Planet Formation in Binaries - Planetary Exploration and Science: Recent Results and Advances - page 319

Fig. 13.10 Accretion vs. Erosion behaviour of a population of kilometre-sized planetesimals in
the habitable zone of the Ǜ Centauri when varying the binary's separation and eccentricity (Taken
from Thebault et al. 2009 , courtesy of the MNRAS)

gets dense enough to be unstable. See, for example, Nelson ( 2000 ) or Mayer et al.
( 2005 , 2010 ), whose main conclusions are that the presence of a close ( 50-60 AU)
companion could greatly hinder the development of instabilities. This issue is thus
far from being settled yet, and further, more detailed investigations are needed to
assess if disc fragmentation can be considered as a viable alternative formation
scenario in close binaries.

13.7 Habitability

Although Earth-like planets are yet to be discovered in the habitable zone (HZ) of
binary star systems, many planet-formation-in-binary studies have rightfully paid
special attention to the HZ (see previous sections). In most of these simulations,
it has been generally assumed that the HZ of a binary is equivalent to the single-
star HZ of its planet-hosting star. Although in binaries with separations smaller
than 50 AU, the secondary star plays an important role in the formation, long-
term stability and water content of a planet in the HZ of the primary; the effect
of the secondary on the range and extent of the HZ in these systems was ignored.
However, the fact that this star can affect planet formation around the primary, and
can also perturb the orbit of a planet in the primary's HZ in binaries with moderate
eccentricities, implies that the secondary may play a non-negligible role in the
habitability of the system as well.