With the question of habitable planets on my mind following Andrew LePage’s splendid treatment of Kepler-452b on Friday, I want to turn to the interesting news out of San Diego State, where astronomer William Welsh and colleagues have been analyzing a new transiting circumbinary planet, a find that brings us up to a total of ten such worlds. Planets like these, invariably likened to the planet Tatooine from Star Wars, have two suns in their sky. Now we have Kepler-453b to study, a world that presented researchers with a host of problems.

Transits of the new world occur only nine percent of the time because of changes in the planet’s orbit. Precession — the change in orientation of the planet’s orbital plane — meant that Kepler couldn’t see the planet at the beginning of its mission, but could after it swung into view about halfway through the mission’s lifetime, allowing three transits. Clearly, this is a system we could easily have missed, says William Welsh (San Diego State), who was lead author on the study. Welsh calls the find ‘a lucky catch,’ and it’s hard to argue: The precession period is estimated to be about 103 years, with the next set of transits not becoming visible until 2066.

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Image: An artist’s impression of the circumbinary extrasolar planet Kepler-453b. Credit: NASA/JPL-Caltech/T Pyle.

It was Welsh, working with San Diego State colleague Jerome Orosz, who used Kepler data back in 2012 to discover the first instance of a two-planet circumbinary system, one of the two worlds being in the habitable zone where liquid water could exist on a solid surface. Kepler-453b turns out to be the third planet identified by the mission as being a circumbinary world in the combined habitable zone of two stars. The world is a gas giant, however, and unlikely to host life as we know it (life as we don’t know it is another matter, but that’s true in our own Solar System as well).

What we’re able to deduce about Kepler-453b so far is that it has a radius about six times that of the Earth, and a mass — not readily measurable with current data — probably less than 16 times that of Earth. The planet’s orbit is 240 days, and the two stars it orbits orbit each other every 27 days. The larger star in much like our own, with 94 percent the mass of the Sun, while the smaller is about 20 percent as massive and much cooler. The Kepler-453 system is 1400 light years away in the constellation Lyra, a young system probably between one and two billion years old.

So are we going to find an Earth-like planet in a circumbinary orbit one of these days? What we’ve learned thus far is that circumbinary planets are not unusual — we’ve already found ten — and the range of configurations is wide. An Earth-class planet in the habitable zone of one of these stars would conjure up those wonderful images of twin stars at sunset, one of them going down before the other, a world of intriguing hues and shadows as the two stars moved through their own orbits, with occasional eclipses thrown in for good measure. Circumbinaries are another reminder of the diversity we’re coming to expect as we build the exoplanet catalog.

The paper is Welsh et al., “Kepler 453 b—The 10th Kepler Transiting Circumbinary Planet,” Astrophysical Journal 5 August 2015 (abstract). An SDSU news release is available.

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