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Half of the stars of our Galaxy are actually binary systems, formed by two bodies orbiting around a common barycentre. Binaries can host exoplanets (i.e. planets which are not part of our Solar System) circling either around one of the two components or around both. The former are called “circumstellar”, the latter “circumbinary”. For binaries as well as for single stars we can define a habitable zone (HZ for short), that is the region of a planetary system in which a hypothetical planet would be able to sustain liquid water on its surface. Binary stars have two kinds of HZs: circumstellar ones, around each of the components, and a circumbinary one, around both. Finally, binaries sport an instability region: if a planet forms inside of that region, it would be either expelled from the system or destroyed by the gravitational forces of the two stars. Depending on the features of the binary system, HZs and instability regions can also be partially or totally overlapped (see Fig.1). In the latter case, the chances to find a planet like our own are drastically reduced. In our paper, recently published in the Astrophysical Journal, we estimated the fraction of binary stars that could host habitable Earth-like planets. In order to do so, we generated a synthetic population of 1 million of binaries that mirror the characteristics of the true binary population which can be found in the Milky Way. For each system we calculated the position and width of the HZs and the instability region, evaluating whether or not they overlapped. Finally, we correlated the presence or absence of stable HZs with various properties of the binary system, such as the mass of the two stars or their average distance. The novelty of our work is the fact that we analyzed the global binary star population rather than some specific case.

Figure 1: The three possible configurations of HZs (green bands) and instability region (hatched bands) in a binary system are shown. In the “type 1” case there is no overlapping, in “type 2” there is a partial overlapping and in “type 3” there is full overlapping. The left-hand panel refers to the circumstellar region (i.e. that around each star) and the right-hand panel refers to the cirbuminary region (i.e. that around both stars). Red symbols mark the position of the two stars.

With this study we both confirmed previous findings and made a discovery. First of all, we confirmed that circumstellar HZs are common, whereas circumbinary HZs are rare. Specifically, we found that the former is present in 80-90% of the cases, while the latter is present in less than 5% of the binaries. Then, we identified the limits in the binary star parameters that allow for the existence of these zones. Finally, we discovered that certain binary configurations can give rise to circumstellar and circumbinary HZs respectively 40% and 900% wider than those around single stars (see Fig.2). This can increase the probability of finding habitable planets, especially if one or both stars in the system have masses smaller than that of the Sun. Our work has practical as well as theoretical implications, especially for astrobiology. The search for Earth-sized planets around distant stars requires a great deal of observational time and thus it is very important to select the most suitable candidates in advance. Establishing a link between known binary star features and the presence of a HZ, we hope to help this selection process.

Figure 2: The width of the circumsecondary and circumbinary HZs with respect to that around a single star. “Circumsecondary” refers to the circumstellar HZ around the least massive star in the binary, called “secondary”. The two panels are density maps: the darker shades of green represent regions where binary systems with “type 1” HZ are more concentrated. The blue crosses represent single instances of “type 2” HZ. In the left-hand panel, the vertical axis represents the width of the circumsecondary HZ, while the horizontal axis represents the mass of the secondary star. In the right-hand panel, the vertical axis represents the width of the circumbinary HZ and the horizontal axis represents the mass ratio between the secondary and the primary (most massive) star in the binary.

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