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April 28, 2009

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Two asteroid belts found in solar system’s young “twin”

Oct. 27, 2008
Courtesy Harvard-Smithsonian 
Center for Astrophysics
and World Science staff

The near­by star Ep­si­lon Erid­a­ni has two rocky as­ter­oid belts and an out­er icy ring, mak­ing it a triple-ring sys­tem, as­tro­no­mers have found.

The in­ner as­ter­oid belt is de­scribed as a near-twin of the one in our so­lar sys­tem. The out­er as­ter­oid belt holds 20 times more ma­te­ri­al, as­tro­no­mers said, and the three rings’ pres­ence im­plies that un­seen plan­ets con­fine and shape them. 

Artist's illus­tra­tion of the Ep­si­lon Eri­dani sys­tem. (Cre­dit: NA­SA/JPL-Cal­tech)

Ep­si­lon Erid­a­ni is slightly smaller and cool­er than the Sun. It lies in the con­stella­t­ion Erid­a­nus at about 10.5 light-years away. (A light-year is the dis­tance light trav­els in a year.)

Vis­i­ble to the un­aided eye, Ep­si­lon Erid­a­ni is the ninth clos­est star to the Sun, and is al­so young­er than it, be­ing an es­ti­mat­ed 850 mil­lion years old.

Ep­si­lon Erid­a­ni and its plan­e­tary sys­tem show re­mark­a­ble si­m­i­lar­i­ties to our so­lar sys­tem at a com­pa­ra­ble age, re­search­ers said. 

It’s like “a time ma­chine to look at our so­lar sys­tem when it was young,” said Mas­si­mo Ma­ren­go of the Har­vard-Smith­son­ian Cen­ter for As­t­ro­phys­ics. Ma­ren­go is co-author of a pa­per on the find­ings, to ap­pear in the Jan. 10 is­sue of The As­t­ro­phys­i­cal Jour­nal.

Our so­lar sys­tem has a rocky as­ter­oid belt be­tween Mars and Ju­pi­ter, about three times as far from the Sun as Earth is. In to­tal, the belt is es­ti­mat­ed to have about as much ma­te­ri­al as 1/20 of Earth’s Moon. 

Us­ing NASA’s Spitzer Space Tel­e­scope, the team of as­tro­no­mers found an iden­ti­cal as­ter­oid belt or­bit­ing Ep­si­lon Erid­a­ni at a si­m­i­lar dis­tance. They al­so found a sec­ond as­ter­oid belt about sev­en times fur­ther off, about the equiv­a­lent of where Ura­nus lies in our so­lar sys­tem. The sec­ond as­ter­oid belt is be­lieved to con­tain about as much mass as our Moon.

This di­a­gram (click for lar­ger ver­sion) com­pares the Ep­si­lon Erid­a­ni sys­tem to our own so­lar sys­tem. The two sys­tems are struc­tured sim­i­lar­ly, and both host as­ter­oids (brown), com­ets (blue) and plan­ets (white dots). (Cred­it: NA­SA/JPL-Caltech)

A third, icy ring of ma­te­ri­al seen pre­vi­ously ex­tends some 35 to 100 times as far from Ep­si­lon Eri­d­a­ni as the Earth-Sun dis­tance, the re­search­ers said. A si­m­i­lar icy res­er­voir in our so­lar sys­tem is called the Kui­per Belt. How­ev­er, Ep­si­lon Eri­d­a­ni’s out­er ring is es­ti­mat­ed to hold about 100 times more ma­te­ri­al than ours. 

When the Sun was Ep­si­lon Erid­a­ni’s age, the­o­rists cal­cu­late that our Kui­per Belt looked about the same as the oth­er star’s. 

Since then, as­tro­no­mers be­lieve much of the belt’s ma­te­ri­al was swept away, some hurled out of the so­lar sys­tem and some sent plung­ing in­to the in­ner plan­ets in an event called the Late Heavy Bom­bard­ment. Sci­en­tists say the Moon shows ev­i­dence of the event: gi­ant craters that formed the lu­nar seas of la­va called ma­re. 

Ep­si­lon Erid­a­ni may some­day un­dergo a si­m­i­lar dra­mat­ic clear­ing, as­tro­no­mers said.

The Spitzer da­ta in­di­cate gaps be­tween each of the three rings sur­round­ing Ep­si­lon Eri­d­a­ni. Such gaps might be caused by plan­ets that mold the rings through their gravita­t­ional force, just as the moons of Sat­urn con­strain its rings, ac­cord­ing to the in­ves­ti­ga­tors. “Plan­ets are the eas­i­est way to ex­plain what we’re see­ing,” said Ma­ren­go.

Three plan­ets with mass­es be­tween those of Nep­tune and Ju­pi­ter would fit the ob­serva­t­ions nice­ly, he added, and ad­di­tion­al, smaller rocky plan­ets are pos­si­ble. A can­di­date plan­et near the in­nermost ring al­ready has been de­tected. A sec­ond plan­et must lurk near the sec­ond as­ter­oid belt, and a third near the in­ner edge of Ep­si­lon Erid­a­ni’s Kuiper Belt, ac­cord­ing to Ma­ren­go.

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The nearby star Epsilon Eridani has two rocky asteroid belts and an outer icy ring, making it a triple-ring system, astronomers have found. The inner asteroid belt is described as a near-twin of the one in our solar system. The outer asteroid belt holds 20 times more material, astronomers said, and the three rings’ presence implies that unseen planets confine and shape them. Epsilon Eridani is slightly smaller and cooler than the Sun. It lies in the constellation Eridanus at about 10.5 light-years away. (A light-year is the distance light travels in a year.) Visible to the unaided eye, Epsilon Eridani is the ninth closest star to the Sun, and is also younger than it, being an estimated 850 million years old. Epsilon Eridani and its planetary system show remarkable similarities to our solar system at a comparable age, researchers said. It’s like “a time machine to look at our solar system when it was young,” said Massimo Marengo of the Harvard-Smithsonian Center for Astrophysics. Marengo is co-author of a paper on the findings, to appear in the Jan. 10 issue of The Astrophysical Journal. Our solar system has a rocky asteroid belt between Mars and Jupiter, about three times as far from the Sun as Earth is. In total, the belt is estimated to have about as much material as 1/20 of Earth’s Moon. Using NASA’s Spitzer Space Telescope, the team of astronomers found an identical asteroid belt orbiting Epsilon Eridani at a similar distance. They also found a second asteroid belt about seven times further off, about the equivalent of where Uranus lies in our solar system. The second asteroid belt is believed to contain about as much mass as our Moon. A third, icy ring of material seen previously extends some 35 to 100 times as far from Epsilon Eridani as the Earth-Sun distance, the researchers said. A similar icy reservoir in our solar system is called the Kuiper Belt. However, Epsilon Eridani’s outer ring is estimated to hold about 100 times more material than ours. When the Sun was Epsilon Eridani’s age, theorists calculate that our Kuiper Belt looked about the same as the other star’s. Since then, astronomers believe much of the belt’s material was swept away, some hurled out of the solar system and some sent plunging into the inner planets in an event called the Late Heavy Bombardment. Scientists say the Moon shows evidence of the event: giant craters that formed the lunar seas of lava called mare. Epsilon Eridani may someday undergo a similar dramatic clearing, astronomers said. The Spitzer data indicate gaps between each of the three rings surrounding Epsilon Eridani. Such gaps might be caused by planets that mold the rings through their gravitational force, just as the moons of Saturn constrain its rings, according to the investigators. “Planets are the easiest way to explain what we’re seeing,” said Marengo. Three planets with masses between those of Neptune and Jupiter would fit the observations nicely, he added, and additional, smaller rocky planets are possible. A candidate planet near the innermost ring already has been detected. A second planet must lurk near the second asteroid belt, and a third near the inner edge of Epsilon Eridani’s Kuiper Belt, according to Marengo.