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How to take snapshots of distant worlds

Roll your mouse over this infrared image of Neptune to see the improvement in resolution attained by using an adaptive optics system. These images were obtained by Palomar Observatory's Hale Telescope.
Roll your mouse over this infrared image of Neptune to see the improvement in resolution attained by using an adaptive optics system. These images were obtained by Palomar Observatory's Hale Telescope.
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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 distant stars.

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 reality.

Coronagraphs

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 be studied.

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 a planet.

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.


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Extrasolar planets, NASA exterrestrial extrasolar planets around nearby stars. SIM Space Interferometory Mission. Keck Interferometer. Terrestrial Planet Finder. Extrasolar planets, or exoplanets. Extraterrestrial. Exo-planets life space, outer space.

Extrasolar planets. Exo-planets. Searching for extrasolar planets. Searching for exo-planets. Earth-like planets in the Milky Way. Exoplanets and extra-solar planets, or exoplanets and extra-solar planets. Planets around others stars are called extrasolar planets. What is an extrasolar planet? Astronomy, or astronomy and finding planets. National Aeronautics and Space AdministrationJet Propulsion Laboratory WebsiteCalifornia Institute of Technology Website JPL Website Home PageJPL Website - EarthJPL Website - Solar SystemJPL Website - Stars and GalaxiesJPL Website - Technology