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Since 1991 there has been great progress in establishing the presence of planets around some of the stars in our galactic neighborhood.  This has been accomplished mainly by observing the “wobble” in the position of a star caused by the gravitational influence of one or more orbiting planets.  There have also been also been some indications of extrasolar planets from observations of the occultation of starlight when the planet passes in front of the star along our line of sight, from gravitational microlensing that changes the intensity of a background star, and from timing of the pulses from neutron stars with planets in orbit.  So far, this work has mainly established the presence of planets having a mass of greater than that of Jupiter, since all of these methods are most sensitive to the presence of very large planets.

At this writing (October, 2006) astronomers have indirectly detected 197 extrasolar planets in 97 different star systems.  The largest of these (HD 202206 b) has 17.4 times the mass of Jupiter and orbits at 0.83 AU (AU=Earth-orbit radius) around a star very similar to our sun (mass = 1.15 M_sol) located 150 light years away in the constellation Capricorn.  Some of the newly discovered planets are “hot Jupiters”, gas-giant planets orbiting in relatively close orbits.  An example is HIP 14810 b, a planet with 3.84 Jupiter-masses and an orbital radius just 0.07 AU from a Sol-like star (mass = 0.99 M_sol) located 172 light years away in the constellation Aires.  Note that 0.07 AU is a very close orbit.  By comparison, Mercury orbits our sun at 0.39 AU.  Specialists in planet formation suggest that there should be no Earth-like planets in such star systems, because the hot gas giants in close planetary orbits would sweep up all of the proto-planetary material, preventing formation of Earth-like planets in orbits with the right amount of solar radiation to support the development of life.

However, sub-Jupiter size planets have been found.  Among the smallest to date is OGLE-05-390L b, a planet of 5.5 Earth-masses with an orbital radius of 2.6 AU (about the location of the asteroid belt in our solar system).  It orbits a relatively dim star (mass = 0.22 M_sol) located 21,000 light years away in the constellation Sagittarius.  However, its larger planetary mass and orbit and the dimmer sun make this planet distinctly non-earthlike.

This body of work has established a high probability that many stars are orbited by planetary systems, but it would be of even greater interest to identify planets that more closely resemble Earth in mass, orbit, and illumination.  There are now proposals for new instruments that would make this possible.  In this column I will focus on only one of them, the diffraction-suppressed occulter proposed by Prof. Webster Cash of the University of Colorado .

Telescopes like the Hubble Space Telescope (HST) and its planned successor, the James Webb Space Telescope (JWST), have adequate image resolution to resolve planets orbiting nearby star systems.  The principal problem with directly observing such extrasolar planets is that the light from any star is many orders of magnitude greater than the reflected light from its planets and will completely swamp light from the planet, rendering direct imaging of the planets impossible.  This problem, however, may be reduced or eliminated through the use of an occulter.

The basic idea of an occulter is straightforward: you simply place a black disk along the line-of-sight to a star system at some distance from a telescope and then observe any planets visible in the star-shadow created by the occulter.  In other words, the occulter blocks the light from the star but not from its planets.  The distance from occulter to telescope is typically around 10⁴ kilometers or more, large enough that the occulter must be located is space and must be carefully navigated into a position where the planets of a candidate star might become visible in the star-shadow over some period of time.  It would probably be most optimally used with space based telescopes like the HST or JWST, but if placed in a movable geosynchronous orbit it might be used with an Earth-based telescope.  However, there is a basic wave-optics problem with the occulter scheme.

Light is an electromagnetic wave.  When such waves are intercepted by a black disk, the waves are diffracted at the edge, and some of the wave fronts change direction, bending around the disk edge to be deflected into the shadow region.  Thus, a truly black shadow is difficult to achieve.  In realistic situations, this diffracted starlight is much stronger than the light from planets around the star, and because of diffraction effects a circular disk is not useful as an occulter in searching for extrasolar planets.

Fortunately, there is procedure called apodization (Latin for “removing the foot”) that can reduce the diffraction from an occulter.  This is done by the smoothing or tapering of the sharp edge of the occulter in such a way that the waves from different regions tend to cancel and diffraction is suppressed.  In optics laboratory applications this is often done photographically by producing a black disc with an edge that has progressively lighter shades of gray as the distance from the disc center increases.  However, such a gray-edge occulter would have many problems in space-based applications.  The production, launch, and deployment of a gray-edge photographic image many meters in diameter would produce many logistic problems, and its long term survival in a space environment under bombardment from cosmic rays, ultraviolet radiation, and micrometeorites would be problematical.

Therefore, Prof. Cash, working under a grant from NASA’s Institute for Advanced Concepts, developed a better idea.  He decided to approximate the gray-shaded edge of the occulter disk with carefully shaped blades that either blocked the starlight or passed it completely, therefore producing what might be called “binary apodization”.  Cash decided to use an average light transmission of the occulter disk that has the value 0 (black) out to some radius R and an average light transmission of 1 - exp{-[(r-R)/S]²ⁿ} at larger radii.  The falloff of the disc edge S is typically about equal to R, and n is typically around 4 to 12.  These parameters and the distance F from occulter to telescope are tuned to optimize the occulting performance for the wavelengths of interest in the range 600 to 1800 nm, arriving at the values R=S=12.5 m and F=5.0 x 10⁷ m.  The result is an occulter shadow that will suppress starlight by a factor of 10^-12 at 600 nm (orange light) and by 10^-6 at 1800 nm (infrared light).

The light absorption is implemented by blocking light at the edges of the occulter with blades that resemble 12 petals of a flower.  These black  “petals” are wide at the base where the join the black disk and taper on a smooth curve to points at a distance of about two or three disc radii, so that the “blackness” or absorption as averaged around the occulter is given by the reduction function given above.

The image that the telescope would see with the occulter in place is not completely free of light from the occulted star.  The image still contains some diffraction where the blades meet, as well as a halo of zodiacal light.  Zodiacal light is starlight scattered from the lens-shaped cloud of dust that surrounds the central star and extends out to well beyond 1 AU.  The zodiacal light is reduced by about a factor of 10 by the occulter, but it is not eliminated.  However, Cash has shown with simulations that, even with the halo of Zodiacal light, at a distance of 23 light years the occulter and the JWST could observe Venus, Earth, Mars, and Jupiter as bright spots in an image taken of the Solar System.

In the environment of space, Cash’s occulter scheme appears to be fairly robust.  Pin-hole penetrations of the black object by micrometeorites should not be much of a problem, since the occulter could be designed to self-seal around small penetrations.  Moreover, even if the occulter developed many pinholes the diffraction effects would spread the penetrating light over an area much larger than the images of interest, reducing its effect.  Since the flower-petal edges are either completely opaque or completely transparent, there are no partially transparent gray regions with transparency that might be degraded by irradiation.  Cash has also calculated that “bites” removed from the edges of the occulter by space debris can be tolerated as long as they are fairly minor.

What about finding indications of life on such extrasolar planets, once they are found?  An occulter system could also be useful in that investigation.  The initial atmospheres of most planets are made of hydrogen, ammonia, and methane, with essentially no free nitrogen and oxygen.  It is only when life is established that the atmosphere is re-processed to form an Earth-like atmosphere with free nitrogen and oxygen.  The reflected light passing through planetary atmospheres has some wavelengths removed by molecules that selectively absorb those wavelengths.  For example, an Earth-like life-bearing planet where photosynthesis is dominant should have absorption lines for O₂ at about 685 nm and 760 nm and for water at 720 nm, 815 nm, and 960 nm.  Similarly, the atmosphere of a Jupiter-like planet with no photosynthesis should have methane lines at 610 nm, 725 nm, 885 nm, and 980 nm and an ammonia line at 790 nm.  Therefore, given sufficient exposure time to do spectroscopy on the light from the planet’s image, it should be possible to identify life-bearing planets.  Moreover, by looking at the time sequence of modulated light from the planetary image it should be possible to determine rotation rate of the planet and the presence (or absence) of continents.

How long will it take for this phase of planet discovery to begin?  That depends on elections, political decisions, and NASA management.  Presently the Bush Administration has been pushing its Mars/Moon initiative without providing any significant new funding to support that activity.  Consequently, the space sciences end of NASA has been feeling the impact.  Projects like planet finding with new technologies may have to wait for a more enlightened Administration.

References

Extrasolar Planets:

NASA/JPL New World Atlas,  http://planetquest1.jpl.nasa.gov/atlas/atlas_index.cfm

Occulter:

Webster Cash, “Detection of Earth-like planets around nearby stars using a petal-shaped occulter”, Nature 442, 51-53 (July 6, 2006);

Webster Cash’s web site, http://casa.colorado.edu/~wcash

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