Planet Formation in Binaries - Planetary Exploration and Science: Recent Results and Advances - page 303

“standard” scenario of planet formation by core accretion that has been developed
over the past decades (e.g. Safronov 1972 ; Lissauer 1993 ; Pollack et al. 1996 ;
Hubickyj et al. 2005 ) is so far restricted to the case of a single star. Of course, this
bias is a direct consequence of the fact that planet-formation theories were initially
designed to understand the formation of our own solar system. For the most part,
however, this bias is still present today, nearly two decades after the discovery of
the first exoplanets. The important updates and revisions of the standard model,
such as planetary migration, planet scattering, etc., that have been developed
as a consequence of exoplanet discoveries have mostly been investigated for a
single-star environment. This single-star-environment tropism does also affect the
alternative planet-forming scenario by gravitational instabilities (Boss 1997 ), which
has witnessed a renewed interest after the discovery of Jovian exoplanets at large
radial distances from their star (Boss 2011 ).
However, the gradual discovery of exoplanets in multiple star systems (Desidera
and Barbieri 2007 ; Mugrauer and Neuhäuser 2009 , and references therein), and
especially in close binaries of separation 20 AU, has triggered the arrival of studies
investigating how such planets could come about and, more generally, about how
planet formation is affected by binarity. The latter issue is a vast and difficult one.
Planet formation is indeed a complex process, believed to be the succession of
several stages (e.g. Haghighipour 2011 ), each of which could be affected in very
different ways by the perturbations of a secondary star.
Not surprisingly, the effect of binarity on each of these different stages is usually
investigated in separate studies. A majority of these investigations have focused on
the intermediate stage of kilometre-sized planetesimal accretion, as this stage has
been shown to be extremely sensitive to stellar companion perturbations. But other
key stages have also been explored, from the initial formation of protoplanetary
discs to the final evolution of massive planetary embryos.
The scope of such planet-in-binary studies has been recently broadened by the
discovery of several circumbinary exoplanets (also known as P-type orbits) by the
Kepler space telescope, most of which are located relatively close to the central
stellar couple. The issues related to the formation of these objects are often very
different from those related to circumprimary exoplanets (also known as S-type
orbits), and in-depth investigations of this issue have only just begun.
Studying how both circumprimary and circumbinary planets form is of great
interest, not only to explore the history of specific planets in binaries but also for
our understanding of planet formation in general. These studies can indeed be used
as a test bench for planet-formation models, by confronting them to an unusual
and sometimes “extreme” environment where some crucial parameters might be
pushed to extreme values. We present here a review of the current state of research
on planets in binary star systems. We focus our analysis on the S-type systems in
which planets orbit one star of the binary.