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It is hard to think of an area of astronomy that has a more exciting or promising future than that of extrasolar planets. In ten years, a new field has been created—with ongoing, frequent, major discoveries—and the pace continues to quicken as more astronomers shift their research interests in this direction. While much has already been learned, so much more remains to be discovered that it boggles the mind. We have learned about some of the properties of a bit more than 100 planets out of the billions of planets that appear to exist in our galaxy alone. The variety of extrasolar planets discovered to date—and those remaining to be found—may approach the variations observed in stellar populations and galaxy types. Ground-based observatories plan to continue to lead the way forward. Both the U.S. National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA) have recognized the importance of this new field of endeavor, and have made far-reaching plans to build a series of ambitious space telescopes that will carry the search for extrasolar planets to its ultimate goal—Earth-like planets capable of originating and sustaining life.

4.1. Ground

Ground-based observatories bear the brunt of current extrasolar planet search activities, ranging from the Doppler spectroscopy programs of the Geneva Observatory, California/Carnegie, University of Texas, and other groups, to the several dozen transit and microlensing search programs underway around the world. The upgraded CCDs on the HIRES spectrograph on the Keck I telescope in Hawaii, the UVES spectrograph on the VLT, and the HARPS spectrometer on the 3.6 m ESO telescope at La Silla have allowed astronomers to push their Doppler precision to higher and higher levels, to the point where residual velocity jitters as small as m are now achievable, comparable to the convective velocity fluctuations in the photospheres of chromospherically quiet target stars. Maintaining these levels of Doppler precision for the next decade will allow the ground-based Doppler surveys to detect planets with masses similar to that of Saturn orbiting at 5 AU, and to detect planets with masses well below Neptune's mass on short period orbits—"hot Earths" could be found.

Figure 7. NASA's Space Interferometry Mission (SIM) is planned for launch around 2011. SIM will be an optical interferometer with a baseline of 9 m, allowing it to detect Earth-like planets orbiting the closest stars.

While NASA's plans for beginning an astrometric planet search with the Keck Interferometer have been delayed by concerns about installing the four 1.8 m Keck Outrigger Telescopes on Mauna Kea, the VLT's plan to combine four 1.8 m auxiliary telescopes with the four VLT 8.2 m Unit Telescopes on Paranal into the VLT Interferometer is well underway. The four VLTI auxiliary telescopes are expected to begin science operations by 2007. The aim is to attain astrometric accuracies of order 20 microarcsec, sufficient to detect Neptune-mass planets on long-period orbits.

Ground-based transit surveys are limited by the Earth's atmosphere to detecting planets with physical radii similar to that of Jupiter in orbit about solar-radius stars. Reaching down to smaller radius planets will require space-based telescopes.

4.2. Space

The Canadian Microvariability and Oscillations of Stars (MOST) microsatellite has initiated the era of space-based transit photometry telescopes. While MOST is limited by its modest aperture (0.15 m) to photometric monitoring of fairly bright stars, it does offer an improvement by a factor of ~10 in photometric accuracy compared to the ground. CNES/ESA's Convection, Rotation, and planetary Transits (Corot) mission is scheduled for launch in 2006. Like MOST, Corot's first priority will be asteroseismology of a relatively small number of stars, but it will also be able to search for transits by planets as small as Neptune-mass on short-period orbits. Corot's 0.3 m telescope will spend roughly five months staring at the same region of the sky, allowing planets with orbital periods of a month or so to be reliably detected.

Corot will be followed in 2008 by the launch of NASA's Kepler Mission, a 0.95 m Schmidt telescope that is specifically designed to detect Earth-like planets. Kepler will stare at a field 105 square degrees in size in Cygnus containing several hundred thousand stars, and will pick out the best ~105 stars for continuous monitoring during the four-year-long prime mission. Kepler's CCDs have a photometric precision sufficiently high to allow detection of an Earth-mass planet by the dimming of a star's light by 0.01%.

Figure 8. NASA's Terrestrial Planet Finder Coronagraph (TPF-C), planned for launch after 2016, will be a 3.5 m x 8 m optical telescope with a coronagraphic design, allowing Earth-like planets to be imaged next to their host stars.

Once the false alarms caused by situations such as unresolved background spectroscopic binaries blended with foreground stars are eliminated by follow-up observations, Kepler should be able to discover several dozen Earth-mass planets orbiting in the habitable zones of their stars. Kepler will be the first mission to determine the frequency of Earthlike planets, a key factor in planning for the next steps, or at least to place an upper limit on this frequency. ESA's Gaia Mission will be similar to Hipparcos: an astrometric survey of the entire sky, but with sufficient mission-end astrometric accuracy to detect giant planets around thousands of stars. Gaia is intended for launch in 2010 on a five-year mission.

NASA plans to launch the Space Interferometry Mission (SIM) around 2011 (Figure 7). The 9 m baseline for SIM's optical interferometer will allow it to achieve a single measurement astrometric accuracy of microarcsec. With repeated observations, SIM's accuracy will allow the detection and determination of the orbits of planets as low in mass as Earth orbiting the closest stars. Being an astrometric telescope, SIM will determine the true masses of the planets it finds, not just lower limits. SIM will help to survey the solar neighborhood for potential targets for subsequent space missions intended to characterize Earth-like planets.

Following SIM, the next major step in NASA's Navigator Program is to launch the first of two Terrestrial Planet Finders. The first TPF mission will be an optical coronagraph (TPF-C), intended for launch around 2016 (Figure 8). TPF-C will have a 3.5 m x 8 m monolithic primary mirror combined with a series of coronagraphic optical elements and adaptive optics deformable mirrors that will strip away the star's direct, diffracted, and scattered light, allowing planets as faint as the Earth to be imaged on habitable zone orbits around nearby stars. TPF-C will not only seek to detect Earth-like planets beyond those discovered by SIM, but it will also have a low-resolution spectrographic capability that will allow it to search for atmospheric signatures of a habitable planet—such as carbon dioxide, water, and ozone.

Figure 9. NASA's Terrestrial Planet Finder Interferometer (TPF-I) is planned for launch after 2020, consisting of four collector spacecraft operating at mid-infrared wavelengths and a combiner spacecraft. ESA has a similar plan for a space telescope named Darwin. A joint NASA/ESA mission is being considered.

Figure 9. NASA's Terrestrial Planet Finder Interferometer (TPF-I) is planned for launch after 2020, consisting of four collector spacecraft operating at mid-infrared wavelengths and a combiner spacecraft. ESA has a similar plan for a space telescope named Darwin. A joint NASA/ESA mission is being considered.

Figure 9 shows an image of NASA's Terrestrial Planet Finder Interferometer (TPF-I), the successor to TPF-C. TPF-I is planned for launch after 2020, and will be a free-flyer with four collector spacecraft and a single beam combiner spacecraft. The collector telescopes will have apertures of ^3.5 m and will operate at mid-infrared wavelengths. With a ^100 m baseline, TPF-I will be able to search for habitable Earths to a greater distance than TPF-C, allowing a larger number of stars to be searched. Once the extrasolar Earths are detected, TPF-I will use its spectroscopic capabilites to search for biosignatures such as carbon dioxide, water, oxygen, and methane. If the last two molecules can be found in the atmosphere of the same planet, that would be a strong indication that the planet is not only inhabitable, but may be even be inhabited, as methane and oxygen would soon combine chemically in the absence of robust production mechanisms for them. Abiotic chemistry may not be able to produce oxygen and methane in abundance and coexistence—some form of life seems to be required, at least based on what we know about Earth. Finding evidence for methanogenic bacteria on an extrasolar planet would have profound implications for the origin of life in the universe.

ESA has similar plans for a free-flying mid-infrared space telescope intended to discover and characterize Earth-like planets: Darwin. The Darwin mission is currently planned for launch around 2015, prior to TPF-I. However, NASA and ESA have been discussing a joint Darwin/ TPF mission for some time now, and considering the costs involved, a combined mission is an attractive possibility.

Once the TPF/Darwin missions fly and begin to send us images of faint dots of light orbiting around nearby solar-type stars, the push will be on to continue to the next logical step, and to consider building a new generation of space telescopes that will make the TPF/Darwin missions look like relatively modest adventures in comparison. Developing a space telescope that will be able to provide even crude images of the surface of an extrasolar Earth is a challenge that will not easily be met, but which will haunt us until we begin on the path toward its eventual development and deployment. Someday we will gaze at the oceans, clouds, and continents of other worlds, and wonder what it would be like to go there ourselves.

This work has been supported in part by the NASA Planetary Geology and Geophysics Program under grant NNG05GH30G and by the NASA Astrobiology Institute under grant NCC2-1056.

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