Encyclopedia of the Solar System 2nd ed

Extrasolar Planets 895

than Saturn, the planet with the lowest mean density in the
solar system. The discovery of the HD 209458 transiting
planet represents another milestone in the field of extraso-
lar planet detection: It demonstrated that the companions
discovered by the radial velocity surveys were indeed gas
giant planetary companions and not more massive (even
stellar) companions seen at a very unfortunate viewing
angle.
Using spectroscopic observations obtained with the
Hubble Space Telescopeoutside and during the transit, it
was even possible to detect the atmosphere of the HD
209548 planet. TheHSTspectra taken during the plane-
tary transit showed a stronger sodium absorption line than
the spectra observed without the planet in front of the star.
This additional absorption is caused by the sodium in the
planet’s atmosphere. However, the amount of atmospheric
sodium was less than expected from theoretical models,
urging the astronomers involved in this study to speculate
that a thick cloud cover prevents us from seeing deeper into
the planet’s atmosphere.
In anotherHSTobservation of the HD 209458 planet, it
was possible to measure hydrogen escaping from the heated
upper layers of the planet’s atmosphere. The escaping hy-
drogen gas forms a kind of cometary coma and tail around
the planet and is blown away by the radiation and particle
wind of the close-by star.
Several additional transiting planets have subsequently
been discovered. The most interesting of these are a planet
in a 2.2 day orbit around HD 189733 and a planet with a
massive rocky core orbiting HD 149026.

3.2.5 GENERAL CHARACTERISTICS OF PLANETS DETECTED BY

RADIAL VELOCITY MEASUREMENTS

Over the past decade the radial velocity technique has

demonstrated its effectiveness in detecting numerous giant

planets and multiplanetary systems around Sun-like stars.

At the time of this writing, more than 150 planetary compan-

ions were found by the cumulative effort of several Doppler

surveys operating in both hemispheres. Several character-

istics of these extrasolar planets differ significantly from the

giant planets in our solar system.

The gas giants found at very small orbital separation are

difficult to explain in terms of their formation. In the clas-

sical picture of planet formation, gas giants can only form

near (and beyond) the ice line in the protoplanetary nebula.

The ice line is the distance from the star where the tem-

peratures in the nebula drop low enough so that ices can

condense out and form massive cores (mixed with rocky ma-

terial and dust grains) onto which nebula gas can accrete

on. That is the reason why the common expectation was to

find gas giants only at large orbital separations similar to our

Jupiter at 5 AU and more. It appears that, in most cases of

extrasolar giant planets, moderate to massive orbital migra-

tion has occurred, which moved the planets from the place

where they have formed to their current location close to the

star.

The other remarkable difference is the high eccentrici-

ties (e) of the orbits of these planets (Fig. 6). Most of the

found extrasolar planets have more elongated orbits than

the planets in our solar system. Especially for planets at

FIGURE 6 The distribution of orbital eccentricities of

extrasolar planets (red and blue points) compared to the

planets in our solar system (green triangles).