892 Encyclopedia of the Solar System
FIGURE 3 The relative flux of planets compared to the Sun’s
emission as a function of wavelength. Four planets of our
solar system and three extrasolar planets (51 Peg b, 70 Vir b,
and 47 UMa b) are shown. The difference in flux ranges from
10 −^6 to 10−^12 in the optical (< 1 μm) and generally improves
toward the infrared (> 1 μm), where the planet’s thermal
emission dominates.imaging improves the situation for high-contrast imaging,
but the real goal is to remove the image of the central star
entirely from the observations.
The most commonly used instrument to perform this
task is the so-called coronagraph. The coronagraph was in-
vented by the French astronomer Bernard Lyot in the 1930s
to study the outer parts of the solar atmosphere (the corona)
without being totally overwhelmed by the intense glare of
the Sun’s disk. He managed to remove the light of the Sun’s
photosphere by introducing an opaque mask (of the same
size as the image of the Sun) into the telescope’s light path
in such a way that it blocked the photons coming from the
disk but not from the surrounding environment. This makes
a coronagraph the ideal instrument for direct imaging of
extrasolar planets. For ground-based searches the highest
image quality is achieved by combining a coronagraph with
an AO system.
However, no optical system is perfect and even coron-
agraphic images contain residual scattered light from the
central star close to the edge of the opaque mask and other
image artifacts produced by diffraction on telescope parts.
This makes the detection very close to the central object
still very difficult. And even for the nearest stars, the ex-
pected angular separations for planetary companions are
small compared to the size of typical coronagraphic masks.
At a distance of 5.2 pc (=17 light years), an analogous planet
to Jupiter would appear 1 arcsec away from the star at max-
imum projected separation. At 10 pc, the maximum separa-
tion is only about 0.5 arcsec. These angles are comparable to
the typical dimensions for coronagraphic masks of current
state-of-the art instruments (e.g., theHubble Space Tele-
scopeinstrument Near Infrared Camera and Multi-Object
Spectrometer NICMOS has a coronagraphic mask with a
diameter of 0.8 arcsec).
The image area around the central obscuration contam-
inated by scattered light is called the halo, and in very short
exposures this halo is resolved into smaller bright and dark
spots called “speckles.” Speckles are interference phenom-
ena produced by atmospheric seeing and by the superposi-
tion of light coming from all parts of a telescope mirror with
imperfect smoothness. At the location of a dark speckle,
the light of the star is canceled out by destructive interfer-
ence. The image of a faint companion can be recovered if
it is located at the position of a dark speckle, where the
light from the star is severely reduced. By taking a great
number of short exposures, the companion can be detected
by a proper data analysis algorithm simply by the fact that
in every image the speckle pattern is different and that a
dark speckle never appears at the location of the compan-
ion. This method is called dark speckle coronagraphy. In
combination with large aperture ground-based telescopes
or the next generation space telescope, this method should
have the sensitivity to detect extrasolar planets around the
nearest stars.
Another technique to achieve high-contrast images is
nulling interferometry. In theory, it is possible to com-
bine the wavefronts arriving at two or more telescopes in
such a way that a wave maximum coming from one tele-
scope is canceled out by a wave minimum from another
telescope. In this way, it produces a null image of the cen-
tral object while it leaves the light from the circumstel-
lar environment unaffected. First trial runs using ground-
based telescopes have already been successfully performed.
A nulling interferometer is currently built for the Large
Binocular Telescope (LBT), which consists of two 8 m
class telescopes mounted side by side on the same sup-
port structure. A space-based nulling interferometer op-
erating in the infrared is planned for the second stage of
NASA’s Terrestrial Planet Finder (TPF) mission and for
the European Space Agency’s DARWIN mission, with the
ultimate goal to image Earth-like planets around nearby
stars.