A brown dwarf, called GL229B, orbits the red dwarf star Gliese 229, located approximately 18 light-years away in the constellation Lepus. Photo: T. Nakajima (Caltech), S. Durrance (JHU)

Astronomers have identified three brown dwarfs of a type never before observed, so filling in what has until now been an elusive 'missing link' in the range of properties of known brown dwarfs. The discovery resulted from a collaboration between astronomers using the United Kingdom Infrared Telescope (UKIRT) in Hawaii and scientists associated with the Sloan Digital Sky Survey (SDSS).

Brown dwarfs are intriguing objects, intermediate between stars and planets. Often picturesquely described as 'failed stars', they are more massive than Jupiter, the largest planet in the solar system, but they fall short of the minimum mass a true star needs - 8% of the Sun's mass. Stars can shine constantly for billions of years because they generate nuclear energy from the fusion of hydrogen into helium. But brown dwarfs cannot sustain nuclear power production. After a modest initial flush, they cool off and become progressively fainter.

Young brown dwarfs are now known to exist in the hundreds in the Sun's neighbourhood. They have surface temperatures that range down from about 3,500 K (3,200 degrees C) to 1,500 K (1,200 degrees C). Over most of this range their appearances are similar to cool stars of the same temperature. However, as the surface of a brown dwarf cools below 1,500 K, a dramatic chemical change takes place: large amounts of methane form, considerably altering the appearance of the brown dwarf.

The first methane-dominated brown dwarf to be discovered was found orbiting a nearby star by astronomers at Caltech in 1995. More have been found by astronomers at Caltech and Johns Hopkins University since early 1999, largely through two ongoing surveys of the night sky - the Sloan Digital Sky Survey operating a single dedicated telescope in New Mexico, and the 2 Micron All-sky Survey (2MASS), which operates one telescope in Arizona and one in Chile. The methane brown dwarfs have turned out to be almost identical to each other. Their spectra are very similar to those of the giant Jupiter-like planets, even though they are considerably warmer.

This image of the GL229B (center) was taken with Hubble Space Telescope's Wide Field Planetary Camera-2, in far red light, on November 17, 1995. Photo: S. Kulkarni (Caltech), D.Golimowski (JHU) and NASA

The three newly discovered brown dwarfs bridge the gap between the young, warmer group and the cooler methane group. They are not identical, but form a sequence linking the warmer more star-like and the cooler more planet-like types.

Teams of astronomers have been searching intensively for such transition objects over the last year. In February 2000, following the discovery of several new brown dwarf candidates by the Sloan Survey, infrared measurements by Dr Sandy Leggett at UKIRT indicated that three of them might be this sought-after type. Infrared spectra were taken at UKIRT by the observing team of Leggett, Dr Thomas Geballe of the Gemini Observatory in Hawaii, Professor Gillian Knapp of Princeton University and Alexander McDaniel, a Princeton University undergraduate student, working with Xiaohui Fan (Princeton graduate student) and Dr David Golimowski and Dr Todd Henry at the Johns Hopkins University.

The spectra clearly revealed that the properties of these three brown dwarfs fall between the warmer and cooler groups previously known. Both methane and carbon monoxide show up weakly. Methane is absent in the warmer group and strong in the cooler group, while carbon monoxide is the other way around - strong in the warmer group and not seen in the cooler group.

A paper reporting these findings will be published in the Astrophysical Journal. Reports are also being presented at a meeting in Jackson Hole, Wyoming (28 May - 1 June) and at the 196th meeting of the American Astronomical Society in Rochester, New York, 4 - 8 June. Detailed analysis of the spectra is under way to deduce more about the nature of these objects, which may resemble Jupiter and Saturn shortly after they formed about 5 billion years ago.