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- 7.1 RADIAL VELOCITY PLANETARY SEARCHES
- 7.2 ASTROMETRIC DETECTION OF PLANETS
- 7.2.1 GROUND-BASED SINGLE-APERTURE ASTROMETRY
- 7.2.2 GROUND-BASED ASTROMETRIC INTERFEROMETER
- 7.2.3 PRECURSOR INTERFEROMETERS
- 7.3 MICROLENSING PLANET SEARCH
- 7.3.1 PREDICTED NUMBERS OF PLANETS
- 7.3.2 A POSSIBLE MICROLENSING PROGRAM
- 7.4 TESTS OF PRECISION PHOTOMETRY
- 7.5 DETECTION OF ZODIACAL CLOUDS AROUND OTHER STARS
- 7.5.1 SINGLE TELESCOPE OBSERVATIONS
- 7.5.2 INTERFEROMETRIC OBSERVATIONS
- 7.6 CHARACTERIZATION OF YOUNG PLANETARY SYSTEMS
- 7.6.1 THERMAL IMAGING FOR THE KECK TELESCOPE
- 7.6.2 HIGH-RESOLUTION NEAR-IR SPECTROMETER
- 7.6.3 SUBMILLIMETER AND MILLIMETER INTERFEROMETRY
- 7.6.4 PROPOSED NATIONAL MM-ARRAY
- 7.7 DIRECT DETECTION AND CHARACTERIZATION OF PLANETARY SYSTEMS
- 7.7.1 ADAPTIVE OPTICS AND PLANET DETECTION
- 7.7.2 OPTIMIZED CORONAGRAPHIC CAMERA FOR A BALLOON-BORNE TELESCOPE
- 7.8 OBSERVING THE CLOSEST STARS
7.4 TESTS OF PRECISION PHOTOMETRY
Modest amounts of time on a large telescope and careful modeling of atmospheric effects could be used to investigate the limits of photometry on thousands of stars observed simultaneously with large format CCDs. While a space mission to detect planetary transits might be a long-term goal of this program, a great deal could be learned about stellar noise sources, the realities of CCD artifacts, and many other systematic errors by taking repeated images toward a number of fields. This program should be coupled to a program of modeling of photometric performance in a realistic space environment.
7.5 DETECTION OF ZODIACAL CLOUDS AROUND OTHER STARS
One characteristic of stars that will affect our ability to detect planets is the amount of circumstellar dust near the star in the target system, the analog of the zodiacal dust in our own Solar System. The exo-zodiacal emission creates both photon noise and a background against which planets must be detected. Finding an Earth-like planet in the presence of exo-zodiacal emission 1 to 10 times brighter than our Solar System's requires a space interferometer consisting of four 1.5-m telescopes on a 75-m baseline. A brighter exo-zodiacal cloud would require a correspondingly larger, more widely separated system.
Our knowledge of the exo-zodiacal emission comes predominantly from IRAS observations, augmented by ground-based observations of the brightest of these systems,  Pictoris. While new observations of this phenomenon will come from the recently launched Infrared Space Observatory (ISO) satellite, these data are not yet available and the picture described here comes from earlier observations. The combined IRAS, optical coronagraphic, and ground-based mid-IR data for Pictoris show that this disk is seen nearly edge-on and consists of an outer region beyond ~80 AU that emits primarily at far-IR wavelengths and an inner region detectable at shorter wavelengths. Interior to 80 AU that the amount of emitting material falls by more than a factor of 10 below an extrapolation of the outer disk profile (Backman, Gillett, and Witteborn 1992; Lagage and Pantin 1994; Burrows et al. 1996). Similarly sharp density discontinuities inside a radius of ~100 AU are inferred toward other stars on the basis of the IRAS spectral energy distributions. Apart from the Sun, nothing is known directly of the dust content of any disks within a few AU of the star, the region of interest for the detection of extra-solar planets!
Pic is among the brightest of IRAS disks, with a surface brightness roughly 104 times that of our Solar System. The faintest excess of any star measured by IRAS corresponds to an amount of material approximately 100 times that of the Solar System. Roughly 15 to 20% of all main-sequence FGK stars have exo-zodiacal clouds at least this bright. When ISO data become available later this year, we will know more about the quantity of exo-zodiacal material as a function of various stellar properties. The sensitivity limits of ISO in its various spectral bands for this purpose are not yet known, but will probably be in the range of 10 to 100 times that of the Solar System for material outside ~50 AU (see section 8.1 for further discussion of the space-based characterization of exo-zodiacal clouds), but will not be directly helpful for the critical inner zodiacal dust.
 
Figure 7-4 gives a possible model for the radial variation of the optical depth and dust temperature for a solar twin. The inner disk might resemble our own zodiacal cloud, as revealed by IRAS and COBE. The outer disk might show enhanced density like that of Pictoris due to particles in the Kuiper Belt.
A critical point is that IRAS, ISO and even Spitzer measure only the outer reaches of exo-zodiacal clouds, beyond about 30 AU, because of the limited spatial resolution of these small telescopes. In all but the brightest cases, knowledge of the exo-zodiacal cloud within a few AU of the star is masked by diffracted starlight. We propose a two-phase attack on the problem of the inner zodiacal cloud using first a single large telescope (6 to 10 m) and
then a ground-based interferometers consisting of two such telescopes.
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