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Mystery world a merged planet?

Jan. 10, 2008
Courtesy Harvard-Smithsonian
Center for Astrophysics
and World Science staff

A “mys­tery ob­ject” or­bit­ing a dis­tant star might have formed from the col­li­sion and merg­er of two de­vel­op­ing plan­ets, as­tro­no­mers say. 

Re­search­ers have long puz­zled over what they call the ob­jec­t’s seem­ingly im­pos­si­ble com­bina­t­ion of tem­per­a­ture, bright­ness, age and loca­t­ion. “This is a strange enough ob­ject that it needs a strange ex­plana­t­ion,” said Er­ic Ma­ma­jek of the Har­vard-Smith­son­ian Cen­ter for As­t­ro­phys­ics in Cam­bridge, Mass., one of two re­search­ers who pro­poses the col­li­sion sce­nar­i­o based on new stud­ies. 

Il­lus­trat­ed in this artist's con­cept, as­tro­no­mers say they may have ob­served the af­ter­math of a col­li­sion be­tween "pro­to­pla­nets." (Cred­it: Da­vid A. Aguilar/Har­vard-Smith­son­ian CfA)
The pro­pos­al was pre­sented Wednes­day at the an­nu­al meet­ing of the Amer­i­can As­tronomical So­ci­e­ty in Austin, Texas.

The ob­ject, known as 2M1207B, or­bits a brown dwarf star lying the di­rec­tion of the con­stella­t­ion Cen­tau­rus, 170 light-years from Earth. A light-year is the dis­tance light trav­els in a year. Com­put­er mod­els in­di­cate the star is eight mil­lion years old, very young for a star, as­tro­no­mers said; thus its com­pan­ion should al­so be no more than that old, since such ob­jects form around the same time.

At this age, the com­pan­ion should have cooled to less than 1300 de­grees Fahr­en­heit (1000 Kelv­in), said Ma­ma­jek and his col­league, Mi­chael Mey­er of the Un­ivers­ity of Ar­i­zo­na. But it’s meas­ured to have nearly twice that tem­per­a­ture. The re­search­ers say fric­tion from a re­cent col­li­sion might ex­plain the ex­tra heat.

“Most, if not all, plan­ets in our so­lar sys­tem were hit early in their his­to­ry. A col­li­sion cre­at­ed Earth’s moon and knocked Ura­nus on its side,” said Ma­ma­jek. “It’s quite likely that ma­jor col­li­sions hap­pen in oth­er young plan­e­tary sys­tems, too.” 

The ap­par­ent merged world is al­so 10 times faint­er than ex­pected for its tem­per­a­ture, as­tro­no­mers said. In 2006, as­tro­no­mers sug­gested the faint­ness is due to a dusty cloud around the star. Ma­ma­jek and Mey­er pro­pose an al­ter­na­tive ex­plana­t­ion: that the ob­ject is smaller than pre­vi­ously es­ti­mat­ed, a bit smaller than Sat­urn. But the new es­ti­mate makes it even tougher to ex­plain how it re­tained its heat so long, un­less one pos­tu­lates a ti­tan­ic col­li­sion, they said.

Our so­lar sys­tem’s plan­ets are be­lieved to have as­sem­bled from dust, rock, and gas, grad­u­ally grow­ing over mil­lions of years. But some­times, two plan­et-sized ob­jects can crash. The Moon is thought to have formed when an ob­ject about half the size of Mars struck the young Earth. 2M1207B might be the prod­uct of a col­li­sion be­tween a Sat­urn-sized gas gi­ant and a plan­et about three times Earth’s size, Ma­ma­jek and Mey­er said. The two worlds would have smacked to­geth­er and stuck, form­ing a larg­er plan­et still boil­ing from the heat gen­er­at­ed in the col­li­sion. 

The hy­poth­e­sis makes sev­er­al pre­dic­tions that as­tro­no­mers can test, the re­search­ers said. Chief among these is a low sur­face gra­vity, a quanti­ty that de­pends on a plan­et’s weight and width. To check this, as­tro­no­mers will need to bet­ter meas­ure the spec­trum of light from 2M1207B, Ma­ma­jek and Mey­ers said; more an­swers should be forth­com­ing with­in a year or two. 

Even if a crash does­n’t turn out to be the right ex­plana­t­ion, oth­er ex­am­ples of col­lid­ing plan­ets are likely to turn up with the next genera­t­ion of ground-based tele­scopes, Ma­ma­jek said: “I would­n’t be sur­prised if some­one finds a clear-cut case in the next 10 years.”

* * *

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A “mystery object” orbiting a distant star might have formed from the collision and merger of two developing planets, astronomers say. Researchers have long puzzled over what they call the object’s seemingly impossible combination of temperature, brightness, age and location. “This is a strange enough object that it needs a strange explanation,” said Eric Mamajek of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., one of two researchers who proposes the collision scenario based on new studies. The proposal was presented Wednesday at the annual meeting of the American Astronomical Society. The object, known as 2M1207B, orbits a so-called brown dwarf star that lies the direction of the constellation Centaurus, 170 light-years from Earth. A light-year is the distance light travels in a year. Computer models indicate the star is eight million years old, very young for a star, astronomers said; thus its companion should also be no more than that old, since such objects form around the same time. At this age, the companion should have cooled to a temperature of less than 1300 degrees Fahrenheit (1000 Kelvin), said Mamajek and his colleague, Michael Meyer of the University of Arizona. But observations show it’s nearly twice that temperature, they added. They postulate that the extra heat might be the result of friction from a recent collision. “Most, if not all, planets in our solar system were hit early in their history. A collision created Earth’s moon and knocked Uranus on its side,” said Mamajek. “It’s quite likely that major collisions happen in other young planetary systems, too.” The apparent merged world is also 10 times fainter than expected for its temperature, astronomers said. In 2006, astronomers suggested the faintness is due to a dusty cloud surrounding its star. Mamajek and Meyer propose an alternative explanation: that the object is smaller than previously estimated, a bit smaller than Saturn. The new estimate makes it even tougher to explain how it retained its heat so long, unless one postulates a titanic collision, they said. Our solar system’s planets are believed to have assembled from dust, rock, and gas, gradually growing over millions of years. But sometimes, two planet-sized objects can crash. The Moon is thought to have formed when an object about half the size of Mars struck the young Earth. 2M1207B might be the product of a collision between a Saturn-sized gas giant and a planet about three times Earth’s size, Mamajek and Meyer said. The two worlds would have smacked together and stuck, forming a larger planet still boiling from the heat generated in the collision. The hypothesis makes several predictions that astronomers can test, the researchers said. Chief among these is a low surface gravity, a quantity that depends on a planet’s weight and width. To check this, astronomers will need to better measure the spectrum of light from 2M1207B, Mamajek and Meyers said; more answers should be forthcoming within a year or two. Even if a crash doesn’t turn out to be the explanation in this case, other examples of colliding planets are likely to turn up with the next generation of ground-based telescopes, Mamajek said. “I wouldn’t be surprised if someone finds a clear-cut case in the next 10 years.”