Extrasolar Planets 897
FIGURE 9 The stellar metallicity distribution of
planet host stars (in red) compared to a volume
limited sample of stars in the solar neighborhood
(black). The observed iron abundance ([Fe/H]) is
given on a logarithmic scale normalized to the Sun’s
metal content ([Fe/H]=0). Stars with [Fe/H]=0.5
contain a little more than three times the amount of
iron, while stars with [Fe/H]=−0.5 have only one
third of the Sun’s iron abundance. Clearly, stars with
detected radial velocity planets tend to be more
metal-rich than the average star in the Sun’s vicinity.only 32% of the mass of the Sun. M dwarf stars have masses
ranging from roughly 55% to about 0.8% solar masses. De-
spite the fact that M dwarfs comprise the majority of stars
in our galaxy, they only form subsets in the target samples
of current radial velocity surveys, due to their faintness.
Gliese 876 was found to have a planetary system of two
Jupiter-type companions in a 2:1 mean-motion resonance
with periods of 30.12 and 61.02 days. This star is so far
the only M dwarf known to have gas giant planetary com-
panions. It is also exceptional in a different way: Because
of its proximity (15 lightyears), it is the ideal target for as-
trometry. In 2002, highly precise measurements obtained
with the Fine Guidance Sensors onboard theHubble Space
Telescopesuccessfully revealed the astrometric signature
of the outer planet. By combining the ground-based ra-
dial velocity data with the space-based astrometric data, a
true mass of 1.9 Jupiter masses was determined for this
planet.
There were also a handful of planets detected orbiting
giant stars. Giants stars are more evolved than solar-type
stars, and their cooler atmospheres have a spectral signature
rich in absorption lines. These stars are thus suitable targets
for the radial velocity technique. The progenitor stars (i.e.,
before they evolve into their current giant status) of most
giant stars are more massive than the Sun and the successful
detection of planetary companions around them is evidence
that planet formation can also occur around more massive
stars. This is not such a big surprise because several thick
dust disks have already been observed around this type of
star.
Another interesting correlation emerging from the ra-
dial velocity census of extrasolar planets is that their del-
tectability is a strong function of the metallicity of the host
star. Astronomers call every element heavier than helium a
metal.Stellar metallicitythus means the abundance of all
chemical elements in a star besides hydrogen and helium.
In general, the element used for the metallicity determi-
nation is iron. By measuring the stellar metal content, we
can probe the primordial chemical composition of the gas
and dust cloud, out of which the star (and presumably its
companions) has formed.
It was found that the mean value of the metallicity distri-
bution of planet host stars is offset with respect to the mean
metallicity of stars in the solar neighborhood. On average,
giant planets are more frequently detected around host stars
that are more metal-rich than the solar neighborhood mean
(Fig. 9). This can be seen as evidence for the core-accretion
model for the formation of gas giants. The efficiency of this
model is sensitive to the abundance of heavier elements in
the protoplanetary disk (more heavier elements→more
cores→more gas giants). Alternatively, this might also be
regarded as evidence that orbital migration is a function of
the metal content of the planet-forming disk, because close-
in planets are easier and faster to detect by radial velocity
surveys.3.2.6 THE HOT NEPTUNES
In 2004, the first radial velocity planets with masses below
the gas giant range were discovered. These planets havem
sinivalues comparable to the masses of the icy giants of our
solar system, Uranus and Neptune. Their very short orbital
periods give detectable radial velocity signals despite their
low mass. Thus, they have been dubbed hot Neptunes.
So far, we know of three of these type of planets. Two
of them reside in already known planetary systems (ρ 1
Cancri andμAra) as the innermost companion and the
third orbits a low mass M dwarf star (Gliese 436). Table 3
summarizes the general characteristics of the three known
hot Neptunes.
The internal composition of these planets remains un-
known. They could represent failed gas giants (i.e., planet
cores that never acquired, or later lost, their massive gaseous
envelopes). Their masses are so low that it becomes unlikely
that they consist mostly of H/He. Theoretical model com-
putations indicate that an H/He Neptune so close to its host
star would be unstable and would have probably evaporated
by now. This supports the notion that the hot Neptunes are