In principle, weighing an astronomical object is easy: just track its orbit around another similar body and apply basic physics to derive its mass. In practice, however, astronomers can use this approach only rarely, and only a handful of brown dwarfs — substellar gas balls that can't sustain nuclear fusion in their cores — have been weighed this way even coarsely. Consequently, to decide whether a pinprick of light like 2M 1207b comes from a "planet" (with less than 1.3 percent of the Sun's mass, or 13 Jupiters), a brown dwarf (13 to 75 Jupiters), or a star (more than 75 Jupiters), one usually must rely on evolutionary models that relate an object's luminosity to its age and mass.

Now, though, an international research team has determined an orbit — and a precise mass — for the youngest brown dwarf yet. There's just one problem: that object, AB Doradus C, isn't a brown dwarf after all. It has an ostensibly stellar mass of 90 Jupiters — up to twice what evolutionary models predict for the 50-million-year-old object given its distance and near-infrared magnitude.

University of Arizona astronomer Laird M. Close discovered AB Dor C on February 4, 2004, using the Very Large Telescope in Chile. When he found the object, it was just 1/6 arcsecond (2.3 astronomical units) from the 7th-magnitude type-K variable star AB Dor A, which is 120 times brighter. His team then combined the adaptive-optics discovery image with existing astrometry from the Hipparcos satellite and radio telescopes to determine AB Dor C's orbit and mass.

Spotting and "weighing" so dim a companion is a noteworthy achievement, astronomers agree. But Close and his colleagues may get more press for the inferences they draw than from their technical prowess. Because AB Dor C's dynamical mass significantly exceeds the predictions of most widely used evolutionary models, Close implies, those models are now suspect. "Some young objects that people are calling brown dwarfs are really low-mass stars," he says, and "things that people are calling free-floating planets, in almost every case, are likely low-mass brown dwarfs."

That doesn't sit well with Isabelle Baraffe (Astronomical Research Center of Lyon, France), one of the reigning model's architects. "We never claimed that our models . . . can be blindly applied to very young objects," she says, and earlier studies already have highlighted similar discrepancies. What's more, she doubts that Close's team really can constrain the primary star's mass and age as precisely as its report in last Thursday's issue of Nature suggests — and AB Dor C's inferred mass depends sensitively on both these quantities. Unfortunately, independent tests may be few and far between until NASA's SIM PlanetQuest (formerly the Space Interferometry Mission) finds and characterizes numerous binaries in young open clusters.