18 Planet-Like Objects Found Far From Stars
Peering into the isolated reaches of space, an international team of astronomers has discovered 18 planet-like objects far from any sustaining star, where no planets should be.
The finding is certain to upset current theories of how planetary bodies form, several experts said. The new objects may, in a sense, be the small seeds of stars that never grew to maturity and that now float rootless in the void.
These celestial free agents, discovered in the belt of the constellation Orion, also are considerably younger than anyone would have predicted, the team of Spanish, American and German astronomers said, in research made public today in Science.
The discovery suggests that planets of all sorts, which until recently could be detected only within our own solar system, may be as common in the universe as pebbles on a beach.
Until now, researchers had discovered more than 50 planets beyond the rim of our own solar system, all in orbit around distant stars. All of them, however, are thought to have formed slowly, over tens of millions of years, as gas and dust in the disc swirling around a star condensed and clumped together.
But these 18 dim, reddish rogues took shape under dramatically different circumstances, said lead researcher Maria Rosa Zapatero Osorio of the Instituto de Astrofisica de Canarias in Tenerife, Spain, who is working at Caltech.
They are not orbiting in a system with a central star like our sun--generally believed to be a prerequisite for planet formation. Moreover, they must have formed very quickly, because they are part of a star cluster, called Sigma Orionis, that is no more than 5 million years old, a fraction of the age of our own sun.
Detected with telescopes in Spain, the Canary Islands and Hawaii, the planet-like bodies that Osorio and her colleagues discovered are about 1,200 light years away, relatively close to Earth as galactic distances go.
Each is between five and 15 times as large as Saturn or Jupiter, much smaller than the darkest, coolest and smallest sort of star, which is known as a brown dwarf.
“Nobody has found this before,” Osorio said. “This is very important. These objects form frequently, very easily in nature. The formation of young, free-floating planetary-mass objects like these is difficult to explain by our current models of how planets form.”
Joan R. Najita, an expert on planet and star formation at the National Optical Astronomy Observatory in Tucson, Ariz., said the discoveries by Osorio and her team will shed new light on how stars and planets form.
The discovery confirms growing suspicions among astronomers about the existence of such relatively small objects in the void between the stars, Najita said.
Her own research on brown dwarf stars, published in the current issue of Astrophysical Journal, suggests that the star fields might be crowded with such drifting objects.
“People will not challenge this result,” Najita said. “These objects have the masses of what we would call planets, but they don’t orbit a star, so they are a different kind of object.”
While astronomers do not dispute the discovery itself, some researchers already are arguing about just what to call these newly discovered “free-floaters.” The debate reflects a more fundamental uncertainty over what precisely constitutes a planet.
“Our objects are not orbiting any star, so they break part of the traditional planet definition; yet they are similar to planets we know,” Osorio said.
Several researchers suggested that, because they do not orbit a star and may not have formed in a conventional planetary system, these objects should be given a new, more precise name that properly reflects their unusual nativity. Astrophysicist Alan Boss, of the Carnegie Institution in Washington, D.C., said: “Nature is throwing us a curveball.”
“By my definition, these guys are planets,” said space scientist Jack Lissauer at NASA’s Ames Research Center in Mountain View, Calif., who is an expert on how planetary systems form.
For Lissauer, size is the deciding factor, and these objects just aren’t big enough to be called anything else. “But even those of us who would call them planets would not say they formed the way the planets in our solar system formed.”
A few of these isolated planets may have been kicked out of conventional solar systems, but Lissauer is confident that most of them formed alone, in the same way as stars.
Like stars, they may have coalesced out of a vast molecular cloud, but then may never have reached a mass sufficient to trigger the self-sustaining fusion reaction that lights the heart of a star.
“The star formation process extends down to much smaller sizes than we thought. That is the significant thing here,” Lissauer said.
So far, astronomers don’t know enough about conditions on these objects to know whether they could support life.
Osorio said her team decided to search the star cluster in this particular area of Orion because the region is relatively close to our solar system, comparatively young, and unusually free of the stellar dust that can make such cool, and therefore faint, objects hard to see. That allowed the astronomers to detect infrared light emitted directly from the planet-like objects.
“It is very difficult to detect these bodies with our current technology,” she said. “My belief is that, as our technology improves, within 20 years astronomers will discover examples of these free-floating bodies in our own solar neighborhood.”
The outskirts of the solar system, from Pluto to the nearest star, are still relatively unexplored, and even something as large as Jupiter could be overlooked.
As astronomers widen their search for other examples of these odd objects, the most important consequence of the discovery, Osorio said, is the way it expands our ideas of the universe we inhabit.
“Our conception of galaxies has changed,” Osorio said. “In just a few years, we have moved from our certain knowledge of just nine planets around our sun to knowing there are many planets around many suns and, now, these free-floating bodies. The stars are not alone.”
