896 Encyclopedia of the Solar System
FIGURE 7 Artist conception of theρ1 Cancri
system. This extrasolar multiplanetary system
contains one massive gas giant (pictured in the
foreground with a hypothetical moon) at a
separation of 5.2 AU, as well as two giant planets
close to the star (at 0.12 and 0.24 AU). After the
discovery of three gas giant planets in this system, a
fourth low-mass planet was found at
a=0.038 AU. (Artwork by Lynette Cook.)larger separations, the eccentricities are distributed quite
uniformly and are practically indistinguishable from the ec-
centricity distribution of stellar binaries. The hot Jupiters
have alle=0 (or close to 0) orbits because tidal forces be-
tween the star and the planet at these small distances tend
to circularize the orbit on much shorter timescales than
the typical lifetime of the star. The origin of the nonzero
eccentricities is not well understood; possible explanations
are a more dynamic formation history than in the case of
the solar system, in which mutual dynamical interaction be-
tween planet embryos pumped up their eccentricities. Also,
planet/disk interactions and gravitational perturbation by
stellar companions could be the cause of the higher eccen-
tricities.
Among the known extrasolar planets, 14 multiple
systems were detected: 12 systems with 2 planets, one with
3 planets, and one, theρ1 Cancri system (Fig. 7), with
4 planets. Some of the planets in these multiple systems
show evidence for mean-motion resonances; their orbital
periods are equal or close to resonance values (e.g., 2:1
or 5:3).
The mass function of extrasolar planets (Fig. 8) shows
a steep rise toward masses of 1 Jupiter mass or less. Thus,
although less massive planets are harder (or impossible) to
detect by the radial velocity technique, we can expect them
to be quite frequent.
About 10% of the stars surveyed by long-term radial ve-
locity programs have detectable giant planets. The majority
of these planets orbit stars of the same spectral type (i.e.,
surface temperature and mass) as the Sun. They usually
have orbital separations less than 5 AU; in fact, about half
of them reside within the first AU from their host star. But
these results also reflect strong observational biases. The
radial velocity technique is more sensitive to close-in plan-
ets, and it takes a monitoring timescale of over a decade to
discover planets beyond 5 AU. Also, stars hotter and more
massive than the Sun are not suitable for the radial velocity
technique because they tend to have higher rotation rates
and much fewer spectral features, which can be used to
measure the velocity. And less massive stars than the Sun
are fainter, and the effort to collect enough photons to en-
sure a sufficient data quality increases significantly. Thus, it
comes as no surprise that Doppler surveys have tradition-
ally focused on Sun-like stars. As radial velocity programs
extend their time baselines and expand their target samples
to fainter and lower mass stars, these observational biases
will be overcome.
The low-mass star Gliese 876 is the famous “exception
from the rule.” The star is a so-called M dwarf with a mass ofFIGURE 8 The extrasolar planet mass function. A strong
general trend toward lower masses is apparent. This trend might
indicate that lower mass extrasolar planets are abundant.
Planetary companions withmsinivalues larger than 10 Jupiter
masses are rarer, despite their better delectability by the radial
velocity technique.