Weather report for a distant 'brown dwarf'? No problem: partly cloudy.
The atmosphere of a brown dwarf 6.5 light-years from Earth rains molten iron and has hurricane-force winds. Here's how astronomers know that – and why that information is useful.
Earthlings have received weather reports from Mars, Venus, Titan, and other objects in the solar system. But a weather report for an object 6.5 light-years away?Skip to next paragraph
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Meet Luhman 16B, a "partly cloudy" brown dwarf boasting searing temperatures, hurricane-force winds, and precipitation that varies from molten-iron rain to hot "snows" of sand-like or rock-like grains.
Luhman 16B's weather is intriguing for what it implies about the nature of brown dwarfs, researchers say. It also points to astronomers' growing ability to directly observe smaller, dimmer objects in the quest to discover and characterize Earth-like planets orbiting other stars.
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Such efforts have received a boost from a new instrument bolted to the back of Gemini South, an 8-meter telescope located high in Chile's Atacama Desert. The new camera, the Gemini Planet Imager (GPI), is designed to overcome the twin hurdles of the distorting effects of Earth's atmosphere and the blinding effect of a host star's light to spot and characterize Jupiter-class planets at Jupiter-like distances from their host stars.
On Tuesday, astronomers released a pair of "first light" images from the new camera, one of which shows the planet Beta Pictoris B, some 62.4 light-years away, orbiting its star. Beta Pictoris B is about eight times more massive than Jupiter and orbits its star at a distance of 8 astronomical units (AU), or eight times the distance between Earth and the sun. Jupiter orbits at about 5 AU.
The camera "is the first of the next generation of instruments designed to directly image extra-solar planets," said Bruce Macintosh, an astrophysicist at the Lawrence Livermore National Laboratory, who led the instrument's design team. He unveiled the images during a briefing at the winter meeting of the American Astronomical Society in Washington.
The approach used to characterize Luhman 16B will fit well with GPI's capabilities, according to Adam Burgasser, an astronomer at the University of California at San Diego, who led the team analyzing Luhman 16B.
"The observations we see on brown dwarfs are also something we are going to be looking forward to as we study exoplanets" and monitor them with GPI, he said during the briefing.
Brown dwarfs are cool, faint objects that failed to gain enough mass to become stars. The gravity from these lightweights isn't strong enough to compress and heat the dwarfs' centers to a level where fusion reactions begin. These reactions release the energy that lights up a star.
But brown dwarfs still emit heat, so they can be detected at infrared and near-infrared wavelengths.
Researchers have noted weather features on brown dwarfs before, typically after observing them with space-based observatories.
A survey of 44 brown dwarfs conducted by a team led by Stanimir Metchev, a researcher at the University of Western Ontario, found that virtually all of them sport clouds.
Although the brown dwarfs in question appear as tiny dots of light, researchers can glean information about the composition of a brown dwarf's atmosphere by capturing its spectrum. And they can detect clouds by subtle, irregular changes in the amount of the brown dwarf's light reaching their telescope, in this case, NASA's Spitzer Space Telescope. The deeper and longer the dips in light, the larger the cloud mass, which in turn implies powerful storms.
For his team's work, Dr. Burgasser focused on Luhman 16B, using ground-based observatories in Chile, Hawaii, Australia, and South Africa to observe the Luhman16AB system at infrared, optical, and radio wavelengths.
Based on their observations, the researchers estimate that about 40 to 60 percent of Luhman 16B is covered by clouds, which display a temperature of some 2,240 degrees Fahrenheit. The space between clouds allowed them to probe a slightly lower layer of the brown dwarf's atmosphere, where the temperatures were several hundred degrees warmer.
The team detected winds ranging in speed from 100 to 400 miles an hour. And as they monitored Luhman 16B's rotation, they could tease from the data how the fraction of the surface covered by clouds changed as the world turned.
"That gives us an idea of the size of the cloud features themselves," Burgasser said. One cloud structure appeared to blanket 15 to 25 percent of the dwarf's surface. Jupiter's Great Red Spot covers 1 percent of that planet's surface.
"The Great Red Spot is not so great," he quipped.
The data led the researchers to conclude that most brown dwarfs sport clouds that condense out of iron vapor, raining molten iron and "snows" of hot sand –and to make the unexpected observation that some of these stellar wannabes rotate more slowly than previously thought.
Variability pointing to cloud cover has been detected from space, Burgasser says, "but it's one of the first times it's been convincingly detected from the ground."
And therein lies one reason researchers are looking forward to using the GPI, another ground-based tool.
Researchers will begin using the camera in earnest later this year after completing a set of tests to ensure that it is operating as expected. Researchers will aim the camera at some 600 stars in the hope of detecting planetary systems that could more closely mimic our solar system than do many of the systems discovered to date.
In addition, astronomers will use the camera to study rings and disks of material surrounding stars and to hunt for evidence of planets being born. Moreover, astronomers will turn the camera on locals such as Saturn's moon Titan, Jupiter's Galilean satellites, and Neptune and Uranus.
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