Image May Be 1st Planet Seen Outside Solar System

Astronomers may have gotten their first direct look at another world beyond our solar system: an enormous planet kicked out of its stellar orbit and now wandering free from its parent stars, NASA announced Thursday in Washington.

“It may be the first image of a planet outside our solar system,” said Susan Tereby of the Extrasolar Research Institute in Pasadena, who discovered the object with the Hubble Space Telescope. Astronomer Steve Strom of the University of Massachusetts hailed the discovery as “a landmark in our quest to understand our origins.”

Until now, astronomers have seen only the effects of planets--gravity-induced wobbles in certain stars or distorted dust rings around others.

The Hubble images of the possible planet are ambiguous, however, without any confirmation of the true nature of the object, its position, mass, brightness or relationship to the parent stars. “It’s on the edge,” said Charles Beichman, project scientist for NASA’s Origins Program. “It’s science in action.”

If the bright point of light picked up by Hubble does turn out to be a planet, that would mark a “watershed event,” said astronomer Anne Kinney of the Space Telescope Science Institute in Baltimore.

Even if it isn’t a planet, but a background star or a failed star, astronomers were fascinated by the image, which shows a glowing filament streaming from stars and seeming to point to a bright object. “It’s an extraordinary image regardless of what that object is,” said astronomer Alan Boss of the Carnegie Institution in Washington. The fact that the candidate planet could be seen at all was pure serendipity, a consequence of its presumably being tossed out of its two-star parent system by a gravitational slingshot. If the planet had remained in orbit around the star, it would have been drowned out by the stars’ light, like a firefly in front of a flame.

If it had wandered farther away from the stars, then there would be no obvious relationship between the object and the two-star system it presumably once orbited. And if it never orbited stars, it couldn’t be defined as a planet.

In order to catch the planet in the act of being ejected, Hubble had to observe the event within a brief span of several thousand years--an eye-blink, cosmologically speaking. “Nature threw us a curveball here,” said Ed Weiler, director of NASA’s Origins Program. It “threw this planet out so we could see it.”

Even so, astronomers may not have related the object to the star system except for the eerie presence of a 130-billion-mile-long glowing filament that, like an umbilical cord, seems to bridge the space between the parent stars and possible planet.

“If it weren’t for that filament, [the astronomers] would be out of their minds [to suggest the object was a planet],” said UCLA astronomer Andrea Ghez. NASA’s Beichman agreed: What raises it beyond the realm of pure speculation, he said, was that the object appears “right at the end of this tail of luminosity.”

The finding has not yet been published in a scientific journal, which made it difficult for other scientists to comment. However, Tereby posted an abstract on the Internet last week in preparation for an upcoming astronomy meeting. The resulting interest was so great, said NASA astronomers, that they felt compelled to proceed with a press conference.

If it is not a planet, the object could well be a brown dwarf--a failed star not quite big enough to ignite. Alternatively, there’s a chance it is merely a background star that happened to be in the image. Tereby calculated that chance at about 1 in 50.

Other astronomers questioned whether Tereby’s estimate of the object’s mass is correct. Based on its apparent age and brightness, she calculated it to be about three times the mass of Jupiter, still small enough to qualify as a planet. However, the camera on Hubble captured only a few narrow bands of infrared light. “That’s not much information,” said University of Arizona astronomer Adam Burrows, who said that such an observation should be based on 20 to 100 bands. Also, he added, the object might look dim because it is obscured by dust.

The object’s age also raised questions. Tereby and her colleagues have assumed that it is the same age as the double stars. That makes it extremely young for a giant planet--only about 300,000 years old, which is below most theoretical estimates of how long it takes to form such an enormous body.

Despite the doubts raised, Kinney and others defended calling the object a planet. “If it looks like a duck and it quacks like a duck, it’s probably a duck,” she said.

Astronomers expect to know a lot more when ground-based telescopes like the Keck in Mauna Kea, Hawaii, will be able to analyze the light from the object to find out more about its composition and other properties.

The double star is about 450 light-years away from Earth, just to the north of the red eye of the constellation Taurus the Bull (although not visible the naked eye). The object is quite far from its parent star--about 1,400 times the distance from the Earth to the Sun.

“Whatever it is, it’s an interesting object,” said Burrows.

Even if it is a planet, this object would not be anything like the Earth or even Jupiter. Having just recently formed, it would still be glowing hot from gravitational contraction. In order to evolve into a mature planet, the object would have to condense and cool down considerably. In other words, it would be a lot like a brown dwarf, further confounding its classification.

Still, the scenario that formed the possible planet may be quite common in star-forming regions of the galaxy. It shows that planets “are perhaps an inevitable outcome of the process that forms stars,” said Strom.

Most young sun-like stars form as doubles. If a third planet / star forms in they same group, explained Kinney, the small fry is usually kicked out of the nest by gravitational instability.

The discovery of the ejected planet, she said, raised the specter that thousands of homeless planets are wandering about in our galaxy.