By Ron Cowen
Monday, October 27th, 2008.
Epsilon Eridani hosts an inner asteroid belt and planet arranged like those in the solar system
In the annals of planethood, astronomers consider the star Epsilon Eridani a member of the fabulous four. Along with Fomalhaut, Beta Pictoris and Vega, Epsilon Eridani is one of the first four stars scientists have found that has an icy ring of debris, an indication that the star has begun the process of forming planets.
Epsilon Eridani just got more fabulous: Researchers have discovered that the star, only 10.5 light-years from the sun, sports two inner asteroid belts in addition to the icy ring on the outskirts of the Epsilon Eridani system.
In both location and mass, Epsilon Eridani’s innermost asteroid belt is a virtual twin of the solar system’s asteroid belt. The second asteroid belt is farther out and about 20 times more massive than the solar system’s belt. This belt circles Epsilon Eridani at a distance roughly that at which Uranus orbits the sun.
Perhaps most intriguingly, a previously detected planet orbiting Epsilon Eridani lies just outside the innermost belt, at an average distance of 3.4 astronomical units from the star. (One astronomical unit is the distance between the Earth and the sun.)
This is the first time that an asteroid belt and a planet orbiting another star have been found in the same arrangement as the solar system’s asteroid belt and Jupiter, notes codiscoverer Massimo Marengo of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. Like Jupiter, the gravity of the planet orbiting Epsilon Eridani may have cleared the region beyond the innermost belt, corralling the material in the inner belt.
In addition, the gaps between Epsilon Eridani’s dual asteroid belts and the icy ring provide new evidence that the star has at least one other, more distant planet that sculpted the star system’s architecture. The 850-million-year-old star, similar in mass to the sun but only one-fifth its age, may be providing a snapshot of what some sunlike systems look like in their youth, notes Marengo.
He and study leader Dana Backman of the SETI Institute in Mountain View, Calif., along with their collaborators, report the findings in the January 10, 2009 Astrophysical Journal. The article can be found online at http://arxiv.org/abs/0810.4564. The researchers also announced these discoveries, which are based on observations with NASA’s infrared Spitzer Space Telescope, in an October 27 press release.
The team observed Epsilon Eridani at several infrared wavelengths, using two cameras and a spectrometer on the orbiting Spitzer observatory. Spitzer directly imaged the icy outer ring, already known to exist, and the outer of the two asteroid belts. The observatory, which has a relatively small mirror, lacked the resolution to take a picture of the inner belt. However, the researchers were able to infer the shape and position of the inner belt from the temperature of infrared emissions — thermal radiation above and beyond what the star itself emits, and which could only come from rocky debris close to the star.
Both belts consist of silicate or rocky debris, with the infrared-emitting particles a few micrometers in diameter, says Marengo.
In 2000, other researchers, detecting a wobble in the motion of Epsilon Eridani, claimed they had discovered a still unseen, Jupiter-mass planet that pulls the star ever so slightly to and fro. The motion indicated that the planet takes an elongated, rather than circular, path around the star. However, such a path would cause the planet to repeatedly cross the newly discovered inner asteroid belt and ultimately destroy it, says Marengo. The very existence of the belt suggests the planet has a nearly circular orbit. Because the star is young and active, the wobble signal has a significant amount of noise associated with it. The discovery of the belts “strengthens the argument that planets orbit the star as well, since in the absence of planets,” asteroids would be distributed in a wider swath rather than a narrow belt, notes Alycia Weinberger of the Carnegie Institution for Science in Washington, D.C.
Clumps in the icy ring, observed at submillimeter wavelengths, already indicated that the star might harbor other, outer planets. The newly found asteroid belts and the gaps between them add to that evidence, notes Marengo. Because any outer planet would lie too far from the star to exert a noticeable gravitational tug, the wobble method couldn’t detect such a planet. But the body might show up in a direct image if it isn’t lost in the glare of its parent star.
Although Marengo and Backman suggest that the Epsilon Eridani system may be a replica of the youthful solar system, “to me the most interesting thing is that this isn't exactly like the young solar system, because it has three belts of comets and asteroids whereas our system only has two,” says Jane Greaves of the University of St. Andrews in Scotland. “This implies that planets can shape systems very differently, and if life emerged in this system in the future, the environment could be very different.” For instance, comets and asteroids could pummel a habitable planet “from all directions, so life might have to evolve very fast to survive,” she says.
In 1960, Epsilon Eridani was one of the first stars that radioastronomer Frank Drake searched for signs of an advanced alien civilization, before researchers realized the star was only a young adult.
“Spitzer has given us just a hint into the phenomenon of multiple dust rings,” comments Lynne Hillenbrand of the California Institute of Technology in Pasadena. “It is only for spectacularly close objects such as Epsilon Eridani that we also have the luxuries … of spatially resolving the emission at several wavelengths,” she notes. “In the future, with better technology, we should be able to image more of these young adult planetary systems.”
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