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Planets with Two Suns Likely Common

In the Star Warssaga, the Skywalker clan has its roots on Tatooine - a desert-covered planet revolvingaround two suns. A theoretical investigation has explored the likelihood forworlds like this to exist.

And it looks like thenearest Tatooine may be closer than a galaxy far, far away.

That's because more thanhalf of the stars in our galaxy have a stellar companion. And yet, of the 130or so currently known exoplanets (none of which are Earth-like),only about 20 of them are around so-called binaries. The percentage may grow higher.The current ratio is affected by an observational bias: planet hunters tend toavoid binaries because the star-star interactions can hide the planetsignatures.

Scientists discussed theissue earlier this month at a gathering of exoplanet hunters at the SpaceTelescope Science Institute in Baltimore.

Bad to good

"A few years ago, itwas thought that [binaries] were a very bad site to search for planets,"says Michel Mayor of the Observatoire de Geneve. "So we carefullyeliminated all binary stars from our sample."

But planets may be just aslikely around binaries as around single stars. Recent numerical simulationshave shown that Earth-like planets, known as terrestrials, form readily indouble star systems.

"The most significantthing we found is that terrestrial planets around certain close and widebinaries can look similar to planets around a single star," said JackLissauer of the NASA Ames Research Center.

Wide binaries are those inwhich the two stars are separated by several astronomical units (AU), which isthe distance between the Sun and the Earth. Planets could orbit around one ofthe pair, or each separately. So far, all the stellar binaries with exoplanetsare wide binaries.

But close binaries, wherethe stars are less than about an AU apart, can potentially have planets inorbit around both stars - presumably as is the case for Tatooine. Theseplanets, however, will be much harder to detect.

Lissauer and his collaboratorshave explored what binary star systems are favorable for planet formation.These limits could be useful in future planet searches.

Simulations

The researchers usedcomputer models that start with 14 large planet "embryos" and 140smaller planetesimals in orbit around one star or both stars of a binary.Evolution of this material is influenced by gravity and collisions. The modelsare followed for the equivalent of about one billion years.

"All of oursimulations have been able to form terrestrial planets," said Amesresearcher Elisa Quintana, who presented a poster on these results at thesymposium.

But not all of the modelsproduce planets around 1 AU, which is often thought to be the most likely habitable zone for life. Quintana varied how thetwo stars revolve around each other to see what configurations allowed forstable planet orbits inside 1 AU.

For wide binaries,Earth-like planets formed as long as the two stars came no closer than 7 AU.Quintana said that about 50 percent of known binaries meet this constraint.

The research group also ransimulations that mimicked Alpha Centauri - the nearest binary system toEarth, where the closest the two stars come is about 11 AU. The secondary starapparently acts like Jupiter does in our solar system - limiting how far outplanets can form. The results showed several terrestrial planets were possiblearound either of the stars.

Planets have not yet beenseen in the Alpha Centauri system, but small mass planets cannot yet be ruledout.

For close binaries, if thetwo stars are about 0.1 AU apart, the planets that form are indistinguishablefrom those seen in simulations with only one star. But as this separationincreases, or the orbit becomes highly non-circular, it is harder forEarth-like planets to exist.

"Perturbations fromthe stellar motions can eject matter into space or into one of the stars,"Quintana said.

The simulation results caninform observers which binaries might be better targets for their telescopes.

Observational hurdles

That said, it will not beeasy to see a planet around a binary, especially those where the stars areclose to each other. Most planets have been found by the radial velocity technique that searches for Doppler shifts inthe light spectra of stars.

"Finding the wobblefrom a planet in a stellar spectrum is hard enough without having another starorbiting the one you are looking at," Quintana said.

An alternative way ofdetecting planets is to look for the eclipse, or transit, of a planet in front of a star.Lissauer said that transiting searches could potentially discover planetsaround close binaries, but "there are complications."

For one thing, two starsare putting out light, so the eclipse of one star is less noticeable. Also, thetransit searches look for certain patterns of dimming and brightening of astar. If there are two stars in a tight orbit, this pattern will be different,so special algorithms will be needed.

But there are situationswhere a binary could provide an advantage for detecting planets. If the twostars eclipse each other, a planet could change when this eclipse happens.

"If the timing of theeclipses is not periodic, maybe a planet is to blame," Lissauer said.

Besides the possibility of transit timing, eclipsing binaries make goodtargets because planets - if they exist - will likely orbit in the same planeas the two stars - meaning they will also eclipse the stars at some point.

Which of these detectionmethods will be most likely to find the first Tatooine-like planet? Lissauer isunwilling to say.

"Predictions aretricky because they deal with the future," he joked.

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