In the Star Wars
saga, the Skywalker clan has its roots on Tatooine - a desert-covered planet revolving
around two suns. A theoretical investigation has explored the likelihood for
worlds like this to exist.
And it looks like the
nearest Tatooine may be closer than a galaxy far, far away.
That's because more than
half of the stars in our galaxy have a stellar companion. And yet, of the 130
or 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 to
avoid binaries because the star-star interactions can hide the planet
signatures.
Scientists discussed the
issue earlier this month at a gathering of exoplanet hunters at the Space
Telescope Science Institute in Baltimore.
Bad to good
"A few years ago, it
was thought that [binaries] were a very bad site to search for planets,"
says Michel Mayor of the Observatoire de Geneve. "So we carefully
eliminated all binary stars from our sample."
But planets may be just as
likely around binaries as around single stars. Recent numerical simulations
have shown that Earth-like planets, known as terrestrials, form readily in
double star systems.
"The most significant
thing we found is that terrestrial planets around certain close and wide
binaries can look similar to planets around a single star," said Jack
Lissauer of the NASA Ames Research Center.
Wide binaries are those in
which the two stars are separated by several astronomical units (AU), which is
the distance between the Sun and the Earth. Planets could orbit around one of
the pair, or each separately. So far, all the stellar binaries with exoplanets
are wide binaries.
But close binaries, where
the stars are less than about an AU apart, can potentially have planets in
orbit around both stars - presumably as is the case for Tatooine. These
planets, however, will be much harder to detect.
Lissauer and his collaborators
have explored what binary star systems are favorable for planet formation.
These limits could be useful in future planet searches.
Simulations
The researchers used
computer models that start with 14 large planet "embryos" and 140
smaller planetesimals in orbit around one star or both stars of a binary.
Evolution of this material is influenced by gravity and collisions. The models
are followed for the equivalent of about one billion years.
"All of our
simulations have been able to form terrestrial planets," said Ames
researcher Elisa Quintana, who presented a poster on these results at the
symposium.
But not all of the models
produce planets around 1 AU, which is often thought to be the most likely habitable zone for life. Quintana varied how the
two stars revolve around each other to see what configurations allowed for
stable 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 ran
simulations that mimicked Alpha Centauri - the nearest binary system to
Earth, where the closest the two stars come is about 11 AU. The secondary star
apparently acts like Jupiter does in our solar system - limiting how far out
planets can form. The results showed several terrestrial planets were possible
around either of the stars.
Planets have not yet been
seen in the Alpha Centauri system, but small mass planets cannot yet be ruled
out.
For close binaries, if the
two stars are about 0.1 AU apart, the planets that form are indistinguishable
from those seen in simulations with only one star. But as this separation
increases, or the orbit becomes highly non-circular, it is harder for
Earth-like planets to exist.
"Perturbations from
the stellar motions can eject matter into space or into one of the stars,"
Quintana said.
The simulation results can
inform observers which binaries might be better targets for their telescopes.
Observational hurdles
That said, it will not be
easy to see a planet around a binary, especially those where the stars are
close to each other. Most planets have been found by the radial velocity technique that searches for Doppler shifts in
the light spectra of stars.
"Finding the wobble
from a planet in a stellar spectrum is hard enough without having another star
orbiting the one you are looking at," Quintana said.
An alternative way of
detecting 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 planets
around close binaries, but "there are complications."
For one thing, two stars
are putting out light, so the eclipse of one star is less noticeable. Also, the
transit searches look for certain patterns of dimming and brightening of a
star. If there are two stars in a tight orbit, this pattern will be different,
so special algorithms will be needed.
But there are situations
where a binary could provide an advantage for detecting planets. If the two
stars eclipse each other, a planet could change when this eclipse happens.
"If the timing of the
eclipses is not periodic, maybe a planet is to blame," Lissauer said.
Besides the possibility of transit timing, eclipsing binaries make good
targets because planets - if they exist - will likely orbit in the same plane
as the two stars - meaning they will also eclipse the stars at some point.
Which of these detection
methods will be most likely to find the first Tatooine-like planet? Lissauer is
unwilling to say.
"Predictions are
tricky because they deal with the future," he joked.
advertisement
