It's a whole new world


By Charles Seife in Washington DC A SNAPSHOT of a lonely planet rejected by its star system and hurtling towards oblivion was released last week by NASA. The picture, taken in the infrared by the Hubble Space Telescope, is the first convincing direct view of an extrasolar planet, the space agency claims. The latest announcement follows hard on the heels of a similar image, released with little fanfare (This Week, 31 January, p 6). Doubts remain over whether either picture really depicts an alien planet. But if they are what they seem, astronomers have their hands on images that some thought would only be taken by future generations of orbiting telescopes (“Searching for alien Earth”, New Scientist, 13 May 1995, p 24). Some 450 light years away in the constellation of Taurus, stars are being born in a cloud of dust. Susan Terebey of the Extrasolar Research Corporation in Pasadena, California, was looking at these stars when she noticed a streamer of radiation in a binary star system. At its end was an object that Terebey and her colleagues believe is a planet about twice the size of Jupiter. “It may be the first picture ever taken of a planet outside the Solar System,” she says. The object, dubbed TMR-1C, seems to have been thrown out of its star system by the unstable gravitational influence of the two central stars. TMR-1C’s distance from these stars is about 1400 times the gap between the Earth and the Sun. As TMR-1C sped away, Terebey argues, it carved a tunnel in the dusty cloud, creating the illuminated streamer. The images come from NICMOS, the Near Infrared Camera and Multi-image Spectrometer, installed on Hubble in February 1997. Infrared can penetrate clouds of dust, so astronomers can see details that are hidden in the visible spectrum. Alan Boss, an astrophysicist at the Carnegie Institution in Washington DC, is convinced by Terebey’s interpretation. “This is the first image of another planet around its star,” he asserts. The earlier claim, made by Al Schulz of the Space Telescope Science Institute in Baltimore, didn’t impress Boss. Schulz’s team also used Hubble and spotted a point of light on two occasions, three months apart. It seemed to be orbiting the Sun’s nearest stellar neighbour, Proxima Centauri, 4.2 light years away. But other researchers have since failed to spot the object. Schultz argues that his object disappeared because its orbit brought it into the glow produced by its parent star. He expects it to become visible again in October. A question mark also hangs over Terebey’s planet. Although she estimates that it is twice the size of Jupiter, it may be much larger. And if its mass is much more than ten times that of Jupiter, it might not be a planet, but a brown dwarf—a failed star, too puny to sustain thermonuclear reactions. Terebey assumed the planet was the same age as its parent stars. She determined its temperature from the spectrum of the radiation it emitted. Because the temperature of a young planet depends on its age and its size, she was then able to estimate its mass. However, other astronomers say there are wide margins for error. “We just don’t know, frankly, that it’s two Jupiter masses,” says Jonathan Lunine of the University of Arizona in Tucson. If Terebey’s object is a planet, it is an interesting one. The two stars in the binary system are only a few hundred thousand years old, but a popular theory of how Jupiter-sized planets form implies that it would take a million years for the dust orbiting a star to coalesce into a rocky planetary core, and then 10 million years more for the core to acquire a gaseous envelope. Boss argues that Terebey’s data support a rival “one step” theory for planetary formation,
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