890 Encyclopedia of the Solar System
This technique only works in combination with stabilized
spectrometers. To minimize any instrumental effects, these
spectrographs have no movable parts and are placed in
pressure- and temperature-stabilized environments. Also,
by using optical fibers, the light path from the telescope to
the instrument is kept as constant as possible. Both tech-
niques have been demonstrated to reach a radial velocity
precision of a few m s−^1 , and in the best cases even1ms−^1.
2.3 Transit Photometry
In the special case that the orbital plane of an extrasolar
planet is close to perpendicular to the plane of the sky, the
planet will appear to move across the disk of the host star.
In our own solar system, this phenomenon can be observed
from the ground for the two inner planets, Mercury and
Venus. Because we cannot spatially resolve the disk of an-
other star, a transiting extrasolar planet can only be observed
as a reduction of the light output coming from the star (i.e.,
by means of precise photometric measurements).
The probability of the visibility of a transit event is a
function of both the radius of the star and the planet and
its orbital separationa:
Transitprob=(
Rstar+Rplanet)aFor a random location in our galaxy, the probability is
less than 1% to observe transits of the inner terrestrial plan-
ets in the solar system and for the outer planets it decreases
from 0.1% (Jupiter) to 0.01% (Pluto). But for giant planets
orbiting at very small separations (a∼0.04 AU), the transit
probability is around 10%. A transit of such a planet pro-
duces a∼1% dip in the so-called lightcurve (i.e., the time
series of brightness measurements) of a star. This effect
can be detected from the ground with state-of-the-art pho-
tometric instruments, which allow a precision of∼0.1%.
Currently numerous ground-based photometric transit
surveys are searching for short-periodic giant planets. These
surveys usually use small-aperture telescopes with a wide
field of view to survey a large amount of stars, typically
hundreds or thousands per CCD image. Their results will
be discussed in Section 3.
Smaller planets will require higher photometric preci-
sions than ground-based photometry can achieve because
of the limitations imposed by our atmosphere. Space-borne
observatories on the other hand should be capable of detect-
ing even the miniscule photometric transit of an Earth-like
planet orbiting a solar type star at 1 AU.
If photometric data of a transit event can be combined
with radial velocity measurements, then the siniambiguity
in the planetary mass is removed. Furthermore, the transit
depth allows an estimate of the radius of the companion
and thus an estimate of the mean density. Comparisons
of high-resolution spectroscopic observations during and
outside a planetary transit could possibly reveal spectral
signatures of the planetary atmosphere. Clearly, we can gain
a tremendous amount of information from planetary transit
observations.2.4 Microlensing
According to Einstein’s theory of general relativity, photons
are affected by the presence of a gravitational field. Be-
cause gravity can be viewed as the changing curvature of
the space–time continuum, the path of a photon follows
this shape and is “bent” when it passes close to a gravita-
tional potential. In certain geometric cases, this can lead to
a focusing of the light from a distant source by a foreground
object. This gravitational bending of light has been mea-
sured directly during total solar eclipses when the effect of
the Sun’s gravitational field can be observed as positional
changes of stars close to the sun’s disk.
In astronomy, this effect is also seen on a much larger
scale: Entire galaxies or even clusters of galaxies are acting as
massive gravitational lenses for the light of more distant ob-
jects in the background. However, as already demonstrated
by our Sun, every object with mass can be a gravitational
lens: a star, a brown dwarf, or even a planet.
Like the transit method, microlensing is caused by a ge-
ometric alignment: when a foreground object (the “lens”)
moves in front of a more distant background object (the
“source”), the light of the source passing close to the lens
is bent toward the observer. The observer can see several
images of the background object separated by milliseconds
of arc, which merge into a full ring called the Einstein ring
at the moment the lens is directly in front of the source. Be-
cause the gravitational lensing of the source magnifies the
image, the total amount of detected light is increased, and
the brightness of the distant source is enhanced. The mag-
nification factor depends on the exact geometric situation,
and the maximum occurs when the lens is at its smallest pro-
jected distance from the source. For microlensing events in
our galaxy and for stars acting as both sources as well as
lenses, the images cannot be spatially resolved, and only
the change in brightness is observed. However, the magni-
fication can be large and theoretically even infinite for point
sources. The position where infinite magnification occurs is
called a caustic. Because stars are not perfect point sources,
the magnification will not be infinite, but it will still be very
large.
If the lens is not a single object but a binary, then the
caustic is no longer a single point but an extended geo-
metric figure symmetric around the binary axis. Thus, the
microlensing technique represents an elegant method to
search for planetary companions to stars in our galaxy. Bi-
nary lenses reveal themselves by a characteristic shape of
their lightcurve (the time series of the brightness measure-
ments during the lensing event). The lightcurve of a binary
lens contains sharp peaks of even larger magnification due