Extrasolar Planets 901
FIGURE 13 Another candidate for a first image of an extrasolar
planet is the companion (b) to the young star GQ Lupi A.
to distinguish young planets from low-mass brown dwarfs.
If GQ Lupi b has indeed formed like a gas giant planet,
then it was probably transported from the denser interior
of the protoplanetary disk, where sufficient planet-forming
material can be found, to its present location of about 100
AU away from the star.
4. Summary and Outlook
The extrasolar planetary systems discovered so far demon-
strate that a surprising variety of planetary systems exists
in our galaxy. Although we now know that other stars also
have planetary companions, and that planet formation is not
unique to our star, most of them have characteristics differ-
ent from the planets in our solar system. The observational
results indicate that the majority of extrasolar planetary sys-
tems might have had a much more dynamic past than our
planetary system. The gas giant planets found at small or-
bital separations are probably a result of massive orbital
migration, while the quasi-random distribution of orbital
eccentricities might be caused by more violent and fre-
quent interactions between planets in the early evolution-
ary stages. The overall mass function of extrasolar planets is
steeply rising toward lower masses, and we can extrapolate
that less massive planets, which are still undetectable by cur-
rent techniques, are abundant in the galaxy. We also know
today that the metallicity of the star- and planet-forming
nebula has an impact on the structure of planetary systems.
Most of the radial velocity planets are detected around stars
that are richer in heavier elements than the Sun. The low-
est mass planets found around solar-type stars have masses
comparable to Neptune and represent the first steps toward
finding terrestrial planets.
We truly live in the golden age of discoveries of ex-
trasolar planets. Detection programs using ground-based
telescopes will continue to improve their sensitivity and to
extend their search spaces, while space telescopes will
play a more important and likely an even dominant role
in this field in the coming years. There are currently
in preparation several space missions that will greatly
increase—and even revolutionize—our knowledge of extra-
solar planets. To reach the ultimate goal in planet search—
to discover Earth-like planet—we will need to use space
telescopes.
The Space Interferometry Mission (SIM) has the goal to
deliver astrometric measurements with an accuracy of 4μ
arcsec. SIM will be positioned in a so-called Earth-trailing
orbit, where it will slowly drift away from our planet to avoid
occultations of parts of the sky by the Earth. The impressive
astrometric precision of 4μarcsec (which is several hun-
dred times better than current techniques) will be attained
by optical interferometry. The spacecraft itself consists of a
fixed 10 m long boom on which three interferometers are
mounted. SIM will be used to survey hundreds of nearby
stars for giant planetary companions at large orbital sepa-
rations. These data will be complementary to the results of
the ground-based radial velocity surveys, which are more
sensitive to close-in planets. SIM will thus greatly expand
the census of Jupiter analogs in the solar neighborhood. It is
difficult to predict what the best case astrometric precision
of SIM will be, but if it will be close to 1μarcsec, then even
the discovery of terrestrial planets orbiting the nearest stars
will become possible.
KEPLER is a NASA mission specifically dedicated to
find Earth-like planets in the habitable zones of other stars.
The mission consists of a photometric space telescope of
1 m aperture, which will continuously monitor a specific
search field in the sky for planetary transits. Unhampered
by the limitations imposed by Earth’s atmosphere, its sen-
sitivity should allow us to detect even the miniscule dip in
a star’s lightcurve caused by the transit of a planet with 1
Earth radius, orbiting at∼1 AU. In order to have a de-
cent chance in finding the transits of extrasolar terrestrial
planets, KEPLER will observe more than 100,000 stars si-
multaneously. After finding one or two transit events (which
will be separated by roughly 1 year for a planet at 1 AU)
for a given target star, the third and fourth transit will be
used to rigorously confirm an orbiting body. KEPLER will
be the first mission that will allow us to estimate the fre-
quency of possible habitable Earth-like worlds in our galaxy.
The European Space Agency will launch a high-precision