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J Observational Requirements for Identification of Extrasolar-system Planets OBSERVATIONAL HISTORY The observational search for extrasolar-system planets is rooted in studies of binary and multiple-star systems in the solar neighborhood. Knowledge of the stellar mass distribution in these systems, especially in the large preponderance (>50 percent) of binary stellar systems, is important to the theory of star formation. Many of the companion stars are unseen and can be detected only indirectly by measurement of stellar reflex motion. Traditionally, astrometric measurements of the motion of nearby stars have been based on data recorded in plate collections spanning several decades. Studies of this nature have produced reports of purported planetary-mass companions to nearby stars, the most famous example be- ing Barnard's star. One interpretation of the astrometric measurements for this nearby stellar system (the second closest to the Sun) proposes the existence of two sub-Jupiter-mass planets with periods of 12 and 20 yr; this interpretation, however, has been questioned on the basis of independent observations. Based on astrometric data a substellar-mass companion, VB 8B, was also proposed for Van Biesbroeck 8 and, as noted in Chapter 4, a putative substellar object was reported to have been detected directly by speckle interferometry but was not found in subsequent infrared imaging. Moreover the astrometric data that initially suggested the presence of a 44
45 substellar-mass companion to VB 8 are now being questioned by later photoelectric astrometric measurements. Recent studies on precision radial velocity measurements suggest the possible existence of orbiting bodies around a small number of nearby stars, one of which fly Cephei by is inferred to have a mass perhaps as small as that of Jupiter. These detections have yet to be confirmed. Although, as noted earlier, the data for HD114762b are promising, and Gliese 569B, GD 165B, and the very recently announced Gurus objects appear to be viable candidates, at present there is no conclusive evidence from any observational technique for even a single star with a substellar-mass companion, and there is considerably more uncertainty regarding current claims of discovery of planeta~y-mass bodies. It is with this background in mind that the committee examines the observational requirements for an ext;rasolar-system planet search. The scientific value of searches for new planets lies in knowing the statistics of their occurrence and their locations as a basis for continuing physical studies. These benefits decline with uncertainty about mass; that is, with ambiguity as to whether the orbiting object is, in fact, a planet. The benefits increase in proportion to the proximity of the discovered sys- tem, which translates into greater accessibility for further research. Beyond proving the "existence theorem" by discovery of planets, each potential search technique provides some physical information about the discovered objects more or less enmeshed with ancillary assumptions required for interpretation. In the next section, the committee discusses possible ap- proaches to direct detection of substellar- and planetary-mass companions. DIRECT DETECTION Any direct~etection system consists of a telescope coupled to ancil- lary instrumentation that analyzes and records the collected radiation. The efficiency with which a telescope performs its functions of gathering, re- laying, and focusing radiation is crucial because of the very low intrinsic brightness of extrasolar planetary material The committee considers for illustration the challenge of observing Jupiter from a nearby star, using two space-borne telescopes in NASAs astrophysics program: the Hubble Space Telescope (MST) and the proposed Space Infrared Telescope Facility (SIRIT;). The committee also considers the limitations on direct detection of a Jupiter-size planet in orbit about another star. Jupiter is selected for this analysis because its brightness and relatively large distance from the Sun optimize the chances of direct imaging. The HST and SIRTF are satellite observatories designed to have diffraction-limited optical quality. HST has a 2.4m aperture and a comple- ment of cameras and spectrographs for recording the ultraviolet and visible
46 light typical generated by stars. SIRTF is expected to have a 0.85-m aperture and to operate in the infrared from 2 to several hundred microns. Dust, rocks, or planets gravitationally bound to a star and warmed by its radiation would emit thermal radiation in SIRTFs spectral range. Indeed, the dominant noise background for most SIRTE: observations would come from solar system zodiacal dust through which the telescope must view astronomical targets in deeper space. In principle, a "Jupiter" might be di- rectly detected by either HST or SIRTF, in reflected starlight or in emitted thermal radiation, respectively. The dominant noise for such detections arises from the portion of the stellar image that overlies the nonstellar signal. The stellar background at any angular distance from the center of the telescopic image of the star is determined by the diffraction pattern of the telescope pupil and by scattering from the telescope optics. 'There is a strong wavelength dependence for both the diffraction image and the energy distributions of the star and the planet. These points are illustrated by the following analysis. There are no stars closer to the Sun than 1 parsec, but there are several hundred within about 10 parsecs. For realism the committee considers the problem of detecting Jupiter in our solar system from an intermediate distance of 5 parsecs. At its greatest elongation from the star, the apparent angular distance to the planet would be 1 arcsec, and its flux at 0.25 I'm wavelength about 10-9 tunes that from the star. At 20 ~m, where Jupiter's thermal emission peaks, the flux ratio is 10-5. Jupiter's 10-5 to 10-9 contribution to the bulk light would be undetectable as an incremental light intensity and thus rules out detection without spatial discnmination. Consider the limitation on spatial resolution imposed by the optical properties of the telescope. For diffraction-limited optics, the dominant component of the HST and SIRTF point-spread functions the telescopic unage of an unresolved astronomical source is an Airy diffraction pattern with the first dark ring at radius 0.026 arcsec for HST at 0.25 Em wavelength, and at 5.9 arcsec for SIRTF at 20 ~m. In the latter case there is no hope of distinguishing Jupiter's image because its separation from the Sun is much smaller than the image size. In the future, direct detection of planets in the infrared, making use of the more favorable contrast than at visible wavelengths, may be possible using interferometric techniques. Moreover one should also note that Jupiter radiates more energy than it absorbs from the Sun (as do Saturn and Neptune as well), and a Jupiter-like object could be observably self-luminous in the infrared if it were located >5 AU from a Sun-like primary. Assessments of detectability become considerably more complex than that given here when this property is included, but potential observability at large orbital distances clearly increases, and searches for
47 companions In the mass range of the giant solar system planets are planned to be a major scientific component of the SIRTF observing program. For HST the image lies at 40 times the Airy radius on the wing of the stellar image profile. The ratio of planetary to stellar flux lying within an Airy radius centered at Jupiter's position is approximately 10-5. (Allis ratio vanes as the third power of the ratio of telescope aperture size to wavelength.) The 10-5 contrast ratio would pose an impossible obse~g challenge for HST. The only HSI instrument with any imaginable potential for adequately sampling the point-spread function at 0.25 I'm wavelength is the faint object camera FOCI. However, that instrument is limited to a low count rate by virtue of real-time photoelectron counting, and from a practical viewpoint the accumulation of the 10~° photoelectrons per resolution element required to achieve a 105 signal-to-noise ratio would be impossible. While a hypothetical planet could not be made substantially larger or more reflective than Jupiter and remain a planet, it could gain contrast against diffracted starlight if it were at a larger distance from the star. The stellar diffraction pattern falls off as the inverse third power of the angular separation, whereas the planet brightness will Daly as the inverse square of this same quantity. However, long before the 10-5 ratio of disadvantage could be recouped, the planet's flux would fall below the detection limit of HST for a point source. Apodization is a technique that in principle can enhance contrast for planetary detection by suppressing the wings of the stellar diffraction pattern. This normally involves masks on the first telescope focal plane and on a reimaged pupil. The first telescope focal plane is reimaged onto the detector. The apodization benefit is countered by any light scattering due to residual roughness or dust on the telescope optics. This light is not localized on the reimaged pupil; it passes the mask and forms the image wings after the pupil diffraction has been suppressed. HST has an anodization capability in the FOC, but the benefit of this feature is expected to be quite limited Hog to light scattering by the residual roughness of the Hers primary and secondary mirrors The committee concludes that if our solar system were to be observed by a HST or SIRTF at a nearby star, Jupiter would not be directly detected. This does not preclude any attempt to utilize Hers potential advantage in spatial resolution over ground-based telescopes in a well-structured search for companions to a selected sample of stars in the solar neighborhood. Technical improvements in direct-imaging capabilities are likely in the next generation of space-borne telescopes and are discussed briefly later in this chapter (under the heading Future Observing Systems). For the present, however, only the ind~rect~etection methods discussed in the following
48 section are capable of exhaustive and sensitive searches leading to valid statistical conclusions. INDIRECT DETECTION Since direct-detection instruments and techniques available now or probable in the near future appear to have limited potential for suney- ing a statistically significant population of stars, and for identifying orbiting planetary-mass objects if they exist, the three technical approaches discussed below rely on indirect planetary effects on stellar light. These include (1) image displacement due to reflex motion (astrometry), (2) Fraunhofer spec- tn~m Doppler shift due to reflex motion (Doppler spectroscopy3, and (3) modulation due to partial occultation of the star by the planet (photom- et~y). In each case the amplitude and timing of an effect depend on the planetary mass Me and the orbit size, given the stellar mass M*, which can usually be determined from its spectral type and luminosity. The period, P. directly yields the sem~major awns, a, of the planetary orbit: a _ (`M,,,. + Mpyl/3p2/3 where P is in years, a is in astronomical units and M. and Ma are in units of Me = 1 solar mass. Figure 5.1 shows the estimated mass distn~ution for observed main- sequence stars in the solar neighborhood. The median mass is about 0.3 M<>, with a 90 percent a pnori probability that the mass lies between 0.1 and 0.8 Me. The turndown below 0.2 Me may be a selection effect since such stars are very difficult to detect due to their faintness, but the distribution is valid for the current application. The population median 0.3 Me is chosen as a typical stellar mass for the calculations below. In order to deal with a sample of a few hundred stars, a typical distance of 10 parsecs is assumed in the numerical examples mat follow. For the baseline analysis, the observable effect of a single planet in a circular orbit is computed, treating Mu and a or P as free parameters. Orbital periods for planets at distances of 5 to 20 AU from a 0.3-M star are 20 to 160 yr. In reflex response to a planet in circular orbit, a star will execute syn- chronized, coplanar circular motion of radius aMpM* ~ about the barycen- ter. On the plane of Me sky, the apparent stellar motion that can be detected by astrometric methods will be an ellipse of (angular) semimajor axis x=aMpM* r where r is the distance to the star. If ~ is in parsecs and a is in astronomical units, then x is in arcseconds. Only the eccentricity of the apparent ellipse,
49 0.15 cam 0.10 - _ z ~ O _ 0.05 O 0.05 ,. it. i i l l M/M<, 0.1 0.2 0.3 0.5 . 3.0 5.0 40.0 1 1 1 1 1 1 ~ 1D 2.0 - · 1 - - ___ . iM,.~ I Minimum I Stellor ~ Ma ss0.08 Mo . : ~2 _ I_. ~___7 8. . , Median ~Q2SM~, ~ .6 _ Hi MASS DISTRI8UTIObI OF MAIN SEQUENCE STARS NEAR THE SUN Tatol number of stars: 0.12pc-3 _ 4 l ~ - l l l l -1.0 -0.5 0.0 log ( M/Mo) 0.5 i.0 FIGURE 5.1 The mass distribution of main-sequence stars in the solar neighborhood. Ike dashed line represents a smoothed compilation of data from several sources by Miller and Scalo (~1979] Astrophys. ~ SuppL So: 41, 513), while the solid line is based on the more recent star counts of Heeled et al. (~19~33] LOU Colloquium No. 76~. I-he ordinate is the number of stars per cubic parsec, per interval of log (M/M<~; the overall space density for both distributions is 0.12 stars parsec~3. Also indicated are the median mass (0.25 to 0.30 Meg and the corresponding absolute visual magnitude, Mv. The turnover in the mass distribution below ~0.2 M<3 (My = 133 is believed to be real, but the true number of very-low-mass stem is probably underestimated because of incompleteness in the available star catalogs. and not the amplitude a, depends on the angle between the orbit pole and the line of sight. For a nominal Jupiter at a distance of 10 parsecs, orbiting a 0.3-M<> star with a semimajor axis of 5 AU, ~ = 1.7 milliarcsec. For a nominal Uranus orbiting the same star at 1 AU, the apparent displacement would be only ~15 microarcsec. This orbital displacement must be distinguished from the displacement due to stellar proper motion, which for a velocity of 10 lan s~ ~ relative to the Sun amounts to 0.2 arcsec yr~ i. This suggests that a successful astrometric detection, even for Jupiter, requires observations over a period of time comparable with the orbital period. In any case, a determination of the orbital period combined with an estimate for M* from
so the spectral type of the star is needed to estimate a, and hence to permit determination of Mp. Ground-based astrometIy using photographic plates achieves a typical accuracy of 100 milliarcsec in determining the relative positions (sepa- rations) of stars. Using photoelectric techniques at ground-based tele- scopes, current studies indicate that a few-milliarcsecond accuracy appears possiblewhich is remarkably good, considering that the center of a typical ground-based "seeing" image (1 to 3 arcsec3 must be determined to 1 part per 1000 in the process. HST is expected to be able to make astrometnc measurements using its fine guidance sensors Weiss) with about this same relative accuracy. The reflex motion of a star in response to an orbiting planet also results in a periodic Doppler shift in stellar spectral features of amplitude AA/A = (V/c) sin i, where ~ is the wavelength, c is the speed of light, i is the angle between the orbit pole and the line of sight, and V is the stellar orbital speed. With G representing the gravitational constant, V = G-~12Mp(M*ay-~/2~~ `, ` 11, \-~/21~ ^- oUlV1pL1V1*"J ala where masses are in M<3 and a is in astronomical units. By identifying and accounting for all extraneous contributions to the line-of-sight motion, it is possible to identify small residual shifts having periodic variation indicative of an orbiting mass. Once again, knowledge of P and M* would yield the value of the semimajor axis, a. The planetary mass could also be determined if sin i were known, but Doppler spectroscopy cannot determine it The average value of sin i is 0.79 for randomly oriented orbits, and this value is used here to determine the sensitivity of Doppler spectroscopy for exploring the domains of a and Mp. For a nominal Jupiter orbiting a 0.3-M star with a semimajor ems of 5 AU, V = 24 m s~i, and for a nominal Uranus at 1 AU, V = 2 m sol. Again, this periodically varying orbital signature must be distinguished against a background of a typical stellar radial velocity of 10 km s~i, and the broadening of spectral lines due to stellar rotation (of the order of 1 hen s~i for a solar-type star) and a host of systematic instrumental shifts. Additional complications may arise from stellar convection and inhomogeneities in brightness across the stellar disk as, for example, in solar flares. By careful application of Doppler spectroscopic techniques using con- ventional coude or echelle spectrographs, it is possible to achieve ~10 m s~i accuracy in the line-of-sight motion. At a Apical resolving power of A)/A = 5 x 104, this implies dividing and determining the centroid of a spectral line to about 1 part in 103. Measurements with an accuracy of this order have been reported, but this is currently restricted to observations
51 of bright stars and requires that systematic error sources be very carefully controlled. In principle, it is possible to search for planets by photometrically monitoring the light from stars, looldng for decreases due to partial occul- tatio~ dung the p~net's transit ~ front of the stellar disk. Me effect is small and difficult to observe. For a nominal Jupiter orbiting a 0.3-M star at 5 AU, a dip in signal amplitude of ~7 percent would be observed for a duration of ~20 hours for a diametric transit (which would occur once every 20 yr). The amplitude of the signal drop scales appro~nmatet,r as Rp 2M* -i 6, where Rat is the planet radius in Jupiter radii, and the duration scales as M* 0 3ai/2 (=e Appends A). A clear ident~cadon by this technique appears unliked. EVALUATION OF PROPOSED INDIRECT TECHNIQUES In this section, with the assistance of briefings by researchers expert in the principal search techniques discussed above, the committee evaluates the expected applicability of proposed and present instrumentation to the search for extrasolar planets. Astrometric Telescope Facility The committee has reviewed proposed plans for an astrometric tele- scope facility (ATF) on the Space Station that would take advantage of the smaller and more stable images available outside the atmosphere. The design of I~ accuracy for ATE is 10-5 arcsec an extraordinary goal, considering that it represents about 1 percent of the Sun's angular size sub- tended at 10 parsecs. Although current studies indicate that this accuracy is technically approachable, uncertainty about possible systematic errors due to offsetting of the stellar light centroid by star spots represents a potential lien against achieving the design goal of 10-5 arcsec. To investigate the planetary domains accessible to an astrometric search, the committee considers the interpretation of a detected 3 dis- placement by an ATF-like facility, that is, the discovery of a stellar ellipse of amplitude x = 3 x 10-5 arcsec. It is assumed that detection is made during a program of observations of several hundred nearby stars, implying an inventory out to at least 10 parsecs or so. Me actual distance to any particular star surveyed would be accurately determined by its annual parallactic motion of amplitude ~10-2 arcsec.) Adopting r = 10 parsecs, and recalling that a is determined from P given M* from the spectral type, the planet mass is also determined uniquely from the obseIved amplitude x . Figure 5.2 shows the locus on the (a, Mp) plane Ending to a
52 -2 - ~ -4 - o 5 -6 ''1 1 -3 -2 log (a /1 AU ) o 1 FIGURE 5.2 Me discovery space for propped astrometuc and Doppler spectroscopic planetary search programs The zone of detectability for a typical 0.3-M<3 star is unshaded. For astrometric searches, a 3~ detection is assumed at ~ distance of 10 parsecs, with ~ = 10-S arcsec. Corresponding limits for an improved ground-based astrometnc search (a = 10-3 arcsec) are also shown. For Doppler searches, a 3cr detection is assumed with an average value of sin i = 0.79 and a = 10 m s 1. Dashed lines show the same detection limits for planets around a 1-M star. Also shown are minimum and maximum values of the semimajor axis, set By the stellar radius and an orbital period of 20 yr, respectively. marginally detectable (3~) displacement for stars of 0.3 and 1 ME >, bounding the region of astrometrically detectable planets. It can be seen that both Jupiter and Saturn would be readily detected at a distance of 10 parsecs, provided the observations spanned a sufficiently long period of time (~20 and ~50 yr, respectively, for a 0.3-M star). For an observing campaign with a duration of approximately 20 yr, the smallest planet detectable using this technique is of the order of 6 Me, about one-half to one-third the masses of Uranus, Neptune, and hypothetical giant planet cores. If a star with a sufficiently large reflex motion were to be discovered, it would be possible in principle to determine both the eccentricity and inclination of the companion orbit. The intrinsically interesting case of
53 multiple-planet systems would pose the further analytic challenge of disen- mngling multiple periodicities in the stellar reflex. Such an analysis, which clearly implies long observing times, would address the central question of whether or not the orbits are coplanar as would be expected on the basis of current theories of solar system formation. It is evident from the above discussion and from Figure 5.2 that an astrometnc accuracy approaching the 1 ~ design goal of the proposed space- based ATE defines an accessible discovery space that includes a relatively broad and scientifically crucial range of planetary masses. For comparison, ground-based photoelectric measurement at a site with excellent seeing (assumed ~ = 10-3 arcsec) could marginally detect Jupiter-mass planets in Jupiter-like orbits around a 0.3-M star ~10 parsecs from the Sun by no means a trivial accomplishment given the intense current interest in such searches and the implications of their results. But technical limitations on ground-based astrometnc accuracy clearly constrain the extent to which it can address the existence, masses, and dynamics of extrasolar planetary systems, and in particular the central question of whether they do or do not resemble our own system. Doppler Spectroscopic Planet Searches 1b explore the domain of the (`a, Mp) plane accessible to a Doppler spectroscopic planet search, the committee considers a search program at a 1 ~ accuracy of 10 m s~ ~ . A circular orbit is assumed for simplicity, with an average value of sin i = Q79, to estimate the radial velocity amplitude. (Note that, unlike the astrometnc signature, the observed Doppler shift does not depend on the star's distance from the Earth.) As before, the committee assumes stellar masses of 0.3 and 1 Me and plots the locus corresponding to a 3~ detection in Figure 5.2. For a 0.3-M star, Jupiter- mass and Saturn-mass planets would be detectable at the stated accuracy if their semimajor axes were respectively ~2 AU and inside 1 AU. Although at the assumed level of precision the Doppler technique is not capable of detecting bodies as small as Uranus or Neptune at any rational orbital distance, it is apparent from Figure 5.2 that the Doppler and astrometric techniques explore complementary domains of discovery space. The former is sensitive to planets with smaller semimajor axes (and thus larger orbital velocities) whereas the latter is not and therein, in the absence of preconceptions derived from the local example about what planetary mass~istance relationships "should" be, lies the value of the Doppler approach. The Doppler discovery space increases with lower stellar mass, but unfortunately a reduction in mass also lowers the luminosity for main- sequence stars and so reduces the number of accessible candidates for
54 ultrahigh-resolution Doppler spectroscopy. The ultimate sensitivity of this technique may be limited not by instrumental considerations, but rather by intrinsic noise in the spectrum due to stellar turbulence. In addition, although the value of sin i can be determined for a high signal-to-noise as- trometric detection, it cannot be deduced from Doppler spectroscopic data alone. Stellar photometry sensitive to star spot rotation and spectroscopy of rotationally broadened Fraunhofer lines may give dues to the value of sin i, but it seems unlikely that these techniques will reliably reduce the associated uncertainties in Me and in the dynamical arrangement of multiple orbits. Several ground-based Doppler-spectroscopic planetary search pro- grams at accuracies near or better than that assumed here are in progress. Reports noted earlier of detections of a substellar companion to HD114762 and a roughly 1-M planet orbiting ~ Cephei (at about 2 AU) both utilized this technique. Neither a major technical breakthrough nor space- based system deployment is required to pursue this approach. Photometric Planet Searches It is clear from the Jupiter transit example (under the heading Indirect Detection) that the photometric technique must deal with events that typi- cally may be of short duration and low frequency, and are detectable only if the observer is in or very near the planet's orbital plane. Because of the low a pnori probability of meeting this requirement and thus the size of the star sample required for a minimally successful photometric planet search (see Appends A), COMPLEX concludes that this technique as currently developed is not yet adequate to determine the statistical occurrence of planets. Detection probabilities could be somewhat enhanced by preselect- ing eclipsing binary stellar systems for photometric observation, presuming that possible planetary orbits would be coplanar. This enhancement would not be sufficiently robust, however, to develop statistical conclusions about the natural occurrence of planetary systems in general. The method has several other potential drawbacks. First, if the obser- vations were ground-based, they would be affected by variations in atmo- sphenc transparency. Second, the intrinsic variability of stars e.g., small variations in stellar flux caused by convection and star spots may confuse interpretation of the data. Some observations from NASAs Solar Maxi- mum Mission show the whole~isk variability of solar flux to be as much as 25 percent of the signal dip expected from a transit by an Earth-size planet (~10-4~. Third, the interpretation of a photometric occultation event as being caused by a planet would not be unambiguous; small stellar objects such as white dwarfs and substellar objects could produce occultation erects not readily distinguishable from those due to planets.
ss FUTURE OBSERVING SYSTEMS COMPLEX has also reviewed other potential approaches to the search for extrasolar planets. These may be grouped under the rubric of inter- ferometry and generally represent attempts to improve resolution without increasing collecting area and hence expense. For the most part these systems exist at the conceptual level, with little or no engineering devel- opment to date. One specific system, Precision Optical Interferomet~y in Space (POINTS), is under active design study. The committee views the design goals and anticipated capability of POINTS as representing a substantial advance over demonstrated capabil- ities of existing systems, or of expected direct engineering descendants of existing systems. Given its future potential, POINTS is, in the committee's opinion, worthy of continued study. The relative state of development of POINTS with respect to ATE, however, places POINTS in a candidate po- sition for detailed followed studies and extended surveys after completion of an initial survey at ~10-5 arcsecs with ATF-like facilities and at 10 m s~i with Doppler spectroscopy. Future application of ground-based infrared interferometry also holds the promise of detection and perhaps even imaging of extrasolar systems at high resolution. Experiments currently in progress at a wavelength of 10 Am are expected to achieve a spatial resolution of ~10 milliarcsec using a baseline of ~100 m. Again, this approach will initially be limited to only the brightest stars in the solar neighborhood. COMPLEX encourages further development efforts in areas such as this, because they are likely to make important contn~utions in the future. The difficulty of direct imaging of planets around nearby stars was addressed earlier in this chapter. Nonetheless, such imaging is likely to be extremely important in the study of precursors to planetary systems de- scn~ed in the next chapter. Ultimately, the availability of large space-based telescopes of high optical quality may permit imaging of extrasolar-system planets. The contrast of a faint companion or circumstellar material against the stellar diffraction wing improves as the third power of the telescope diameter. Further, if dust contamination and residual surface roughness of the telescope optics can be suppressed, extraneous scattered light can be reduced, and innovative image tailoring by anodization can provide ad- ditionally enhanced contrast. Improved management of astronomical light within telescope systems would significantly augment the range, spatial specificity, and overall effectiveness of extrasolar planetary studies. RECOMMENDATIONS REGARDING TECHNIQUES FOR PLANEI SEARCHES General recommendations concerning the programmatic and technical elements of a scientific strategy for detection and study of extrasolar plan-
56 Stan materials are presented in Chapter 8. Here the committee sets out those recommendations that specifically relate to the technical considera- t~ons discussed in this chapter: · (2f the three different technical approaches to indirect ¢nonimagingJ detection of remote planetary systems examined here in detad astrometry, Doppler spectroscopy, and photome~COMPLEX recommends that Snowily attention be given to the first two of these technics es. · To achieve the significant improvement in as~ometnc accuracy needed to address serious the detection am1 study of e~rasolar planetary systems, space-based ins~umentaiion, with its potential of a more than: 100-fold gain in sensitivity over current g~ound-based instruments, will ultimately be required. COMPLEX believes that the promise of significant advancements in planetary science Justifies finisher invesagaiion of the technical issues involved, and rec- ommends active development toward timely Earth-orbital deployment of such a capability. The committee farther recommends that in the interim, ongoing g~ound-based searches be continued at their state-of-the-art accuracy, and that potential for improvement of this accuracy be investigated and implemented if technically and financial) feasible. . Doppler spectroscopy extends the range of potential planetary detec- nons to planets that have a Jupiter (and possibly SamrnJ mass in relatively small radius orbits and would no! be detectable by astrometnc techniques. This complementary technique does not require space-based facilities, and pilot invesagatzons are already weld under way. COMPLEX recommends that Doppler spectroscopic searches be continued at their swe-of-the-art accuracy. · Interferomewc techniques from the optical to 1000 Am will evenmal~ yield pow~rfi~l tools for detecting condensed objects and mapping dust dis~i- bunons around nearby stars, and for detailed imaging of relevant objects and regions. COMPLEX encourages continued development of promising Earth- based and space-based instruments and techniques of this me for follow-on defiled study of 0arasolar planetary systems. · ~~ regard to unaging instruments, given Weir present importance in the study of preplanetary precursor systems and their ultimate potential for direct imaging of evolved planets, COMPLEX urges that the design of fixture telescopesincorporatediffrac~n controland techniques for the reduction of light scattering due to dust contamination and to residual errors in mirror figure. Appropriate technology to achieve these goals should be developed arid implemented us space-based telescopes. Where appropriate and feasible, consideration should be given to improving the optical performance of existing telescope facilities envisioned for such studies