Binary stars are frequently used in Astrophysics. It is one of very few ways to control some variables. Both components were born around the same time, from the same gas cloud.
The masses and radii of single stars cannot be directly measured. Eclipsing binaries allow this. They can also be used to measured distances, even to nearby galaxies.
Several hypothesis are proposed for planet formation but are hard to distinguish. Close binaries will affect the disc of material from which planets will form.
Those systems therefore provide a different planet formation environment. Comparing the type of planets found in circumbinary configurations, compared to those in single star systems
could unveil which of the proposed scenarios is most likely.
Creating two stars close to each other is also problematic. Several ways have been proposed, but all are fairly disruptive for any gas or dust in the system. Yet
the presence of planets demonstrates that conditions were sufficiently mild to allow the establishment of a disc massive enough to form planets. Finding out which binaries have what type of planets
may help to find how binary stars form, and shed light on the general process of stellar formation.
Beating the odds of transits
The geometric probability for a planet to transit a single star is strongly dependent on its distance to it. This is why transit surveys (notably WASP)
are mostly confined to short period planets. However the transit technique provides a valuable and practical means to analyse the composition of an exoplanet's atmosphere.
The probability for a remote observer to witness the Earth transit the Sun is only 0.4%.
Circumbinary systems can bring the probability of transit to about 95%.
find out how
Papers on circumbinary planets
as illustrated on the left hand side. This means their orbit spans a large area on the sky and consequently, has a high probability to intersect the binary plane, during which transits can happen. The planet would then enter "transitability". Selecting for eclipsing binaries further increases the odds of transits.
For a transit to happen the planet's disc must cover one of the two moving stars. This renders the occurrence of events infrequent and very sensitive to the system's parameters.
On the graph, the dark bands represent transit times (on stars A and B) for cases where the mutual inclination only was changed, by one degree (the grey region is when transitability happens). Work is underway to predict these events, test their robustness and seek them on existing systems.
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transits independent of orbital distance
The probability of transit is mostly dependent on the inclination of the planet to the binary plane, and only very weakly to the orbital distance. This means
that despite the difficulty of catching a transit, the binary geometry is our most practical means to observe intermediate distance planets in transit and get to study