January 21, 2003
Nature defies categorization. Think of the platypus, a curious creature that lays eggs like a bird but suckles its young like a mammal. When first discovered by British explorers in the rivers of Eastern Australia in the late 18th century, this duck-billed, furry beast became a nightmare for taxonomists. Over the last few decades, astronomers have dealt with a similar quandary -- brown dwarfs, a class of celestial objects riding the bumpy dividing line between stars and giant planets.
The existence of brown dwarfs was predicted by theory in the early 1960s. University of Virginia astronomer Shiv Kumar envisioned a continuum of different-sized objects forming out of the gravitational contraction of gas and dust -- from giant, ultra-bright stars all the way down to tiny, star-like spheres that barely flicker.
These latter theoretical objects were first dubbed "black stars" or "infrared stars" before American astronomer Jill Tarter coined the term "brown dwarf" in 1975. The "brown" in brown dwarf is somewhat of a misnomer since these objects appear more red than brown. However, the term "red dwarf" was already taken to designate a certain class of stars.
The first conclusive observational evidence of a brown dwarf, Gliese 229B, didn't come until 1995. Soon thereafter some astronomers referred to brown dwarfs as "failed stars" while others dubbed them "super planets." An announcement earlier this month highlighted the discovery of a brown dwarf in the Sun's neighborhood, just over 12 light years from Earth.
According to present theory, planets and stars differ in two dramatic ways: how they form and how they shine.
Planets emerge from the coalescence of small rocky or icy bodies in a disc around a star -- a so-called proto-planetary disc. Stars form from the gravitational collapse of a molecular cloud of gas. But recent discoveries of large gaseous planets orbiting snugly around their parent stars has made even this seemingly solid distinction questionable. And brown dwarfs cloud the picture all the more: the star formation-like mechanism thought to create them leaves many with masses closer to big extrasolar planets -- outside our solar system -- than any stars.
Stars shine from thermonuclear reactions within their cores, whereas planets tend to be more passive reflectors of this fiery light from their parent suns. However, upon closer examination even this difference dims. Stars, especially those with close, bright companions, also reflect starlight. Planets, particularly giant gaseous ones like Jupiter, may emit more of their own light than they reflect. And brown dwarfs may, at one time or another, both generate their own twinkle from nuclear fusion and reflect starlight.
Astronomers continue to get caught up in these same arguments about stars and planets. This serves to highlight the particular confusion when trying to characterize brown dwarfs, which "don't allow for easy pigeon-holing, and scientists are pigeon-holers," says Dr. Davy Kirkpatrick, an infrared astronomer at the California Institute of Technology.
Brown dwarfs have masses that range between 10 and 70 times the mass of the planet Jupiter. In theory, an object with a mass of less than about 8 percent of the mass of the Sun, or 75 times the mass of Jupiter, cannot sustain thermonuclear fusion of hydrogen in its core. However, brown dwarfs of sufficient mass produce energy from the fusion of heavy hydrogen, or deuterium, into helium nuclei within their cores, but this early fire peters out after only a few million years. After that, these brown dwarfs - like those that never managed to stoke a nuclear fire within -- emit a faint infrared glow as they continue to contract and release gravitational energy.
NASA's Space Infrared Telescope Facility, managed by JPL and launching this spring, should be particularly adept at picking up this faint glow, thereby allowing astronomers to further zero in on these astronomical oddities.
One way to distinguish between different types of celestial objects is to look inside them. Stars mix the gaseous contents of their cores and surfaces through convective heating and cooling. Planets, particularly large gaseous ones like Jupiter, form as smaller solid bodies clump together, and emerge with chemically differentiated interiors: a solid metal core and gaseous outer layers. Current theory sees a brown dwarf's internal structure as more similar to a star. Unfortunately, astronomers have been limited in their ability to see inside these curious objects to tell for sure. With the penetrating vision of the Space Infrared Telescope Facility, it may be possible to tease apart the internal structures of brown dwarfs.
Once the infrared-sensitive Space Infrared Telescope Facility is launched and operating, it should easily pick up the heat signature of yet undetected brown dwarfs, and thereby dramatically increase their tally. Armed with these new observations, astronomers should not only appreciate the idiosyncrasies of a cosmic platypus, but also some of the subtleties behind star and planet formation.
Contacts: JPL/Paul Morledge (818) 354-0850