Planet Formation in Binaries - Planetary Exploration and Science: Recent Results and Advances - page 305

2006 ), HD 41004 (Zucker et al. 2004 ), Cephei (Hatzes et al. 2003 ; Neuhäuser
et al. 2007 ; Endl et al. 2011 ), HD 196885 (Correia et al. 2008 ; Chauvin et al. 2011 )
and Ǜ Centauri B (Dumusque et al. 2012 ).
As soon as their number became statistically significant, the characteristics
of these planets in binaries have been investigated in order to derive possible
specificities as compared to planets around single stars. Desidera and Barbieri
( 2007 )andRoelletal.( 2012 ) have shown that whilst the distribution of planets
in wide ( 100 AU) systems is identical to that of planets around single stars, the
characteristics of exoplanets in close binaries are significantly different. The main
trend seems to be that planetary masses increase with decreasing stellar separation.
According to Roell et al. ( 2012 ), the minimum planet mass scales approximately
as .10 AU=a bin /M Jup . However, these trends should be taken with caution, as the
number of planets in tight binaries is still very limited. Furthermore, the probable
detection of the Earth-sized planet around Ǜ Cen B in late 2012 might significantly
weaken this result. As for the global occurrence of planets in binaries, Roell et al.
( 2012 ) have found that multiplicity rate amongst planet-hosting stars is 12 %,
approximately four times smaller than for main-field solar-type stars (Raghavan
et al. 2010 ). But as pointed out by Duchêne ( 2010 ): “...the small sample size,
adverse selection biases, and incompleteness of current multiplicity surveys are
such that it is premature to reach definitive conclusions”. As a consequence, future
surveys should probably increase both the number of exoplanets in close binaries
and the number of stellar companions in known exoplanet systems.
Besides this statistical exploration, another crucial issue that has been investi-
gated early on is that of the long-term orbital stability of these planets in binaries.
The reference work on this issue remains probably that of Holman and Wiegert
( 1999 ), 1 who derived empirical expressions for orbital stability as a function of
binary semimajor axis a B , eccentricity e B and mass ratio . Later studies have
shown that, reassuringly, all known exoplanets in multiple systems are on stable
orbits (e.g. Dvorak et al. 2003 ; Haghighipour et al. 2010 ), although the case for HD
41004 is not fully settled yet, as it depends on the yet unconstrained eccentricity of
the binary orbit (Haghighipour et al. 2010 ).
An important recent development in terms of observations is the discovery of
several exoplanets in P-type orbits. The first confirmed such planet orbits around
the cataclysmic binary DP Leonis (Qian et al. 2010 ), but most circumbinary planets
around binaries with main-sequence stars have been detected by the Kepler space
telescope (Doyle et al. 2011 ; Welsh et al. 2012 ; Orosz et al. 2012a , b ; Schwamb et al.
2013 ; Kostov et al. 2013 , 2014a , b ). Here again, dynamical studies have shown that
all known circumbinary planets are on long-term stable orbits.
However, even if the question of long-term stability seems to be settled for
all known exoplanets in binaries, the question of their formation is a much more
complex issue. It is true that, for many S-type planets, the stellar separation is so
1 Although similar pioneering work on this issue had already been performed a decade earlier by
Dvorak ( 1984 , 1986 ) and Dvorak et al. ( 1989 ).