SANDS Project:
Stars, plANets & Discs in multiple Systems

Protoplanetary discs behave as astronomical wizards that turn small sand into planets. Although no magic is involved, the process of planet formation remains mysterious and its fine details are unknown. This constitutes one of the most active fields of research in current day astrophysics. Young stellar objects were first discovered in the 1950s. Since then, major advances in the field have followed at a phenomenal pace and we now know that stellar formation process leads to a very high fraction of binarity (Reipurth et al. 2014). This very fact — largely overlooked — is at the core of the SANDS proposal.

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The presence of circumstellar discs was confirmed by direct imaging in the mid 1990s (e.g. HH 30) along with the first detections of exoplanet (starting with 51 Peg b). As a consequence circumstellar discs naturally became protoplanetary discs, the place where planets are born. This idea received spectacular direct support from the discovery of Beta Pic b (Lagrange et al., 2010) and the planetary system in HR 8799 (Marois et al., 2010) which are still located in their discs, although both are older systems with evolved debris discs. Last year, the first very young planet — still forming and still embedded in its “parental” disc — was directly imaged in PDS 70 (Keppler et al. 2018). These discoveries, possible thanks to new generations of high resolution instruments on large telescopes, also mean that the fine details of the protoplanetary discs are also being uncovered. For instance, the recent survey DSHARP of highly structured discs with ALMA (Andrews et al., 2018) caused a revolution in the field of planet formation.

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Several thousands of exoplanets have been confirmed as a result of major efforts and large surveys. Systems with multiple planets are now known and comparison of exoplanetary architectures with our own Solar System are emerging. Interestingly, our Sun is isolated and its planetary system likely evolved in isolation for most of its lifespan. The radial velocity surveys to search for exoplanets are usually severely biased against binary, except for a few rare exceptions (Eggenberger & Udry, 2010; Martin et al., 2019), in the hope to keep the radial velocity signal as clean as possible. Large transit surveys with wider field imaging (like Kepler) are more free from that bias. Perhaps surprisingly, exoplanets were found around several binary stars: either circumstellar or circumbinary, and also including multiple planetary systems as in Kepler 47 (Orosz et al., 2019). We have to face it: the process of planet formation is very resilient!

 

Understanding how these planets formed largely relies in our capacity to comprehend gas and dust dynamics in circumstellar, circumbinary, or even more exotic protoplanetary discs. In summary, how are the initial conditions for planet formation affected by stellar multiplicity? The aim of the SANDS project is to become the foundation of a comprehensive picture of the sites of planet formation in multiple systems.