A flying observatory has taken the first ultrasharp images of rings of cold debris around sunlike stars. The doughnut-shaped rings appear to be extrasolar analogues of the Kuiper belt, the outer solar system’s reservoir of comets and other frozen bodies.
The newly observed rings are either left over from the planet-making process or were generated when planets collided. Astronomers used the European Space Agency’s infrared Herschel Space Observatory, which sports the largest light-collecting mirror in space and is exquisitely sensitive to cold, sand-grain-sized dust, to photograph the belts.
”The Herschel images are the highest resolution far-infrared measurements ever made for debris disks” like the Kuiper belt, says infrared astronomer George Rieke of the University of Arizona in Tucson, who was not involved in the study.
René Liseau of the Chalmers Institute of Technology in Sweden, Carlos Eiroa of the Universidad Autónoma de Madrid in Spain and their colleagues posted their findings on debris belts surrounding two sunlike stars online May 19 and reported evidence of belts around more stars May 20. Some of their findings will also appear in an upcoming Astronomy & Astrophysics.
One of the sunlike stars, called q1Eridani or HD 10647, lies 57 light-years from Earth and has a Jupiter-sized planet orbiting it at about twice Earth’s distance from the sun. The bright ring surrounding the star is a frigid 30 kelvins, lies an average of 85 astronomical units from the star (1 AU is the Earth-sun distance) and is about 40 AU wide. In comparison, the solar system’s Kuiper belt, which lies beyond the orbit of Neptune, resides about 30 to 55 AU from the sun.
Strong infrared emissions from q1Eridani, recorded as far back as 1983 with the Infrared Astronomical Satellite, had already indicated the presence of an infrared-emitting belt of debris. Although expected, “it’s nonetheless lovely to see” an actual belt, says Alycia Weinberger of the Carnegie Institution for Science in Washington, D.C. “Herschel is the first telescope to have the spatial resolution and sensitivity” at an infrared wavelength of 100 micrometers to resolve infrared emissions into bona fide belts or disks, she adds.
A much fainter belt appears to surround the star Zeta2 Reticuli, about 39 light-years from Earth, Liseau and his colleagues report. The belt lies at an average distance of 100 AU from the star, which is not known to harbor a planet.
Weinberger says the existence of this second belt is on shakier ground. “The high level of asymmetry [of the imaged belt], very cold temperature of the dust and possibility of confusion with a background object all give me an uneasy feeling,” she says.
Nonetheless, the Herschel images provide the best estimate of the amount of mass in a debris disk and the size of the grains that populate it, says Weinberger. The observatory also has the best chance of glimpsing a tenuous, far-away Kuiper belt similar to the solar system’s, Weinberger adds.
Astronomers believe the solar system’s Kuiper belt formed several billion years ago, when some of the outer planets, then packed tightly together, were suddenly hurled into the path of existing planetary debris, pushing the debris outward and sculpting it into a ring-shaped reservoir. By comparing the many examples of Kuiper belts that Herschel is expected to find with the locations of massive outer planets around sunlike stars, astronomers may learn if a similarly violent story unfolded in other planetary systems, Weinberger says.
