The U.S. Hubble Space Telescope has detected dusty evidence around a nearby star that suggests at least one hidden planet is circling it. It is one more indication that planets are common outside our solar system. The observation might offer insights into Earth's formative years.
The star Fomalhaut is one of the nearest and brightest in the night sky, only 25 light years away. Its strong presence has made it an object of curiosity since at least ancient times.
Not until 1984, however, did astronomers using infrared wavelength telescopes discover a ring of dust grains surrounding it, probably supplied by collisions between comets and asteroids. Dust suggests that planets are forming out of it or might already have, so researchers have been struggling to get a sharp view of the ring.
But only now have they succeeded in a crisp observation in the visible wavelengths, thanks to a powerful new camera aboard the Hubble Space Telescope. For the first time, the sighting shows that the center of the dust ring is a very long distance away from the star, more than two-billion kilometers away, not concentric with it. It is also a narrow ring, not diffuse or wide.
One member of the observing team, Paul Kalas of the University of California at Berkeley, says the finding supports the long-held notion that at least one unseen planet is moving around Fomalhaut in an oval orbit and reshaping the dust ring with its gravitational pull.
"What we should see is a flattened disk of dust with virtually no interesting features," he explained. "But if you insert a planet into the system, then the planet will sweep up material in its vicinity and also block dust from spreading inward toward the star. So what you would see is a narrow belt or a ring of dusty material, and this is what we see in these new Hubble images of Fomalhaut."
Another classic sign of a planet is seen in the ring's inner edge. It is sharper than the outer edge, indicating a planet is sweeping out material.
Fomalhaut is an infant star, just 200-million-years-old. When our sun was that young, Earth and the other planets had just formed from the dust around it. A scientist not involved in the discovery, astrophysicist Jane Greaves of the University of St. Andrews in Scotland, says the new observations could help us better understand what occurred in the early stages of a solar system like ours.
"It is intriguing to speculate about whether a planet like the Earth might be forming here," she said. "Certainly, looking at their images, it is obvious there was lots of raw material around from which you could construct a planet. So this could be a great opportunity to look at the final stages of what you might think of a planet assembly line."
The finding ushers in another method of searching for planets outside our solar system, a growing activity in astronomy for the last 10 years. Currently, astronomers infer that one is orbiting a star by observing its gravitational effect on the star's movement or by seeing the star dim briefly, suggesting a planet is passing in front of it. This latest method analyzes the dust cloud around a star.
The discovery came for the U.S. astronomers after they had scoured the sky for dusty disks around stars with ground and space telescopes since the early 1990s. It was finally made possible by the ultra-high resolution Advanced Camera for Surveys aboard the Hubble Telescope, installed when space shuttle astronauts last serviced the observatory in 2002. It is 10 times more powerful than the camera it replaced.
Team member Mark Clampin of the U.S. space agency says a crucial feature of the camera, called a coronagraph, masked the overpowering starlight so that its bright edge glow would not hide the extremely dim dust ring.
"The basic challenge of this kind of observation is trying to image a very low surface brightness disk against the glare of its bright parent star," he explained. "We want to do this in the visible because with Hubble we can get some of the sharpest images of these debris disks from the reflected light that we see, but to do this requires an instrument that has very excellent contrast to pull off this kind of an observation."
Mr. Clampin predicts that Formalhaut will continue to be the object of intense observation with Hubble and its successor space observatory, scheduled for launch in 2011.
