May 29, 1998
NASA's Hubble Space Telescope has given astronomers their first direct look at what is possibly a planet outside our solar system -- one apparently that has been ejected into deep space by its parent stars.
The discovery, made by Susan Terebey of the Extrasolar Research Corporation in Pasadena, CA, and her team using Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS), further challenges conventional theories about the birth and evolution of planets, and offers new insights into the formation of our own Solar System.
Located in the sky within a star-forming region in the constellation Taurus, the object, called TMR-1C, appears to lie at the end of a strange filament of light that suggests it has apparently been flung away from the vicinity of a newly forming pair of binary stars.
At a distance of 450 light-years, the same distance as the newly formed stars, the candidate protoplanet would be ten thousand times less luminous than the Sun. If the object is a few hundred thousand years old, the same age as the newly formed star system which appears to have ejected it, then it is estimated to be 2-3 times the mass of Jupiter, the largest gas giant planet in our Solar System.
Also possible is that the object is up to ten million years old, the same age as other young stars nearby, in which case it may be a giant protoplanet or a brown dwarf star. A brown dwarf star is a small star that has failed to sustain nuclear fusion.
The candidate protoplanet is now 130 billion miles from the parent stars and predicted to be hurtling into interstellar space at speeds up to 20,000 miles per hour (10 kilometers/sec) -- destined to forever drift among the Milky Way's starry population.
Hubble researchers estimate the odds at two percent that the object is instead a chance background star.
"If the results are confirmed, this discovery could be telling us gas giant planets are easy to build. It seems unlikely for us to happen to catch one flung out by the stars unless gas giant planets are common in young binary systems," said Terebey.
"The results don't directly tell us about the presence of any terrestrial planets, like Earth," she adds. "However, we believe gas giants do influence the formation of much smaller rocky planets."
Current models predict that very young giant planets are still warm from gravitational contraction and formation processes. This makes them relatively bright in infrared light compared to old giant planets such as Jupiter. Even so, young planets are difficult to find in new solar systems because the glare of the central star drowns out their feeble glow. Young planets ejected from binary systems would therefore represent a unique opportunity to study extrasolar planets with current astronomical technology.
The discovery also challenges conventional theories that predict gas giant planets take millions of years to coagulate from dust in space. Instead, it favors more recent ideas that large, low-density planets may condense out of gas very quickly, at the same time their parent star does.
"This observation pushes back the clock on planet formation and offers short time scales which allow us to see how things form. This provides valuable new clues to the origin of our Solar System," says Terebey.
The candidate protoplanet was accidentally discovered by Terebey and colleagues while studying Hubble infrared images of newly formed protostars in a molecular cloud in Taurus. The exquisite sensitivity and sharpness of NICMOS clearly revealed the object's pinpoint image. However, it might have been dismissed as a background star if not for the presence of a bizarre 130-billion-mile-long filamentary structure that bridges the space between the binary pair and the candidate protoplanet.
"I said to myself, 'This is really weird, what in the world could it be?'" recalls Terebey. She speculates it could be a tunnel the runaway object burrowed through a dust cloud surrounding the stars. This created a "light tube" which channels light from the stars deep inside their dusty cocoon - like a light beam traveling through a length of fiber optic cable.
This brought Terebey to the tantalizing possibility that the planet had been flung into deep space by a gravitational "slingshot" effect from its parent stars. This could have happened if the planet's orbit allowed it to rob momentum from the stars and pick up so much speed that it escaped the system, similar to the way spacecraft perform gravitational "slingshot" maneuvers to pick up speed by flying close by a planet.
"We know that many triple star systems eventually toss out the lowest mass star. And we can predict the speed at which the object should be moving, based on the separation of the binary stars," said Terebey.
Future observations call for images taken at a later date, to confirm the object's predicted movement across the sky. In addition, the spectrum of the object will tell whether the object is a background star, brown dwarf, or something whose spectrum is less easy to predict, such as a giant protoplanet.
"We will just have to wait and see if future observations confirm this picture," said Terebey. "However it turns out, we have come to appreciate that protoplanet ejection by young binary stars ought to happen, and it offers a new way to search for giant planets."
"These future observations will be critical in verifying that this object is truly a planet and not a brown dwarf," said Dr. Ed Weiler, Director of the Origins Program at NASA Headquarters, Washington, DC. "We are sharing this preliminary data with the public at a very early stage in the research process because of its potential importance and because of the compelling nature of the image. If the planet interpretation stands up to the careful scrutiny of future observations, it could turn out to be the most important discovery by Hubble in its 8 year history".
The members of the research team include Susan Terebey (Extrasolar Research Corp.), Dave Van Buren, Deborah L. Padgett, Jet Propulsion Lab, Pasadena, CA (JPL), Terry Hancock (Extrasolar Research Corp.), and Michael Brundage, JPL.
Editor's Note: The original news release, with related links and material, can be found at http://oposite.stsci.edu/pubinfo/pr/1998/19/pr.html.
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