The confirmation of a planet circling two stars, recounted in these pages yesterday, is actually the result of a long process. Jean Schneider (CNRS/LUTH – Paris Observatory) noted in a follow-up comment to the Kepler-16b story that investigation of such systems dates back to 1990 (see citation below), while Alex Tolley has pointed out that the great space artist Chesley Bonestell was painting imaginary planets orbiting binary stars fully sixty years ago. So the idea isn’t new, but the confirmation was obviously useful, and in more ways than we might have expected.

For one thing emerging from the Kepler-16b paper is that the smaller of the two stars in this binary system, an M-dwarf, is now the smallest low-mass star to have both its mass and radius measured at such precision. The question of stellar mass and M-dwarfs is significant because a new paper by Philip Muirhead (Cornell University) and colleagues goes to work on the parameters of low-temperature Kepler planetary host stars and finds stellar radii that are roughly half the values reported in the Kepler Input Catalogue. The authors believe these values correlate better with the estimated effective temperatures (Teff) of these stars and suggest a striking possibility:

The effective temperatures, radii and masses of the KOIs imply different planet-candidate equilibrium temperature estimates, such that 6 planet-candidates are terrestrial-sized and have equilibrium temperatures which may permit liquid water to reside on the planet surface, assuming Earth-like albedos and re-radiation fractions. Scaling the Earth’s equilibrium temperature of 255 K by the orbital semi-major axis, stellar Teff and stellar radius of the KOIs in this letter, we find that KOIs 463.01, 1422.02, 947.01, 812.03, 448.02 and 1361.01 all have equilibrium temperatures between 217 K and 261 K: the limits of the habitable zone as described in Kasting et al. (1993).

This one has struck a nerve and it’s easy to see why, as we are suddenly looking at six Earth-like planets in the habitable zone of their stars. I’ve received quite a few links to the paper (and thanks to all who sent them, as this is often how I find interesting work!), but we first have to note a few qualifiers. The authors point out, for example, that this work assumes “the same albedo, re-radiation fraction and greenhouse effect” as are found in our own system, an assumption that may well be challenged for a terrestrial planet orbiting a red dwarf star.

I’m also cautious because the physical parameters of exoplanet-hosting stars are so crucial to our understanding of the detected exoplanets themselves. Here we run into issues, and the authors are quick to point this out. We have detailed information about the Sun, for example, that helps us calibrate models for Sun-like stars, so our analyses of mass, effective temperature, radius and other values seems logical and well-founded. But M-dwarfs are a different story because few such stars are both bright enough and close enough for us to obtain accurate parallaxes and direct measurement of their radius. The authors also note a discrepancy between radii as measured in eclipsing binaries and the predictions of at least some stellar evolution models.

The authors go on to say this:

Although there remains a monotonic correspondence between spectral type (the observational parameter) and effective temperature, Teff, the calibration of this relationship is not as advanced as it is for solar-type stars. M dwarf atmospheres are fully convective, rich in molecular absorption features and depart substantially from blackbody emission at all wavelengths…, so the empirical effective temperature scale is particularly challenging.

Muirhead and team went at their work using the TripleSpec Spectrograph at Palomar, observing 84 Kepler ‘objects of interest’ (KOIs) with effective temperatures (as described by the Kepler Input Catalogue) of less than 4400 K. The resultant mass and radius estimates derived in this paper reduce the size of the planet candidates to the Earth-analogue worlds reported here. This would obviously be a significant finding, but I think we have to wait for a response from the Kepler team, and in particular those involved with the Kepler Input Catalogue, to put the work into perspective. An error of this size would be extreme and, as at least one commenter has noted here, such an error should have shown up in the work on Kepler-16b, yet evidently did not.

I’m a writer, not an astrophysicist, so I’m intrigued but waiting for follow-up work to sort this out. This is, after all, how science works, an interplay of data and analysis that is adjusted as new data emerge. We’ll soon learn whether we have to modify our views of other Kepler candidates to match this result. In the meantime, I’m interested to learn what readers think of the Muirhead team’s analysis.

The paper is Muirhead et al., “Near-Infrared Spectroscopy of Low-Mass Kepler Planet-Candidate Host Stars: Effective Temperatures, Metallicities, Masses and Radii,” submitted to Astrophysical Journal Letters (preprint). On early work on circumbinary planets, see Schneider & Chevreton, “The Photometric Search for Earth-sized Extrasolar Planets by Occultation in Binary Systems,” Astronomy & Astrophysics 232, pp. 251-257 (1990). Abstract available.