The Milky Way’s closest-held secrets may soon come to light, when the scientists behind the European Space Agency’s Gaia spacecraft release the mission’s second batch of data on April 25. Designed to create the best-ever three-dimensional map of our galaxy, Gaia is poised to deepen our appreciation not only of the Milky Way but also of its most mysterious denizens: brown dwarfs, strange objects that lie uncomfortably between astronomers’ standard definitions of stars and gas-giant planets.

By more precisely mapping the movements of more than a billion stars as they travel through the galaxy, the Gaia telescope will allow astronomers to tease out the presence of the stars’ smaller, dimmer companions that cannot yet be directly seen. Eventually, the mission should yield millions of these nonstellar objects around the stars it surveys—and help determine if they should be classified as brown dwarfs or planets. “The sample of brown dwarfs that Gaia may find will really be a potential revolution for this field,” says Gaia team member Alessandro Sozzetti, an astronomer at the Turin Astrophysical Observatory in Italy.

Often called “failed stars,” brown dwarfs are not quite massive enough to shine as ordinary stars do by “burning” hydrogen via fusion reactions in their cores. But they are not just oversize planets, either. Unlike gassy worlds such as Jupiter, brown dwarfs are heavy enough to faintly shine by burning deuterium, a stable chemical form of hydrogen. “Brown dwarfs are fascinating missing links,” says Jackie Faherty, a brown dwarf researcher at the American Museum of Natural History who is unaffiliated with the Gaia mission.

Precisely because they reside in the hazy midpoint between stars and planets, brown dwarfs can serve as crucial data points for reaching a deeper understanding of other worlds. They are relatively similar to hot Jupiters, gas-giant planets with bloated, scorched atmospheres from their hours- to days-long orbits around their parent stars. Such planets are, in essence, being cooked from the outside-in. Brown dwarfs, on the other hand, are heated from the inside-out, using deuterium fusion to reach similar temperatures and atmospheric chemistry. And they are far easier to study than hot Jupiters, often floating far from their companion stars or even alone in space rather than hovering like the latter in the overpowering glare of a stellar inferno.

Planet or Failed Star?

The official classification of a brown dwarf depends on its mass, and dates to calculations first performed in the 1960s. Once an object exceeds 80 Jupiter masses, so the estimate goes, it should ignite as a star because of hydrogen fusion in its core. Meanwhile any object less than 13 Jupiter masses should fail to trigger deuterium fusion and thus would be a planet. Anything between those bounds, then, should be a brown dwarf.

Calculating the mass of a star, planet or anything in-between depends on several measurements, including determining the tilt of its orbit with respect to our view from Earth. In all but the rarest of circumstances such measurements remain relatively imprecise, leaving many brown dwarfs with uncertain masses that overlap the dividing lines between star and planet.

The clearest illustration of the resulting confusion may be HD 114762 b, an object announced as a brown dwarf in 1989 by Harvard–Smithsonian Center for Astrophysics astronomer David Latham and several colleagues. When Latham and his co-workers first identified it, astronomers had yet to discover either brown dwarfs or planets around other stars. HD 114762 b was, at the time, a lone oddball that defied easy categorization. The object weighed in at approximately 13 Jupiter masses, as far as Latham could tell, and boasted a close-in orbit much like Mercury’s in our own solar system. Had it been discovered today rather than nearly 20 years ago, it would probably be called a hot Jupiter. But at a time when scientists were expecting no giant planets so close to stars, HD 114762 b’s discoverers decided its unusual characteristics made it more likely to be a theorized brown dwarf than an exoplanet.

In hindsight many astronomers today recognize HD 114762 b as a planet. It is even listed as a confirmed world in NASA's official exoplanet catalogue, despite lingering uncertainties about its mass. That classification would make it the first discovered exoplanet, announced three years before the current titleholder, a system of planets discovered in 1992 orbiting a stellar remnant called a pulsar.

Brown Dwarf Bonanza

Planet, brown dwarf—who really cares about the difference? Emily Rice, a brown dwarf researcher at the City University of New York, argues the distinction is very important to science. “It really does reflect our understanding of the objects and how they formed, what they are and how they fit into our understanding of the universe,” she says. “It’s important to classify these objects in a way that reflects our understanding and keep that up to date.”

HD 114762 b’s parent star happens to be among the billion-plus suns that Gaia now tracks through the Milky Way. With each measurement, the spacecraft improves astronomers’ knowledge of exactly how this star moves in three dimensions around our galaxy, allowing them to disentangle any wiggles and wobbles due to the object’s gravitational pull from the larger, purely stellar motion. With that refined data in-hand, they can then pin down the tilt of the object’s orbit—and tease out its exact mass.

Presently about halfway through its five-year primary mission, Gaia is scanning the entire sky twice per year, capturing data on each star in its sample 70 or 80 times in total across a half decade. Even so, Sozzetti says, its impending data release next week is unlikely to resolve the outstanding mystery of HD 114762 b on its own. It will probably take until the end of Gaia’s primary mission to reach the required precision to nail down the object’s mass. At that point, Latham and his colleagues will know if their oddball officially turns out to be a brown dwarf or the first known exoplanet. “I've been waiting since ‘88,” Latham says. "I can wait a couple more years."”

In the meantime Gaia should still reveal a wealth of information about brown dwarfs with its latest release by virtue of simply finding and tracking many more candidates that astronomers worldwide can then focus on for further study.

Rice and Faherty, along with colleague Kelle Cruz, lead what Faherty calls the largest brown dwarf research group in the world. When Gaia's data is released, members of the group will gather together to pore over it, hoping to find new objects and better orbital constraints on ones already known. “April 25 is going to be a huge day for science in general,” Faherty says. “It’s like being handed everything you ever wanted to know about brown dwarfs.”