Two Temperate Super-Earths in a Five Planet System

Kepler-296 is a binary star system consisting of two red dwarf stars - Kepler-296A and Kepler-296B. NASA’s Kepler space telescope previously detected the presence of five transiting planets associated with the Kepler-296 system. However, it is unclear whether the five planets orbit Kepler-296A or Kepler-296B since both stars are quite close to one another. Kepler-296A (primary star) is more massive and more luminous than Kepler-296B (secondary star), and it is responsible for ~3/4 of the system’s total luminosity.

Using statistical and analytical analysis, Barclay et al. (2015) show the five planets most likely orbit Kepler-296A, the primary star. The five planets around Kepler-296A are termed “super-Earths” because they are somewhat larger in size than Earth. Moving out from Kepler-296A, the five planets are identified as Kepler-296Ac, b, d, e and f.

- Planet c: orbital period of 5.8 days, 2.00 times Earth’s diameter and 14.8 times Earth’s insolation.
- Planet b: orbital period of 10.9 days, 1.61 times Earth’s diameter and 6.5 times Earth’s insolation.
- Planet d: orbital period of 19.9 days, 2.09 times Earth’s diameter and 2.90 times Earth’s insolation.
- Planet e: orbital period of 34.1 days, 1.53 times Earth’s diameter and 1.41 times Earth’s insolation.
- Planet f: orbital period of 63.3 days, 1.80 times Earth’s diameter and 0.62 times Earth’s insolation.

One reason why the five planets most likely orbit only one of the two stars in the binary system (Kepler-296A in this case) is that the ratios of the orbital period between successive pairs of planets are all ~1.8 (1.88, 1.83, 1.71 and 1.86). This means that the planets underwent convergent migration in the past, and migrated into the current configuration of nearly equal orbital period ratios. If one or more planets orbit the other star Kepler-296B, such a chain of nearly equal orbital period ratios would be very unlikely.

Of the five planets, Kepler-296Ae and Kepler-296Af, respectively, receive 1.41 and 0.62 times the amount of insolation Earth gets from the Sun. This amount of insolation places both planets within or at least close to the habitable zone around Kepler-296A. Based on the definition by Kopparapu et al. (2013), Kepler-296f, the outermost of the five planets, is probably a better candidate for a habitable planet since it falls in the “conservative” habitable zone, while Kepler-296e falls into the “optimistic” habitable zone.

References:
- Barclay et al. (2015), “The Five Planets in the Kepler-296 Binary System All Orbit the Primary: A Statistical and Analytical Analysis”, arXiv:1505.01845 [astro-ph.EP]
- Kopparapu et al. (2013), “Habitable Zones Around Main-Sequence Stars: New Estimates”, arXiv:1301.6674 [astro-ph.EP]