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 (鈥淪earching for alien Earth鈥�,
快猫短视频, 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. 鈥淚t may be the
first picture ever taken of a planet outside the Solar System,鈥� she says.
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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鈥檚
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鈥檚 interpretation. 鈥淭his is the first image of another
planet around its star,鈥� he asserts.
The earlier claim, made by Al Schulz of the Space Telescope Science Institute
in Baltimore, didn鈥檛 impress Boss. Schulz鈥檚 team also used Hubble and spotted a
point of light on two occasions, three months apart. It seemed to be orbiting
the Sun鈥檚 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鈥檚 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鈥攁 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. 鈥淲e just don鈥檛 know, frankly, that it鈥檚 two Jupiter
masses,鈥� says Jonathan Lunine of the University of Arizona in Tucson.
If Terebey鈥檚 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鈥檚 data support a rival 鈥渙ne step鈥� theory for planetary
formation, which allows the material orbiting a star to form a planet in a
thousand years or less.