Binaries are twice the trouble. The shifty geometry of planets that orbit two stars means we’ve missed about 75 per cent of these worlds – but we are playing catch-up.
Planets that orbit two stars are truly, intriguingly alien – they have varied seasons, and formed under different circumstances from planets in our own solar system. They are also trickier to discover and study: unlike planets around single stars, they shift their orbital paths over just a few years.
The Kepler telescope has spotted 10 of these worlds by watching them transit, or cross in front of their stars from our point of view. Transits around just one star run like clockwork: once you know how long the planet’s year is, you can predict exactly when it should next pass in front of the star.
But binaries have more moving parts. The planet could orbit its stars in the same plane, or adopt its own separate plane. The angle of its orbit with respect to the stars’ plane shifts with each trip around the binary. Sometimes the planet will transit as it goes around. Sometimes it won’t.
Since the easiest way to find the planet is when it transits, we really need to know when to look – a tricky problem only solved by number crunching on high-powered computers.
In 2015, David Martin at the Geneva Observatory in Switzerland started looking for an easier way. First, he and his colleagues devised equations to calculate whether a circumbinary planet would transit at all – an easier technique than running simulations.
“It showed you’d get loads of transits, but it didn’t really let you know when,” Martin says.
Now, he has calculated when a planet’s orbit crosses in front of the orbital path of its stars. That gives astronomers specific windows in which they have a good chance of spotting a transit.
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