One of the grand challenges of NASA's search for new worlds is to develop
technologies that will allow us to obtain the first images of planets circling
While the parent star is the source of light that will make any
planet visible, its glare is between a million and 10 billion times brighter
than the faint little speck we are looking for. Therefore, any detailed
study of extrasolar planets will require methods to cover up or otherwise
control the glare of the parent star so that we can study its immediate surroundings.
Another challenge stems from the fact that, compared to the separation
between most things in the universe, planets are located extremely close
to their parent stars. For this reason, we need very high resolution to
separate the planet from its nearby host.
The following is an overview of several techniques in development
that could overcome these obstacles and make extrasolar planet imaging a
Originally invented to study the Sun, a coronagraph is a telescope designed
to block light coming from the solar disk, in order to see the extremely
faint emission from the region around the Sun, called the corona. It was
invented in 1930 by B. Lyot to study the Sun's corona at times other than
during a solar eclipse. The coronagraph, at its simplest, is an occulting
disk in the focal plane of a telescope or out in front of the entrance aperture
that blocks out the image of the solar disk, and various other features to
reduce stray light so that the corona surrounding the occulting disk can
However, this technology is now being refined and adapted for the
purpose of studying the region around distant stars in search of planets
themselves or spectral evidence of planets. One challenge with this approach
lies in the diffraction of light around the edges of the occulting shape,
which detracts greatly from the potential angular resolution of the image.
The diffraction pattern of a simple round telescope, for example,
is a series of concentric rings with a bright central spot. Blocking the
light from a star in order to see an orbiting planet requires suppressing
the first several bright rings without blocking out the planet. By using
a different shape, the diffraction pattern can be controlled so that the
starlight is much dimmer closer to the center in some areas, and brighter
in others. The telescope can be rotated about its line-of-sight so that
the planet image passes in an out of the regions where the starlight is dim.
Managing this diffraction pattern isn't too difficult -- there are
a number of options available to accomplish this. So, the technologies under
study include various tricks to block out as much of the starlight as possible,
while managing the diffraction pattern such that the planet can be seen peeping
out from beyond the diffraction bands.
One idea is to make the aperture square, instead of round, and use
a cross for the occulting shape. The diffraction bands are thus perpendicular
to the aperture edges, and a planet could be visible in the diagonal areas
of the field where the diffraction pattern is somewhat suppressed.
Other proposed solutions for dealing with scattered light within
the telescope include novel-shaped apertures, odd-shaped pupils, pupil masks
to suppress some of the diffraction, and deformable mirrors.
A more critical issue for TPF is wavefront control, which must be
mastered in order for a visible-light TPF to work. This includes correcting
for imperfections in the optics, which scatter light and degrade image contrast.
To appreciate the difficulty the phenomenon of diffraction presents
to the development of a coronagraph technology for studying other solar systems,
see Find out More: A closer look at diffraction .
Another possibility is to combine techniques of coronagraphy with
interferometry. A coronagraph could also incorporate a spectrometer, so
that chemical signs of life could be sought within the light reflected from
Interferometers and nulling
An alternative way to get a picture of a distant planet is to replace
one large mirror with a number of smaller mirrors and combining their light
in a process called interferometry.
Using optical interferometers to study distant planets would allow
for smaller mirrors, which can obtain a resolution equal to a single telescope
as big as the largest separation between the individual telescopes.
To get enough of this information to build up a good picture, the
interferometer must rotate around to different relative positions and repeat
the "exposures." As well as taking a picture, an interferometer can obtain
spectra of the targets it is looking at.
Interferometers provide extremely good angular resolution. That
means they are very good at sorting out which light waves come from which
part of the star system. Additionally, an interferometer can be "tuned"
so that the light coming from the exact center in the field of view (where
the star is) will be blanked out or nulled, while the light from any other
area will be viewed normally.
The Keck Interferometer, for example, will use nulling techniques to search for planets around other stars.