Kepler-47 Is Amazing

Figure 1. Kepler-47, a binary consisting of one Sun-like star and one M dwarf, orbited
by two planets similar to Neptune & Uranus. Credit: NASA/JPL-Caltech/T. Pyle

Now we know why the Kepler team let Kepler-38 squeak out with no hoopla whatsoever. They were saving their thunder for Kepler-47, announced just today with a flourish of press conferences, news headlines, artist’s renditions (Figure 1), videos, and a publication in the high-profile journal Science. As with Kepler-38, the lead author for Kepler-47’s discovery paper is Jerome Orosz.

And what makes Kepler-47 so special? It’s the first circumbinary system with more than one planet. We now have a binary with a true planetary system. And not only that – we also have our first circumbinary planet (Kepler-47b) with a radius smaller than Neptune’s. And what’s more – the tendency for circumbinaries to be smaller and less massive than Saturn continues, offering the possibility that we may eventually find Earthlike planets around binary stars.

The virtual ink on my summary table of binary systems was barely dry, but I’ve already had to update it to accommodate new data. Now that’s exoplanetary science for the twenty-first century!

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, Orosz et al. 2012a, Orosz et al. 2012b, and the Kepler Mission site.

Kepler-47 fits seamlessly into the accumulating sample of circumbinaries. Like Kepler-16 and Kepler-38, the binary pair consists of one Sun-like star (very similar in mass to our Sun, and probably sharing a similar spectral type) and one M dwarf. The larger and cooler of the two planets, Kepler-47c, is similar in radius to Kepler-38b, and similar in period and semimajor axis to Kepler-34b. Both planets occupy an orbital space resembling that of the four inner planets of our Solar System, as well as that of the four other circumbinary planets.

But we can also find some firsts, along with some superlatives. Kepler-47b is the first known circumbinary planet that is almost certainly less massive than Uranus, given its radius of about 3 Rea. It also has the smallest semimajor axis, the shortest period, and the lowest eccentricity. And although the previous four circumbinaries all orbited very near their system’s main sequence habitable zones, Kepler-47c is the first such planet that clearly occupies that region.

Unfortunately, given its super-Uranian radius, planet c is undoubtedly a gas dwarf with a hydrogen atmosphere, and thus not a suitable environment for oceans, coral reefs, polar bears, or roses. Nevertheless, the discovery paper throws us an astrobiological bone: “large moons, if present, would be interesting worlds to investigate” in terms of habitability (Orosz et al. 2012b). Who knows, maybe we will eventually have enough transit data to identify such moons – see Kipping et al. 2012.

The binary primary also displays unique features, since the two stars’ shared orbit has the shortest period and the smallest separation and eccentricity of the entire sample. I predicted just the other day that planetary eccentricity would scale with binary eccentricity, but so far the jury is out on Kepler-47. The inner planet follows the trend, but the outer planet’s orbit is not yet sufficiently constrained to determine its true configuration.

Like Kepler-38b, and unlike the first three circumbinary planets, Kepler-47b and c have been observed only in transit across the larger of the two stars. Also like Kepler-38b, it is currently impossible to set a firm value on the two new planet’s masses. We can be reasonably sure, nevertheless, that they are not rocky planets like Earth and Venus, given radii that place them in the family of gas dwarfs (see Between Earth and Uranus, PartII).

Orosz and colleagues hold out the possibility that Kepler-47 may host more planets, given the presence of additional “transit-like” signals in the data. Nevertheless, they caution that the larger star is prone to starspots, which muddle our chances of attaining high-precision analyses. Still, as they note, “previously detected transiting circumbinary planet systems show no evidence for more than a single planet,” so Kepler-47 should maintain its halo of magic and mystery for at least a little while longer.

If previous years are any guide, today’s splashy news is just the first of many wonders to come, given the tendency of exoplanetary teams to save their goodies for an autumn release, in somewhat the same way that Hollywood saves most of its Oscar candidates for Christmas. Stay tuned.

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.

Kipping DM, Bakos GA, Buchhave L, Nesvorny D, Schmitt A. (2012) The hunt for exomoons with Kepler (HEK). I. Description of a new observational project. Astrophysical Journal 750, 115.

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

Orosz JA, Welsh WF, Carter JA, Fabrycky DC, Cochran WD, Endl M, and 33 others (2012b) Kepler-47: A Transiting Circumbinary Multiplanet System. SciencExpress 28 August 2012.

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.