* Astronomy

Astronomers bring a new hope to find 'Tatooine' planets

Astronomers could discover a plethora of planets around binary star systems (stars that rotate around each other) by measuring with high precision how stars move around each other, looking for disturbances exerted by possible exoplanets.
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Title: Most Sub-Arcsecond Companions of Kepler Exoplanet Candidate Host Stars are Gravitationally Bound
Author: Elliott P. Horch, Steve B. Howell, Mark E. Everett, David R. Ciardi

Using the known detection limits for high-resolution imaging observations and the statistical properties of true binary and line-of-sight companions, we estimate the binary fraction of Kepler exoplanet host stars. Our speckle imaging programs at the WIYN 3.5-m and Gemini North 8.1-m telescopes have observed over 600 Kepler objects of interest (KOIs) and detected 49 stellar companions within ~1 arcsecond. Assuming binary stars follow a log-normal period distribution for an effective temperature range of 3,000 to 10,000 K, then the model predicts that the vast majority of detected sub-arcsecond companions are long period (P>50 years), gravitationally bound companions. In comparing the model predictions to the number of real detections in both observational programs, we conclude that the overall binary fraction of host stars is similar to the 40-50\% rate observed for field stars.

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Title: Exoplanets Bouncing Between Binary Stars
Authors: Nickolas Moeckel, Dimitri Veras

Exoplanetary systems are found not only among single stars, but also binaries of widely varying parameters. Binaries with separations of 100--1000 au are prevalent in the Solar neighbourhood; at these separations planet formation around a binary member may largely proceed as if around a single star. During the early dynamical evolution of a planetary system, planet--planet scattering can eject planets from a star's grasp. In a binary, the motion of a planet ejected from one star has effectively entered a restricted three-body system consisting of itself and the two stars, and the equations of motion of the three body problem will apply as long as the ejected planet remains far from the remaining planets. Depending on its energy, escape from the binary as a whole may be impossible or delayed until the three-body approximation breaks down, and further close interactions with its planetary siblings boost its energy when it passes close to its parent star. Until then this planet may be able to transition from the space around one star to the other, and chaotically `bounce' back and forth. In this paper we directly simulate scattering planetary systems that are around one member of a circular binary, and quantify the frequency of bouncing in scattered planets. We find that a great majority (70 to 85 per cent) of ejected planets will pass at least once through the space of it's host's binary companion, and depending on the binary parameters about 45 to 75 per cent will begin bouncing. The time spent bouncing is roughly log-normally distributed with a peak at about 10^4 years, with only a small percentage bouncing for more than a Myr. This process may perturb and possibly incite instability among existing planets around the companion star. In rare cases, the presence of multiple planets orbiting both stars may cause post-bouncing capture or planetary swapping.

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Title: Eccentricity of radiative discs in close binary-star systems
Authors: F. Marzari, C. Baruteau, H. Scholl, P. Thebault

Discs in binaries have a complex behaviour because of the perturbations of the companion star. Planet formation in binary-star systems both depend on the companion star parameters and on the properties of the circumstellar disc. An eccentric disc may increase the impact velocity of planetesimals and therefore jeopardize the accumulation process. We model the evolution of discs in close binaries including the effects of self-gravity and adopting different prescriptions to model the disc's radiative properties. We focus on the dynamical properties and evolutionary tracks of the discs. We use the hydrodynamical code FARGO and we include in the energy equation heating and cooling effects. Radiative discs have a lower disc eccentricity compared to locally isothermal discs with same temperature profile. As a consequence, we do not observe the formation of an internal elliptical low density region as in locally isothermal disc models. However, the disc eccentricity depends on the disc mass through the opacities. Akin to locally isothermal disc models, self-gravity forces the disc's longitude of pericenter to librate about a fixed orientation with respect to the binary apsidal line (\pi). The disc's radiative properties play an important role in the evolution of discs in binaries. A radiative disc has an overall shape and internal structure that are significantly different compared to a locally isothermal disc with same temperature profile. This is an important finding both for describing the evolutionary track of the disc during its progressive mass loss, and for planet formation since the internal structure of the disc is relevant for planetesimals growth in binary systems. The non-symmetrical distribution of mass in these discs causes large eccentricities for planetesimals that may affect their growth.

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