New observations from
NASA's Spitzer Space Telescope indicate that the nearest planetary system to our own has
two asteroid belts. Our own
solar system has just one.
The
star at the center of the nearby system, called
Epsilon Eridani, is a younger, slightly cooler and fainter version of the sun. Previously,
astronomers had uncovered evidence for two possible planets in the system, and for a broad, outer ring of icy comets similar to our own
Kuiper Belt.
Now,
Spitzer has discovered that the system also has dual asteroid belts. One sits at approximately the same position as the one in our
solar system. The second, denser belt, most likely also populated by
asteroids, lies between the first belt and the
comet ring. The presence of the
asteroid belts implies additional planets in the
Epsilon Eridani system.
"This system probably looks a lot like ours did when life first took root on
Earth," said Dana Backman, an
astronomer at the
SETI Institute, in Mountain View, Calif., and outreach director for
NASA's Sofia mission. "The main difference we know of so far is that it has an additional ring of leftover planet construction material." Backman is lead author of a paper about the findings to appear Jan. 10 in the
Astrophysical Journal.
Asteroid belts are rocky and metallic debris left over from the early stages of planet formation. Their presence around other stars signals that rocky planets like
Earth could be orbiting in the system's inner regions, with massive
gas planets circling near the belts' rims. In our own
solar system, for example, there is evidence that Jupiter, which lies just beyond our asteroid belt, caused the
asteroid belt to form long ago by stirring up material that would have otherwise coalesced into a planet. Nowadays, Jupiter helps keep our asteroid belt confined to a ring.
Astronomers have detected stars with signs of multiple belts of material before, but Epsilon
Eridani is closer to
Earth and more like our
sun overall. It is 10 light-years away, slightly less massive than the
sun, and roughly
800 million years old, or one-fifth the age of the sun.
Because the
star is so close and similar to the sun, it is a popular locale in science fiction. The television series
Star Trek and
Babylon 5 referenced
Epsilon Eridani, and it has been featured in novels by Issac Asimov and Frank Herbert, among others.
The popular
star was also one of the first to be searched for signs of advanced alien civilizations using radio telescopes in 1960. At that time, astronomers did not know of the star's young age.
Spitzer observed
Epsilon Eridani with both of its infrared cameras and its infrared spectrometer. When
asteroid and
comets collide or evaporate, they release tiny particles of dust that give off heat, which
Spitzer can see. "Because the system is so close to us,
Spitzer can really pick out details in the dust, giving us a good look at the system's architecture," said co-author Karl Stapelfeldt of
NASA's Jet Propulsion Laboratory, Pasadena, Calif.
The
asteroid belts detected by Spitzer orbit at distances of approximately 3 and 20
astronomical units from the
star (an astronomical unit is the average distance between Earth and the sun). For reference, our own
asteroid belt lies at about 3 astronomical units from the sun, and Uranus is roughly 19 astronomical units away.
One of the two possible planets previously identified around
Epsilon Eridani, called Epsilon Eridani b, was discovered in 2000. The planet is thought to orbit at an average distance of 3.4
astronomical units from the star -- just outside the innermost asteroid belt identified by Spitzer. This is the first time that an
asteroid belt and a planet beyond our
solar system have been found in a similar arrangement as our
asteroid belt and Jupiter.
Some researchers had reported that
Epsilon Eridani b orbits in an exaggerated ellipse ranging between 1 and 5 astronomical units, but this means the planet would cross, and quickly disrupt, the newfound asteroid belt. Instead, Backman and colleagues argue that this planet must have a more circular orbit that keeps it just outside the belt.