By definition, neither brown dwarfs nor planets attain a mass high enough to initiate sustained thermonuclear burning. The manner of their birth serves to define a difference between the two types of substellar mass objects. A brown dwarf forms as a collapsing fragment of a molecular cloud exactly as do more massive stars. Planets, on the other hand, form via an agglomeration of gas, ice, or more refractory materials in a protostellar disk. Astronomers are finding both planets and brown dwarfs at an accelerating pace.
Searches in clusters like the Pleiades, as well as the Deep Near-Infrared Survey of the Southern Sky (DENIS) and 2-Micron All Sky Survey (2MASS), are now identifying large numbers of brown dwarfs, as confirmed by the presence of easily-destroyed lithium in follow-up optical spectra. Since near-infrared searches can detect only the hottest brown dwarfs (2500#DXGT#T#DXGT#1000 K), these brown dwarfs are the most massive or the youngest of the class of substellar objects.
Smaller or older objects will be detectable only in long-wavelength surveys like that to be conducted by the Wide Field Infrared Explorer (WIRE) and eventually by the Space Infrared Telescope Facility (SIRTF). WIRE, to be launched in February 1999, will be able to detect brown dwarfs as cool as 300 K, implying masses as low as 20 Jupiter masses. The combination of the ground- and space-based searches will lead to a full understanding of the lowest-mass objects that the star formation process can produce.
The next decade of research will also lead to great advances in our knowledge of planets orbiting nearby stars. Two instruments will complete a census of planets around stars in the solar neighborhood. The four outriggers of the Keck Interferometer will measure the positions of stars with 10- to 20-microarc-sec accuracy, surveying 1,000 stars for planets as small as Uranus in a jovian orbit. The Space Interferometer Mission (SIM) will push this accuracy down by almost a factor of 10, enabling searches for planets of a few Earth masses around the closest stars and looking for Jupiter-mass planets around stars as far away as 1 kpc.
These astrometric results will complement ground-based radial-velocity studies, which to date have been the most fruitful method for finding planets. Other indirect methods that may prove useful in determining the incidence of planets are microlensing and transit photometry. Both techniques are limited to studying distant stars.
In a few cases it may be possible to detect the visible or near-infrared light from planets directly despite the glare of the parent star. Ground- or space-based telescopes equipped with coronagraphs using sophisticated wavefront control to reduce scattered and diffracted light might image planets directly around the closest stars. Whether the Next Generation Space Telescope (NGST) will be equipped with such instrumentation is at present unknown, but subsequent large telescopes could be.
Finally, there is the ultimate goal of finding and characterizing other Earthlike planets. The mid-infrared is the natural wavelength band in which to carry out these investigations because of the improved contrast between the planet and its parent star, and because of atmospheric tracers such as H2O, CO2, O3, and CH4 that are found uniquely in the 7- to 17-μm band and that may serve as markers of habitable, or even of inhabited, planets. Progress is being made in developing the performance requirements for the Terrestrial Planet Finder (TPF) and for developing the key technologies needed for such an instrument.