CoRoT May Have Detected a Binary Planet

CoRoT (French: COnvection ROtation et Transits planétaires; English: COnvection ROtation and planetary Transits) is a space-based observatory built by the French Space Agency (CNES). It detected an unusual transit event identified as SRc01 E2 1066. This single transit event has a depth of 4 percent and a remarkably long duration of 66 hours, more than twice the expected duration of the transit of Jupiter across the Sun. Additionally, the light curve of the transit event also features a “bump” in the middle.

From the long transit duration and the “bump” in the transit light curve, Erikson et al. (2012) suggests that the event was either the transit of a gas giant planet in front of a giant star or the transit of a distantly-orbiting gas giant planet in front of a Sun-like star. In both cases, the planet happened to pass in front of a starspot on the surface of its host star. Since the surface of a star at a starspot is cooler and less bright than the rest of the star’s surface, a planet passing in front of a starspot will block a slightly smaller proportion of the total starlight, resulting in a “bump” in the transit light curve.

Figure 1: Artist’s impression of a gravitationally bound pair of gas giant planets.

However, SRc01 E2 1066 could also be the transit of a gravitationally bound pair of gas giant planets. In this scenario, the “bump” in the transit light curve was due to one planet mutually eclipsing the other during the transit event, resulting in a slight drop in the total amount of starlight blocked by the binary planet, hence the “bump” in the light curve.

S. Ida et al. (2015) showed that SRc01 E2 1066 can be well fitted by a binary planet model. The model assumes a Sun-like host star, with P1 and P2 denoting the two planets. P1 has 0.221 times the radius of its host star and P2 has 0.157 times the radius of its host star. Both planets are tightly bound, separated by only 2.6 times the sum of their physical radii. The binary planet orbits at a distance of 12.5 AU from its host star.

If instead SRc01 E2 1066 was due to the transit of a single planet across a Sun-like host star, then the planet should have 0.187 times the radius of its host star and should orbit at a distance of 25 AU. Note that in the single planet model, the planet also needs to pass in front of a starspot to create the “bump” observed in the light curve.

Figure 2: Light curve for CoRoT target SRc01 E2 1066. The single planet fit (dotted red) and binary planet fit (solid purple line) are also shown. S. Ida et al. (2015).

References:
- A. Erikson et al. (2012), “Planetary transit candidates in the CoRoT-SRc01 field”, Astronomy & Astrophysics, Volume 539, A14
- S. Ida et al. (2015), “Extrasolar Binary Planets II: Detectability by Transit Observations”, arXiv:1504.06365 [astro-ph.EP]