Nearby Star Hosts Three-Ring Circus

A nearby star, visible with the unaided eye, is ringed with two rocky asteroid belts and an outer icy halo, making it a three-ring cosmic circus. The inner asteroid belt appears to be a virtual twin to the belt in our solar system.

The presence of the separate rings of material around the nearby star, called Epsilon Eridani, suggests unseen planets lurk there, where they confine and shape the rings, say the researchers.

If there were in fact rocky planets within the inner gap between the star and asteroid belt, the worlds would likely reside within the star's habitable zone where temperatures would be such that life could survive.

Located 10.5 light-years from Earth in the constellation Eridanus, the star is the ninth closest to the sun. The sun's three nearest known stars are gravitationally bound in a system called Alpha Centauri that's located 4.36 light-years away. (A light-year is the distance light travels in one year, or about 6 trillion miles, or 10 trillion km.)

Epsilon Eridani is slightly smaller and cooler than the sun. And it's also younger. While the sun is an estimated 4.5 billion years old, Epsilon Eridani has been around for just 850 million years.

"Studying Epsilon Eridani is like having a time machine to look at our solar system when it was young," said researcher Massimo Marengo, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Massachusetts.

Rocky rings

Astronomers had known about the star's outer icy ring, but they were surprised when NASA's Spitzer Space Telescope revealed two rocky rings between the icy halo and the star.

The inner asteroid belt looks identical to ours in terms of material, and it orbits at 3 astronomical units (AU) from Epsilon Eridani — the same distance between the sun and the rocky asteroid belt between Mars and Jupiter. (An astronomical unit equals the average Earth-sun distance of 93 million miles, or about 150 million km.)

Epsilon Eridani's second asteroid belt is 20 AU from the star, or about where Uranus is in relation to our sun, and it is crowded with as much mass as Earth's moon.

The outer icy ring, previously observed, extends about 35 AU to 100 AU from Epsilon Eridani and is similar in composition to our Kuiper Belt, a region of icy objects beyond Neptune. Eridani's outer ring holds about 100 times more material than ours, however.

New exoplanets?

The rings formed when the system was very young, likely when collisions between planets and other smaller bodies resulted in small bits and big chunks of debris that took shape as the asteroid belts and icy ring, the researchers suggest.

And the gaps between these rings were likely shaped by planets whose gravitational forces could remove any excess material flung from the belts, while also keeping the shape of the rings. Planets in our solar system exert similar shaping effects.

"The big planets that are now keeping those gaps are determining the geometry of the system of rings," Marengo told SPACE.com.

He and his colleagues propose three planets with masses between those of Neptune and Jupiter could be in orbit about Epsilon Eridani. A Jupiter-mass exoplanet was detected in 2000 by the radial velocity method in which astronomers look for wobbling motion of a star due to the gravitational tug of a planet. That planet is located near the edge of the innermost ring.

A second planet must lurk near the second asteroid belt, and a third at about 35 AU near the inner edge of Epsilon Eridani's Kuiper Belt, the researchers say. Terrestrial planets could reside inside the innermost asteroid belt as well, though there currently is no clear indication of that, Marengo said.

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