Space Studies Board

Strategy for the Detection and Study of Other Planetary Systems and Extrasolar Planetary Materials: 1990-2000

The fundamental scientific rationale for investigation of extrasolar planetary materials has been set out in the Introduction to this report. Within this context, basic research goals may be defined for each of the broad major research areas regarded by COMPLEX as essential components of this new field of planetary exploration.

1. The search for and study of evolved extrasolar planetary systems. Scientific and technological techniques appear to be at or near the thresholds of sensitivity and precision needed to detect and study large planets in other stellar systems. The initial emphasis in this effort will focus on stars in the neighborhood of the Sun, at distances out to 10 to 100 parsecs (1 parsec = 3.26 light years = 2 x 10⁵ AU = 3 x 10¹³ km) to sample an adequate population, and in the mass range of 1 M

or less because the amplitudes of stellar reflex motions are largest for low-mass stars and thus are most readily detected and precisely measured. Roughly 500 stars with a median mass of about 0.3 M

lie within 10 parsecs of the Sun. The primary initial goals are to confirm the presence and frequency of, or limit the existence of, extrasolar planets in the Jupiter- to Uranus-mass range, and to study the dynamics of discovered systems. To achieve these goals requires the following:

Development of instrumentation to enable sensitive search techniques, and application of these techniques to a stellar population sufficiently large to address decisively the fundamental question of existence or absence of extrasolar planetary systems;
Determination of statistical distributions of occurrence, if multiple detections are made, among samples of stars of various masses and evolutionary states; and
Measurement of physical and dynamical properties, including distributions of masses and orbital parameters.

2. The study of systems of dust and gas associated with young stars and considered likely to be planet-forming environments. We know that such systems exist. Requisite observational capabilities for first-order characterization are or can be provided by existing or planned ground- and space-based observatories. Planetary materials in our own system provide some basis for interpretation of data. The general goals of this study are as follows:

Obtain statistics on the occurrence of dust systems among young pre-main-sequence and main-sequence stars of different types and ages;
Characterize dust systems condensed as disks by determining dimensions, masses, and structural elements such as radial distributions of densities, temperatures, and orbital velocities, degrees of asymmetry, occurrence and orientation of jets, ratios of dust to gas, and broadband compositional features; and
Investigate the time scales for the apparent evolution of circumstellar disks around solar-type pre-main-sequence stars from massive, optically thick structures to low-dust-mass, optically thin disks in which preplanetary or planetary bodies may have accreted or may be in the process of accreting.

3. Supporting theoretical and laboratory studies. Real understanding of preplanetary and planetary systems requires a close interplay between new observations and theoretical and laboratory advances in areas related to the origin and physical and chemical evolution of molecular clouds, accretion disks, and planetary bodies, including the planets and accessible planetary materials (e.g., meteorites) of our own solar system. This general goal requires both new theoretical comparisons among such systems and related laboratory experiments, including considerations of the following:

Formation of low-mass stars, substellar objects, and planets and the differences and similarities of their formation conditions;
Various types of binary systems and planetary systems and the relations among them;
Fragmentation of a collapsing and rotating assemblage into multiple objects and their subsequent evolution;
Physical and chemical characterization of stellar nebulae through time, silicate-carbon dust grains and icy-organic grain mantles, the precipitation of solid matter from cooling nebular gas, the conditions under which dust particles and planetesimals accrete or erode, and the evolution of accretion disks from initiation of dust agglomeration into subplanetary objects through subsequent planet-building stages; and
Definition of properties of planetary systems in the process of formation that could be subject to astronomical observation.