The number of stars with possible planets keeps going up. The astronomy books I read as a kid operated under the assumption that we needed to look at Sun-like stars to find planets, and that meant single rather than double or triple systems. The tantalizingly close Alpha Centauri stars were all but ruled out because of their assumed disruptive effects on planetary orbits. No, find a nice G-class star all by itself and there you might have a solar system something like our own and, who knows, a second Earth.

Today we’re fitting binary stars into the planetary picture with ease. Astronomers see little reason to rule them out. Consider what David Trilling (University of Arizona) has to say about the matter in an upcoming paper: “There appears to be no bias against having planetary system formation in binary systems. There could be countless planets out there with two or more suns.” Just imagine the possible sunsets.

A double sunset

Image: Our solitary sunsets here on Earth might not be all that common in the grand scheme of things. New observations from NASA’s Spitzer Space Telescope have revealed that mature planetary systems — dusty disks of asteroids, comets, and possibly planets — are more frequent around close-knit twin, or binary, stars than single stars like our sun. That means sunsets like the one portrayed in this artist’s photo concept, and more famously in the movie Star Wars, might be quite commonplace in the universe. Credit: NASA/JPL-Caltech/R. Hurt (SSC).

Countless planets with two or more suns. I like that, evoking as it does the awestruck wonder I used to feel when looking at the Midwestern sky at night, the air crystalline and aswarm with stars that I was convinced sheltered planets. But that was conjecture. Today we have 200 confirmed exoplanets, of which some 50 orbit one member of a wide binary system. The work Trilling is involved in now looks at closer binaries and involves the hunt for the debris disks that indicate a possible planetary system.

Using the Spitzer Space Telescope, the team examined 69 binary systems between 50 and 200 light years from Earth. The results are heartening indeed: Forty percent of these systems have debris disks, a figure which is actually somewhat higher than the comparable number for single stars. The implication is that planetary systems are as common around binaries as they are around single stars.

Where the work really gets interesting is when you look at tight binaries, where stars are three or less AU apart. In cases like these, the Spitzer work finds debris disks even more frequent, with the disks orbiting both members of the stellar pair rather than just one. A planet orbiting a tight binary could have the kind of sunset Luke Skywalker saw in Star Wars, a video of which — interwoven with the Spitzer findings — can be seen here.

Where planets seem least likely to form is in binaries with intermediate spacing, defined here as between three and fifty AU. The Centauri stars show a mean separation of 23 AU, placing them squarely in this category. We’ve determined that stable planetary orbits exist around such stars, but the question now turns to whether planet formation is likely.

The Spitzer data from this study draw the matter into question, implying that the best scenario for planets is tight binaries or widely spaced ones. But be aware that an Italian study we’ll examine next week argues that there “…is no significant dependence of planet frequency on the binary separation, except for a lower value of frequency for close binaries.” Obviously, the issue of binary separation is still in play.

And note this interesting fact from the Trilling paper:

The incidence of debris disks around main-sequence A3-F8 binaries is marginally higher than that for single old AFGK stars. Whatever combination of nature (birth conditions of binary systems) and nurture (interactions between the two stars) drives the evolution of debris disks in binary systems, it is clear that planetesimal formation is not inhibited to any great degree.

There goes our old G-class Sun-centrism again (forgive the coinage). For in addition to finding them in binary systems, we’re seeing debris disks, clear markers of possible planet formation, occurring around stars in a wide range of spectral types. The paper is Trilling et al., “Debris Disks in Main-Sequence Binary Systems,” Astrophysical Journal 658 (April 1, 2007), pp. 1289-1311, with abstract here.