"Long before it's in the papers"
November 11, 2009

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Stars’ chemistry could give away planetary presence

Nov. 11, 2009
Courtesy ESO
and World Science staff

A sur­vey of stars with and with­out plan­ets has turned up an easy way to de­tect which ones have them, as­tro­no­mers have found: the stel­lar chem­is­try of­ten gives it away.

Re­search­ers said the find­ing not only could lead to big sav­ings of mon­ey and time in fu­ture plan­e­tary searches, it al­so points to a so­lu­tion to a long­stand­ing rid­dle: why our Sun has much less of the el­e­ment lith­i­um than most oth­er stars an­a­lyzed.

Instruments used in re­search that pro­poses a new way to find plan­ets based on the lith­ium abund­ance in their star. At up­per left is the Eu­ro­pe­an South­ern Ob­ser­va­to­ry’s 3.6-meter tel­e­scope at La Silla moun­tain, Chil­e. At up­per right is the tel­e­scope it­self. Be­low is the at­tached High Ac­cu­ra­cy Ra­di­al Ve­loc­i­ty Plan­et Search­er, part­ly open so that some of the high-precision com­po­nents in­side can be seen. The in­stru­ment is de­signed to de­tect plan­ets in or­bit around stars by means of ac­cu­rate ve­loc­i­ty mea­sure­ments.

It turns out this is “be­cause it has plan­ets,” Garik Is­raelian of the As­t­ro­phys­ics In­sti­tute of the Ca­nar­ies in Ten­er­ife, Spain, lead au­thor of a study on the find­ings pub­lished this week in the re­seach jour­nal Na­ture.

Low lith­i­um lev­els have been no­ticed for dec­ades in the Sun, and as­tro­no­mers won­dered why, Is­raelian said. Mean­while, “For al­most 10 years we have tried to find out what dis­tin­guishes stars with plan­e­tary sys­tems from their bar­ren cousins.” 

The two ques­tions, it now seems, are in­ter­re­lat­ed.

The con­clu­sions were based on an anal­y­sis of 500 stars, in­clud­ing 70 plan­et-host­ing stars, that were stud­ied with an in­stru­ment called a spec­trograph at­tached to the Eu­ro­pe­an South­ern Ob­ser­va­to­ry’s 3.6-meter tel­e­scope at La Silla moun­tain, Chil­e.

The as­tro­no­mers looked in par­tic­u­lar at Sun-like stars, al­most a quar­ter of the whole sam­ple. They found that the ma­jor­ity of stars host­ing plan­ets have less than 1 per­cent of the amount of lith­i­um shown by most of the oth­er stars. 

It’s be­lieved that stars don’t pro­duce sig­nif­i­cant amounts of lith­i­um on their own, but in­stead simply in­her­it some of the lith­i­um formed at the birth of the un­iverse. Most stars there­fore have about the same amount of lith­i­um, and it stays there, un­less some­thing hap­pens to de­stroy it.

Planet-bearing stars, thus, have ap­par­ently “been very ef­fi­cient at de­stroying the lith­i­um,” said re­search team mem­ber Nuno San­tos of the Un­iver­si­dade de Porto, Por­tu­gal. “We can al­so prove that the rea­son for this lith­i­um re­duc­tion is not re­lat­ed to any oth­er prop­er­ty of the star, such as its age.”

Astro­nom­ers can ana­lyze the chem­istry of a star based on the light it gives off.

Scientists aren’t sure ex­actly why the pres­ence of plan­ets would pro­voke lith­i­um de­struc­tion. “There are sev­er­al ways in which a plan­et can dis­turb the in­ter­nal mo­tions of mat­ter in its host star, there­by re­ar­range the dis­tri­bu­tion of the var­i­ous chem­i­cal el­e­ments and pos­sibly cause the de­struc­tion of lith­i­um,” said Mi­chel May­or of the Ob­serv­a­to­ry of Ge­ne­va Un­ivers­ity, Switz­er­land, a par­ti­ci­pant in the re­search. The ques­tion needs fur­ther work, he added.

The Eu­ro­pe­an South­ern Ob­serv­a­to­ry, a proj­ect sup­ported by 14 Eu­ro­pe­an coun­tries, car­ries out an am­bi­tious pro­gram fo­cused on the de­sign, con­struc­tion and opera­t­ion of pow­er­ful ground-based tel­e­scopes.

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A survey of stars with and without planets has turned up an easy way to detect which ones have them, astronomers have found: the stellar chemistry often gives it away. Researchers said the finding not only could lead to big savings of money and time in future planetary searches, it also has pointed a way to a solution to a longstanding riddle. Astronomers have wondered why our Sun has much less of the element lithium than other stars analyzed. It turns out this is “because it has planets,” Garik Israelian of the Astrophysics Institute of the Canaries in Tenerife, Spain, lead author of a study on the findings published this week in the reseach journal Nature. Low lithium levels have been noticed for decades in the Sun, and astronomers wondered why, Israelian said. Meanwhile, “For almost 10 years we have tried to find out what distinguishes stars with planetary systems from their barren cousins.” The two questions, it now seems, are are interrelated. The conclusions were based on an analysis of 500 stars, including 70 planet-hosting stars, that were studied with an instrument called a spectrometer that analyzes stellar chemistry based on light. The instrument is attached to the European Southern Observatory’s 3.6-meter telescope at La Silla mountain, Chile. The astronomers looked in particular at Sun-like stars, almost a quarter of the whole sample. They found that the majority of stars hosting planets have less than 1% of the amount of lithium shown by most of the other stars. It’s believed that stars don’t produce significant amounts of lithium on their own, but instead simply inherit some of the lithium that was formed at the birth of the universe. Most stars therefore generally have the same amount of lithium, and it stays there, unless something happens to destroy it. Planet-bearing stars, thus, have apparently “been very efficient at destroying the lithium,” said research team member Nuno Santos of the Universidade de Porto, Portugal. “We can also prove that the reason for this lithium reduction is not related to any other property of the star, such as its age.” Astronomers still aren’t sure exactly why the presence of planets would provoke lithium destruction. “There are several ways in which a planet can disturb the internal motions of matter in its host star, thereby rearrange the distribution of the various chemical elements and possibly cause the destruction of lithium,” said Michel Mayor of the Observatory of Geneva University, Switzerland, a participant in the research. The question needs further work, he added. The European Southern Observatory, a project supported by 14 European countries carries out an ambitious program focused on the design, construction and operation of powerful ground-based telescopes.