Zeroing in on Extrasolar Planets
23 Oct 2002
Smallest Known Extrasolar Planet Possibly Found
By Robert Roy Britt
New research reveals what could be the smallest planet ever found outside our solar system in a long, Pluto-like orbit around its star.
No planet has ever been found this far from another star.
Clumps of dust around a young, nearby star could signal a planet smaller than Saturn, the astronomers say. The knots of material, arranged in a ring around a star called Epsilon Eridani, were spotted in a previous study, but researchers could not pin down why they existed. Several scientists suspected planets might be involved.
Alice Quillen, assistant professor of physics and astronomy at the University of Rochester, said she can't be certain the relatively small world is there, but her new computer model suggests it is.
"I think the clumps are probably caused by a planet," she said in a telephone interview.
The study, done with undergraduate student Stephen Thorndike, appears in the current issue of Astrophysical Journal Letters.
Epsilon Eridani is about 10 light-years from Earth, close by astronomical standards. It is a young star, still circled by a disk of dust leftover from the star's birth. A similar disk circled our own Sun in the early years and was the stuff of which our solar system formed.
Astronomers say Epsilon Eridani may be a window to our own past, a view of our solar system shortly after the planets formed. Epsilon Eridani has long been an object of great interest, even being monitored for radio signals that might signal intelligent life. Researchers now believe, however, that life would struggle to evolve to an intelligent level in such a young system, where comets and asteroid would routinely pummel a planet.
Another, larger planet has already been found around Epsilon Eridani. The roughly Jupiter-mass object orbits the star every five years.
The new planet candidate circles the star every 280 years. It is about twice as massive as Neptune, Quillen estimates, or about one-tenth the heft of Jupiter. She said it could be smaller. It is thought to be less than 1 billion years old. The planets in our solar system are all more than 4 billion years old.
Several research groups have been probing stellar dust disks in recent years in search of newborn planets. Just last week, a different team announced strong evidence for a Saturn-sized planet.
Quillen said her computer model is different in that it promises to find planets very far from their host stars in highly elliptical, or non-circular, orbits, like the path taken by Pluto.
To date, the only firm, widely agreed upon extrasolar planet discoveries have involved an entirely different search technique, however, in which astronomers watch stars for slight wobbles that indicate a planet, whose gravity is tugging on the star. But this so-called wobble method requires monitoring a star for years until the planet makes nearly a complete orbit. About 100 planets have been detected this way. Most are as big as Jupiter or larger. No planet outside our solar system has ever been photographed.
Quillen's method, like others that look at dust, can see signs of a planet with a single snapshot.
Here's how it works: Dust tends to spiral inward in any new solar system. Larger dust particles can get gravitationally trapped into particular positions along a planet's orbit, forming clumps. Telescopes gathering infrared and submillimeter light can see the star's energy reflected off this dust, and higher concentrations of it appear brighter.
Quillen worked backwards to make her finding. She used a computer to model how a planet on a Pluto-like orbit might sculpt dust. Then she searched existing observations for a star with dust signatures that might match her printouts.
"The fact that the dust around this star closely matches what we expected to see if a planet were present doesn't mean we know for sure that a planet is really there," Quillen said. "The images of Epsilon Eridani that we matched with our model are five years old. If Epsilon Eridani were re-observed then the clumps should have moved. The rate that they move will pin down the likely location of the planet."
If Quillen's planet turns out to be real, it would be less massive than any yet discovered. One that's slightly heavier was found using the wobble method earlier this year. Both planets would almost surely be gaseous, not rocky like Earth.
The research was funded in part by the National Science Foundation.