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The Question
(Submitted January 22, 1998)
Binary stars are popular in science fiction. Star Wars'
Tatooine, for example.
I'm wondering about the orbit of a planet around binary
stars. Is it possible?
Would the greater combined mass of two stars be more likely
to pull in surrounding material, hence making the formation
of planets less likely in a binary system?
Would the combined heat and radiation from binary stars
mean that habitable planets would have to have a VERY
large orbital radius?
The Answer
There are stable orbits for planets in binary star systems. There are
various stability criteria which say when an orbit is stable. One such
criteria (and I don't know the actual numbers) says that if all orbits are
circular and the stars are the same size, then the planet must orbit one of
them at less than /some fraction/ of the inter-star distance, or must orbit
both combined at more than /whatever/ times the inter-star distance.
Figure-eight orbits are unstable, and can eject the planet from the system.
If you have two Sun-like stars at the center of the system, a planet would
be the same temperature as Earth if it were at sqrt(2) = 1.4 astronomical
units away, rather than Earth's 1 AU. This distance is closer than Mars's
orbit (1.6 AU). Most stars are dimmer than our Sun, so the orbit could be
even smaller.
The high energy astronomers at NASA don't know much about this subject, so
we asked an expert: Eric Mamajek of Pennsylvania State University:
The solar-like stars 16 Cygni B and 55 Cancri A have been found to have
Jupiter-size extrasolar planets orbiting them. So we do have indirect
proof, through Doppler spectroscopy methods (Marcy & Butler, SFSU, Lick
Observatory), that planets indeed form in binary systems.
The formation mechanisms for forming stars and planets are very
different. Planets require accretion to form, specifically accretion in a
protoplanetary disk around a young star. Stars can form from the collapse
of a molecular cloud core on their own, however planets can only form in
the disk around a star. (Pulsar planets are likely formed "posthumously"
around pulsars, and are a different beast all together). The main problem
with forming planets in multiple star systems is dynamic ejection...
stars can simply toss planetesimals out of the system all together (or
even accrete them). An example of this is the Kirkwood gaps in the
asteroid belt where Jupiter doesn't allow asteroids to exist in certain
orbits, and conversely it "shepherds" asteroids in to certain other
orbits. A companion star would have a similar effect, except there would
be a lot less "shepherding" orbits. The vast majority of binary stars have
eccentric orbits. It is difficult for bodies to exist in a system with
two very massive bodies in an eccentric orbit. They can only exist very
close to each star, or very far from both stars.
An excellent example in the lines of the Tatooine example is the nearby
solar-like stars Alpha Centauri A and B. They orbit each other at an
average distance of 23 AU, however the eccentricities of each orbit bring
them to as close to 11 AU and as far as 35 AU. Numerical simulations by
Paul Weigert at University of Toronto have shown that each star has a
"safe zone" about 3 AU in radius in which planets could safely survive
for billions of years. Objects placed further out from each star than
about 3 AU are dynamically ejected in a matter of millions of years or
less. Alpha Cen A is about 1.5 times as luminous as our Sun, and Alpha
Cen B is about .45 times as luminous as our Sun, and if you do the simple
physics, one can see that a "habitable zone" exists around BOTH stars
within the 3 AU dynamic "safe zone." Indeed, it could be possible that
BOTH Alpha Cen A and B have planets conducive to life. Theoretical models
age them anywhere from 3-8 Gyr... plenty of time for life to develop if
the planets have the right conditions...
David Palmer and Maggie Masetti
for Ask a High-Energy Astronomer
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