Circumbinary Number Four

 Figure 1. Massive planet orbiting a binary consisting of one Sun-like star and one M dwarf.
Credit: David A. Aguilar, Harvard-Smithsonian Center for Astrophysics.

The recent Summer Olympics in London reminded us that coming in first is very different from coming in fourth. Both the difference and the distinction were confirmed just a week after the closing ceremonies, with the quiet appearance of a preprint by Jerome Orosz and colleagues reporting the detection of Kepler-38b, the fourth known planet to orbit both members of a binary star system. 

 Figure 2. Orbital configuration of the Kepler-38 planetary system. A is a Sun-like star; B is an M dwarf; and b is a likely gas dwarf planet similar in size to Uranus & Neptune. Credit: NASA

The first circumbinary planet was Kepler-16b, announced with much fanfare in the autumn of 2011, just shy of one year ago. The discovery paper by Laurance Doyle and colleagues (including many on the same team that announced Kepler-38b) was embargoed in advance of its splashy publication in the September 16 issue of Science, with simultaneous headlines in all the major media and nifty videos on YouTube and Vimeo.

For circumbinary number four, however, media coverage has been nonexistent, and we have yet to see any artist’s renditions or videos relating to Kepler-38b. (Searching for them was nevertheless fun – check out this beautiful new video of the ensemble of Kepler candidates by Alex Parker, a postdoctoral researcher at the Harvard-Smithsonian Center for Astrophysics.)

Although numbers are still small, we can begin to identify trends in the data:

Figure 2. Data on all known circumbinary systems. Star masses are given in units of Solar mass (Msol); binary and planet periods are given in days; semimajor axes (a) are given in astronomical units (AU; 1 AU = 93 million miles); planet masses are given in Earth masses (Mea); planet radii are given in Earth radii (Rea); and system distances are given in parsecs (1 parsec = 3.26 light years). All data from Welsh et al. 2012 and Orosz et al. 2012.

In all cases, the largest object in the system is a Sun-like star in the mass range of spectral types G and K (0.65-1.05 Msol). In half of these systems, the stellar companion is another Sun-like star, while in the other half the companion is an M dwarf. Binary semimajor axes are small, ranging from about 0.10 AU to 0.25 AU, while orbital eccentricities are surprisingly diverse, ranging from 0.10 to 0.52.

All four circumbinary planets are smaller than Saturn but larger than Uranus and Neptune, meaning that they occupy a valley in the distribution of exoplanetary radii. This distribution exhibits one peak around 3 Rea and another around 13 Rea (see Figure 8 of Between Earth & Uranus, Part II). Three out of four circumbinaries qualify as compact, lightweight gas giants on the basis of their masses (40-110 Mea) and bulk compositions (more than 50% hydrogen), while Kepler-38b is still smaller and probably even less massive. By contrast, the median mass of all known extrasolar gas giants is about 480 Mea (1.5 Jupiter masses or Mjup). Since a Jupiter-sized planet (~12 Rea) would be much easier to observe in transit than any of the known circumbinaries, the tendency for such planets to be small is now well established (Orosz et al. 2012).

All four circumbinaries have semimajor axes between 0.45 AU and 1.1 AU (a parameter space shared by Mercury, Venus, and Earth) and all have eccentricities in the range of 0.01-0.20 (comparable to the range of 0.007-0.206 for the same three planets). So far, the shape of each planet’s orbit seems to correlate with that of the parent binary, so that more eccentric binaries host more eccentric planets.

Notably, all known circumbinaries have orbital periods that are “only slightly longer than the minimum needed to guarantee dynamical stability” (Orosz et al. 2012). A Jupiter-sized planet would need a considerably wider orbit for long-term stability, but such a configuration would make potential transits even less likely to be detectable.  

The known circumbinaries therefore comprise a fairly homogeneous sample. Kepler-38b is a bit of an outlier, primarily because of its small radius, which is only about 50% that of the largest known circumbinary (Kepler-34b). It also has the shortest period, the smallest semimajor axis, the smallest eccentricity, and the smallest and least eccentric binary orbit of the circumbinary planets discovered to date.

Unfortunately, the transit data for Kepler-38b are inferior in quality to those of its three siblings. In particular, the planet is observed to transit only the primary star (Kepler-38 A) and not the secondary (Kepler-38 B). Further, although occultations of the planet by Star A are known to occur, they are “undetectable given the noise level” (Orosz et al. 2012). Thus we have fewer constraints on this system than we do for the other three.

The shortcomings in the available data are most vividly expressed by the fact that the mass of Kepler-38b, and thus its bulk composition, remain unknown. Although the discovery team was able to place an upper limit of 122 Mea on the planet’s mass, its known radius of 4.35 Rea is actually more informative anyhow, since this value demands at least a minimal contribution from hydrogen. Notably, the planet Kepler-11e has a radius of 4.52 Rea and a mass of only about 8 Mea, with its radius inflated by a deep atmosphere of low molecular weight. Kepler-38b could be even less massive, with an even deeper atmosphere, or it could be so enriched in metals as to attain a mass 10 times larger. Alternatively, this planet could be similar in composition to Uranus and Neptune. Such is the interpretation favored by the discovery team, who propose a “reasonable mass” of 21 Mea on the basis of empirical mass-radius relations (Orosz et al. 2012).

The most recent release of Kepler data, on July 28, has yet to filter into the astrosphere, while the next release is only two months away, on October 28. We can anticipate many more exciting Kepler planets to be announced (and others to be more precisely constrained) between now and the end of the year.

REFERENCES

Doyle L, Carter JA, Fabrycky DC, Slawson RW, Howell SB, Winn JN, Orosz JA, and 42 others. (2011) Kepler-16: A Transiting Circumbinary Planet. Science 333, 1602-1606.

Orosz JA, Welsh WF, Carter JA, Brugamyer E, Buchhave LA, Cochran WD, and 25 others. (2012) The Neptune-Sized Circumbinary Planet Kepler-38b. Astrophysical Journal, in press. Abstract: http://adsabs.harvard.edu/abs/2012arXiv1208.3712O.

Welsh WF, Orosz JA, Carter JA, Fabrycky DC, Ford EB, Lissauer JJ, et al. (2012) Transiting circumbinary planets Kepler-34 b and Kepler-35 b. Nature 481, 475-479. With online supplementary material.