Between planets and stars exist a strange set of objects with masses between 20 and 150 times the mass of Jupiter. Brown dwarfs (often called "substellar objects") have masses roughly 20-80 times the mass of Jupiter, but radii that are about the same size. These objects are unable to fuse hydrogen into helium, however, they are capable of fusing deuterium in their centers, and in the sense of energy generation, are more akin to stars than planets. Very low mass (VLM) stars are very small, cool stars with masses between 80 and 150 times the mass of Jupiter, and while they can fuse hydrogen into helium, they represent the smallest possible object that would be referred to as a "star" in the classical sense.
Both BDs and VLMs are intrinsically rare; as close companions to main sequence stars, they are less common than both giant planets and low-mass stars. Since they bridge the gap in mass, radius and temperature space between the realms of stars and planets, they are critical in understanding the formation and structural properties of both. Their intrinsic faintness and rarity make studying them in detail an ongoing challenge that many of the current and near-future astronomy programs will be able to address.
Doppler detection of short-period (a < 2 AU) BD's and VLM stars.