Binary Vision

Double sunset on an imaginary Earthlike planet orbiting a pair of G-type stars with a small separation. This image was created to announce the publication of Trilling et al., "Debris Disks in Binary Systems" (Astrophysical Journal, 658: 1289-1311; April 1, 2007). Trilling's group reported the remarkable finding that debris disks -- the remnants of planet-forming nebulae -- are marginally more common around close binary stars (semimajor axis < 3 AU) than around single stars. Some commentators then jumped to the conclusion that more habitable worlds may form around close binaries than around single stars. However, as simulations by Quintana & Lissauer (2006) have shown, planetary systems centered around two suns can form only if the binary separation is less than 0.2 AU and the binary orbit is circular. Thus these postulated systems must be quite rare. Image courtesy NASA/JPL-Caltech/University of Arizona/R. Hurt.

Three suns would be even more exotic than two, and in fact one of Sol's close neighbors is a spectacular triple star system known as Omicron Eridani (also 40 Eridani and Keid), located only 5 parsecs away (16.3 light years). The system consists one orange K1 star of about 0.89 Msol accompanied by a pair of lower-mass stars at an orbital separation of about 400 AU. This distant binary contains of a relatively small white dwarf star and a still smaller red dwarf of class M4.5 and mass 0.20 Msol. (Data from SolStation.com.) The imaginative rendition, above, depicts a hypothetical Earth-like planet in orbit around the orange star, with the distant binary pair appearing much larger and brighter above the horizon than they actually would. The Omicron Eridani system has acquired a degree of pop-culture celebrity as the fictional home of Mr. Spock and the race of Vulcans in the Star Trek universe. Image copyright Paddy McGee.

Only 7 parsecs away (23 light years) is the triple star system of Gliese 667 ABC, which consists of 2 K dwarfs in a close binary orbit circled by a distant M dwarf (GJ 667C). The European Southern Observatory recently announced the discovery of a rocky or icy Super Earth planet accompanying the M dwarf star. The new planet has a minimum mass of 5.7 Earth masses and an orbital period of about 7 days. Such a tight orbit guarantees that the starward hemisphere of GJ 667C b will be infernally hot. (This artist's view of the planet, however, suggests a balmier, more Earthlike environment.) Visible in the distance is the central pair of K-type stars; the planet's red primary star is out of frame. More information is available here. Image credit: European Southern Observatory/L. Calcada.

In 2000, a large brown dwarf of about 50 Mjup was discovered in a distant orbit (semimajor axis ~1500 AU) around the three central stars (Burgasser et al. 2000). The brown dwarf's orbital configuration has not yet been reliably constrained (a rough estimate of its period is 40,000 years), and its evolutionary origin remains unknown. The entire system of Gliese 570 is depicted above in a painting by Robert Hurt (courtesy NASA/JPL-Caltech/R. Hurt). The brown dwarf appears in the foreground, accompanied by a hypothetical rocky planetoid.

The system of ADS 16402, located at a distance of 139 parsecs (453 light years), consists of a pair of almost identical G0 stars with a wide separation of 1550 AU. Star B has one detected planet known as HAT-P-1b, a transiting Hot Jupiter of 0.53 Mjup with a puffed-up radius and an orbital period of only 4.5 days (see Bakos et al. 2007). Image courtesy David A. Aguilar - Center for Astrophysics.