Most planets orbit a single star, but some orbit two stars at once—and these strange worlds are rewriting the rules of how planets form. The CandY project will systematically study these circumbinary planets for the first time, using new analytical methods and three observing campaigns to detect dozens of them, including rocky ones as small as Earth. The team will also analyse their atmospheres, something never done before for planets with two suns. This matters because current planet-formation theories were built on planets orbiting single stars. Binary stars disrupt the usual process of core accretion, so comparing circumbinary planets to ordinary exoplanets reveals which formation mechanisms are truly fundamental and which are secondary. The project will also exploit a unique advantage: circumbinary planets are more likely to transit their stars, making atmospheric studies easier, and they are the only transiting exoplanets where seasonal atmospheric changes can be observed. This is fundamental science. It will not directly improve a satellite or a power grid. But understanding how planets assemble under different conditions sharpens our picture of planetary systems across the galaxy—and that knowledge, like earlier work on exoplanet atmospheres, may one day underpin technologies or observational strategies we cannot yet foresee.
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Planets orbiting both stars of a binary system -circumbinary planets- are challenging what we think we know about how exoplanets are assembled, and how their orbits subsequently evolve. Because they orbit one another, binary stars disturb the process of core accretion that creates planets. But, instead of changing the number of planets created, the two stars instead affects circumbinary planets' masses and orbital separations, thus shining an unexpected light onto which planet formation processes are truly important. With its ambitious programme, CandY will consolidate circumbinary planets as an unavoidable corner of exoplanetary science, crucial for the study of planet formation, and the study and interpretation of exoatmospheres. To better our understanding of planet formation, the CandY project will perform the first systematic study of circumbinary planets. CandY will develop a suite of new analytical methods, and conduct three ambitious observing campaigns that will lead to the detection of dozens of new circumbinary planets, including some all way down to rocky masses. In parallel, the CandY team will study the atmospheric composition of circumbinary planets for the very first time in order to reveal yet more clues about these exotic worlds. During the CandY project, we will record every similarity, and any difference between the population of circumbinary planets and the population of exoplanets orbiting single stars. This systematic exercise will create a leap in our understanding about how planets are produced and which processes dominate planet assembly and their subsequent orbital migration. In addition, circumbinary planets have unique orbital properties amongst exoplanets. Their orbital dynamics boost their probability to experience transits easing exo-atmospheric investigations. Moreover, circumbinary exoplanets are the only transiting planets for which seasonal atmospheric variations can be studied.
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