The Sciences

Works in Progress

When it's a planet that's not a planet

By Karen WrightJan 1, 2002 12:00 AM


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Long ago, Greek astronomers watched a few errant lights move across a backdrop of stars and called them planets, after their word for wanderer. It seemed like a good idea at the time. But in the past few years astronomers have found planetlike objects in the sky that more rightly deserve to be named after the Greek word for headache. They number in the hundreds in familiar constellations such as Orion, Perseus, and Cygnus, but they're too small to be stars and too far away from any star to be ordinary planets. Their mere existence challenges standard theories of star and planet formation.

Astrophysicist Alan Boss of the Carnegie Institution of Washington, in D.C., is enjoying the problem. "It's exciting when people think they know what's going on and nature proves otherwise," he says. Not long ago Boss knew what planets were: clumps of rock, dust, and/or gas that orbit stars under the spell of gravity and don't generate light. He knew what stars were too: hot, heavy, hydrogen-burning balls of gas that do emit light. Stars are hot and dense enough to fuse the nuclei of hydrogen atoms; huge gaseous balls like Jupiter remain planets because they are not dense enough to sustain nuclear fusion. Theorists calculate that Jupiter would have to be at least 75 times bigger to burn hydrogen. So all stars must be heavier than 75 Jupiter masses, and all planets must be a lot lighter than that.

The telescope's cylindrical enclosure (shown under construction) sheds heat better than the traditional dome structure. The Subaru Telescope's mirror—27 feet wide—is the world's largest.Photograph courtesy of Subaru Telescope/National Astronomical Observatory of Japan

Theories of star formation also predicted the existence of brown dwarfs, which occupy the middle ground between planets and stars. Brown dwarfs aren't heavy or hot enough to burn hydrogen, so they don't shine like stars. But they're formed the same way stars are—from the collapse of a cloud of gas. And they can burn deuterium, a heavier form of hydrogen. They're also a lot heavier than Jupiter. Theorists believe that the fusion of deuterium doesn't occur in anything lighter than 13 Jupiter masses. So, according to this scheme, there is a comforting order to the cosmos. Stars are heavier than 75 Jupiters, brown dwarfs are heavier than 13 Jupiters, and planets are the small dark things that orbit stars.

Because brown dwarfs burn deuterium, they generate enough heat to be detected by infrared-sensing instruments. Astronomers began looking for their dim beacons decades ago in hopes of confirming their ideas about the birth of stars. At the same time, another faction of scientists searched the skies for planets orbiting stars other than the sun—so-called extrasolar planets that could advance our understanding of planet formation. With the aid of the Hubble Space Telescope and sophisticated optics in earthbound telescopes, astronomers finally nabbed both quarries. On the same day in 1995, separate groups in Italy and the United States announced the discovery of both a planet outside our solar system and a brown dwarf.

But astronomers found something else along the way to those discoveries—dozens of faint, free-floating objects lighter than the lightest brown dwarf and heavier than the heaviest planets. Scanning the constellation Perseus a few years ago, Joan Najita of the National Optical Astronomical Observatory in Tucson, Arizona, happened on an unexpected bounty of low-mass brown dwarfs in a young star cluster, and her observations suggested that even lighter objects were roaming alone out there. Some turned up when Maria Rosa Zapatero-Osorio of the California Institute of Technology conducted a survey of a star-forming cluster in Orion. Her team reported a year ago that "isolated giant planets form commonly in nature and may be significantly populating the galactic disk and the solar neighborhood." Sure enough, a doctoral student at the University of Tokyo then reported finding more than 100 "isolated planetary-mass objects" in the constellation Cygnus, about 2,000 light-years away. In addition to their strange size—5 to 15 Jupiter masses—the objects seemed to be gravitational free agents, not bound to any star.

More reports of superplanets and orphan planets followed. But some astronomers questioned using the word planet to describe the sightings. "Calling them planets implies swarms of Jupiters stripped from their parent stars, a scenario currently unsupported by any evidence," a group complained last year in a letter to the journal Science. Others wondered if the celestial nomads could be called stars if they didn't provide any illumination.

Japan's Subaru Telescope in Mauna Kea, Hawaii, detected a star-forming region (above) on February 13, 2001, that contains a newborn floating object (white circle), a star (blue), and a brown dwarf (green).Photograph courtesy of Subaru Telescope/National Astronomical Observatory of Japan

Neither label seems to fit. Planets are thought to form only in the vicinity of stars, so the notion of an isolated planet is heretical. In its infancy, a star is surrounded by a plane of cosmic detritus called a circumstellar disk. Planets congeal from this disk, and because the disk is gravitationally bound to the star, the planets stay stuck there too. The notion of a tiny star seems implausible because nobody thought the gravitational collapse of a gaseous cloud could yield a measly 10-Jupiter mass.

Nonetheless, hypotheses are evolving to explain the new observations. Recently theorists proposed models in which planets can get ejected from their orbits into empty space. And the latest models of star formation allow for pint-sized rejects. "In a way, it's a turf battle: Who do these things belong to?" says Boss, who believes the misfits are sub-brown dwarfs escaped from stellar nurseries. Glenn Schneider, an exoplanet specialist at the University of Arizona in Tucson, is inclined to view them as planets: "Give a billion years of cooling and evolution, and these objects may be indistinguishable from planets. But from the perspective of formation, they would still be distinct."

The dust might settle when astronomers know more. Analyses of reflected spectra, for example, would show what the objects are made of and thus their origins. Measuring velocities might help too. "If they're ejected planets, they should be moving fast," says Najita. Neither she nor her colleagues, however, are rushing to do the work. "The evidence you'd need to prove one way or another will probably take some time to accumulate," she says.

Meanwhile, astronomers need time to clarify their terminology. Is a star called a star because of the way it forms or because of its ability to radiate light? Is a planet only a planet if it orbits a star? Last year one team at the Harvard-Smithsonian Center for Astrophysics found warm, dusty disks around some of the free-floaters—evidence, they claim, that the objects are stars. But another group at the Space Telescope Science Institute in Baltimore reported sighting planets on the loose that weigh less than Jupiter and could be numerous enough to account for 10 percent of a star cluster's mass. And a University of California at Berkeley expert reports finding a planet with 17 to 40 times the mass of Jupiter.

Right now, it seems like the answers are still light-years away.

The Space Telescope Science Institute offers information about the planet-sized drifters, as well as animations and links:

Visit to learn more about Japan's Subaru telescope.

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