The movie Star Wars showed double suns on the planet Tatooine, and science fiction novels have for years postulated worlds with double or multiple suns. A planet with double suns is now a reality. A paper published in the September 16, 2011 issue of Science describes a planet, Kepler-16b, orbiting a binary star system. The team of 49 scientists led by Laurance Doyle discovered this first circumbinary planet using NASA's Kepler spacecraft.
NASA's Kepler Mission
The Kepler mission is designed to discover extrasolar planets by repeatedly measuring the brightness of 155,000 stars in the direction of the summer triangle, in the constellations Cygnus and Lyra. If a planet passes in front of one of the stars (transits the star), the star's brightness will dim slightly. A star's brightness can also decrease if one of the stars in a binary system eclipses the other. Astronomers call such systems eclipsing binaries.
Eclipsing Binary, Kepler-16
Kepler mission data initially identified Kepler-16 as an eclipsing binary system. The hotter (4450 Kelvin) primary star (Kepler-16A) has 0.69 times the mass of the Sun and 0.65 times the Sun's radius. The cooler secondary star (Kepler-16B) has 0.20 times the Sun's mass and 0.23 times the Sun's radius. (Notice that the stars in a binary system are designated by upper case letters and the planets by lower case letters.) In Kepler-16 the hotter primary star is also the larger and more massive star, but that is not the case in all eclipsing binary systems.
During the primary eclipse, which occurs when the primary star is partially eclipsed by the secondary star, Kepler-16's brightness decreases by 13%. During the secondary eclipse, the secondary star is completely eclipsed by the primary star and the total brightness decreases by only about 1.6%. This small decrease tells astronomers that the secondary star is both small in size and a very cool faint star. The times of the primary and secondary eclipses indicate that the two stars in Kepler-16 orbit each other every 41 days in an elliptical orbit. The stars are an average of about 33.6 million kilometers (about 21 million miles) apart. In more technical terms: the orbital semi-major axis is 33.6 million kilometers.
Discovering a Planet Orbiting Kepler-16
Had Kepler-16 only had the primary and secondary eclipses, the Kepler scientists, primarily interested in discovering new planets, would have paid scant attention. The data however also revealed tertiary eclipses that decreased the system's brightness by 1.7% as a third object in the Kepler-16 system transited the primary star.
Finally astronomers observed much weaker quaternary eclipses that decreased Kepler-16's brightness by only 0.1%, when the third body transited the secondary star. The times of these eclipses told astronomers that the third body orbits the entire binary system every 229 days at an average distance of about 106 million kilometers (65 million miles) from the stars. This orbit is well outside the stellar orbits.
Studying the eclipses allows astronomers to deduce some, but not all, of the properties of the stars in an eclipsing binary system. A more complete picture emerges when astronomers combine eclipse data with spectral data. The Doppler shifts in the stellar spectra tell astronomers the stellar orbital speeds and allow them to more accurately deduce the stellar properties, particularly the stellar masses.
Astronomers used a 1.5 meter telescope on Mount Hopkins near Tucson, AZ to observe spectra of Kepler-16. The secondary star was too faint for this telescope to detect its spectrum, so astronomers got less information than would have been possible had they detected both stellar spectra. For example they were unable to obtain the temperature of Kepler-16B, the secondary star. Combining the ground based spectral data with the Kepler satellite eclipse data, however, allowed Doyle's team to deduce a fairly complete picture of the Kepler-16 system.
Both Kepler-16A and Kepler-16B are low mass cool red dwarf stars. The brighter sun visible from Kepler-16b would be a little redder than our Sun, while the fainter sun would glow with a faint red light.
Despite comparisons to the fictional planet Tatooine, Kepler-16b is really more similar to Saturn than Tatooine. Kepler-16b has about one third the mass and about 3/4 the radius of Jupiter. Unlike Tatooine, Kepler-16b is not likely habitable. Like Saturn, however, its density is less than that of water. Hence if there were a sufficiently humongous bathtub, Kepler-16b would float.
The planet orbiting two stars in Kepler-16 has received the most media attention and is most appealing to the general public. There is however more of significance in the Kepler-16 discovery. The two stars in Kepler-16 both have very small masses. Very low mass stars in an eclipsing binary system are rare. Astronomers can only measure stellar masses when the stars are in such binary systems. Kepler-16 has therefore presented astronomers an unusual opportunity to directly measure the properties, such as the mass, of very low mass stars.
Binary stars are sufficiently common that the existence of circumbinary planets is not surprising. However knowing that something is likely to exist is less satisfying than actually discovering it. Astronomers have finally discovered a planet with two suns.
Further Reading
Doyle, L.R., et al., 2011, Science, v. 133, p. 1602.
Detecting Planets Orbiting Stars
Paul A. Heckert - I have a Ph.D. in astrophysics, over 30 years experience teaching physics and astronomy, and over 60 published research articles.
