that planetary-mass objects would exist in reasonable number as free-floaters.”

“The definition of a planet has changed with time as our knowledge in science has improved,” says Zapatero Osorio. “We may have to revise this definition just as our ancestors did.”

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