Planetary Sciences News

Record-Setting Winds on a Nearby Brown Dwarf

Infrared and radio observations reveal zonal winds moving faster than 2,000 kilometers per hour on a “failed star” in our celestial neighborhood.

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The high-speed winds of hurricanes and tornadoes frequently wreak havoc on Earth. But now scientists have used infrared and radio observations to deduce that much, much faster flows—over 600 meters per second, well off the Saffir-Simpson scale—encircle a brown dwarf roughly 35 light-years away. These results offer a rare glimpse of the atmospheric properties of a distant world and pave the way for measuring the wind speeds of exoplanets, the team suggests.

Winds Here and Elsewhere

Latitudinally flowing “zonal winds” are found throughout the solar system; examples include our own planet’s trade winds and Jupiter’s iconic multihued cloud bands. However, getting a handle on the speeds of these winds on worlds other than Earth takes a bit of sleuthing.

Two sets of measurements are necessary, said Katelyn Allers, an astronomer at Bucknell University in Lewisburg, Pa. One set must trace the rotational period of the world, and the other must trace the rotational period of its atmosphere. By comparing these periods and knowing the size of the planet or brown dwarf, a velocity difference—the speed of the wind on that world—can be calculated, said Allers.

A “Failed Star” in the Lion

Allers and her colleagues did just that for 2MASS J1047+21, a brown dwarf roughly 35 light-years away in the constellation Leo the Lion. Brown dwarfs are “failed stars”: objects with masses between those of gas giant planets like Jupiter and full-blown stars like the Sun. (The mass of 2MASS J1047+21 isn’t precisely known, but it’s probably between 16 and 68 times that of Jupiter, researchers believe.)

Allers and her collaborators used the Karl G. Jansky Very Large Array in New Mexico to observe 2MASS J1047+21 at radio wavelengths. These observations, which trace the brown dwarf’s magnetic field, reveal the world’s rotational period. The researchers calculated a period of 1.751–1.765 hours, where the uncertainty comes from using different techniques to measure the brown dwarf’s period, said Allers.

The scientists also collected data at midinfrared wavelengths with the Spitzer Space Telescope. These observations trace atmospheric features like clouds and hot spots on 2MASS J1047+21, but it’s impossible to know for sure what structures are present, said Allers. “We can’t really easily distinguish between those different possibilities.”

The researchers tracked low-level but periodic changes in the brown dwarf’s brightness over two separate observing sessions in 2017 and 2018 totaling 21 hours. Interestingly, they recorded consistent sinusoidal variability for both observing epochs, which means that “whatever feature is causing this variability, it lasted for at least a year,” said Allers. “It’s some sort of long-lived phenomenon.” Using their infrared data, Allers and her colleagues calculated a period of 1.734–1.748 hours for 2MASS J1047+21’s atmosphere.

A Minute Faster

On the basis of their measurements, the scientists concluded that 2MASS J1047+21’s atmosphere makes a complete revolution about 0.017 hour, or 61 seconds, faster than the brown dwarf’s interior makes a complete revolution. Armed with knowledge of 2MASS J1047+21’s radius—about 67,000 kilometers, barely smaller than Jupiter—they calculated the brown dwarf’s wind speed: about 650 meters per second.

That’s far, far faster than the strongest hurricanes and tornadoes on Earth (a 70-meter-per-second wind qualifies as a category 5 storm), and it even exceeds the fastest winds recorded in the solar system, said Allers. “It’s certainly higher than what we get for Jupiter.” These results were published in April in Science.

“This is a very nice study,” said Dániel Apai, a planetary scientist at the University of Arizona not involved in the research. “It provides a new method to assess wind speeds on brown dwarfs.”

In the future, Allers and her colleagues hope to extend their analysis of brown dwarf wind speeds to other wavelengths. Data at various wavelengths are valuable because they probe different depths of a world’s atmosphere, said Allers. “That allows you to look at atmospheric dynamics as a function of depth.”

Looking to Planets

Another goal is to use the same technique to measure wind speeds on planetary mass bodies. “The method that we’ve used here can, in principle, be applied to exoplanets,” said Allers. But that’ll require precise radio- and infrared-derived rotational periods for relatively small, faint worlds, the research team concedes. That’s on the edge of what’s possible with current technology, but upcoming telescope facilities like the James Webb Space Telescope, the Owens Valley Long Wavelength Array, and the Square Kilometre Array will “knock that out of the park,” said Allers.

—Katherine Kornei (@KatherineKornei), Science Writer

Citation: Kornei, K. (2020), Record-setting winds on a nearby brown dwarf, Eos, 101, https://doi.org/10.1029/2020EO142824. Published on 16 April 2020.
Text © 2020. The authors. CC BY-NC-ND 3.0
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