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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 an Astrophysicist

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