Astronomers using NASA's James Webb Space Telescope have discovered that a brown dwarf (an object more massive than Jupiter but smaller than a star) may display aurorae, like the familiar Northern Lights. It's an unexpected mystery because the brown dwarf, known as W1935, is an isolated object in space with no nearby stars to produce the aurora.
This artist's concept depicts brown dwarf W1935, 47 light-years from Earth. Astronomers using NASA's James Webb Space Telescope discovered infrared emissions of methane from W1935. This was an unexpected discovery because the brown dwarf is very cold and lacks a host star; therefore, there is no apparent source of energy to heat its upper atmosphere to make the methane glow. The team speculates that the methane emission may be due to the process that produces the auroras, shown here in red. Image source: NASA, ESA, CSA, Leah Hustak (STScI) editor
Auroras on Earth are produced when high-energy particles from the sun are captured by our planet's magnetic field. These particles fall layer by layer into the atmosphere near the Earth's poles, colliding with gas molecules to form an eerie, dancing curtain of light. Since W1935 has no stars producing stellar winds, external interactions with interstellar plasma or nearby active moons such as Jupiter's Io may help explain the observed infrared emission.
Astronomers used NASA's James Webb Space Telescope to study 12 cool brown dwarfs. Two of them - W1935 and W2220 - appear to be similar twins in composition, brightness and temperature. However, W1935 shows emission of methane rather than the expected absorption signature of W2220. The team speculates that the methane emissions may be due to the process that produces auroras. Source: NASA, ESA, CSA, Leah Hustak (STScI)
Astronomers using NASA's James Webb Space Telescope have discovered infrared light emitted by methane on a brown dwarf (an object more massive than Jupiter but smaller than a star), possibly caused by energy in its upper atmosphere. This was an unexpected discovery because the brown dwarf, named W1935, is very cold and lacks a host star; therefore, its upper atmosphere has no apparent source of energy. The team speculates that the methane emissions may be due to the process that produces auroras.
The findings will be presented at the 243rd meeting of the American Astronomical Society in New Orleans.
To help explain the mystery of methane's infrared radiation, the research team turned to the solar system. Methane emission is a common feature of gas giant planets such as Jupiter and Saturn. The heating of the upper atmosphere that produces this radiation is associated with auroras.
On Earth, auroras are produced when high-energy particles blown into space by the sun are captured by the Earth's magnetic field. They enter the atmosphere along the magnetic field lines near the Earth's poles, collide with gas molecules, and form an eerie dancing light curtain. Jupiter and Saturn have similar auroral processes, including interactions with the solar wind, but they also receive auroras from nearby active moons such as Io (Jupiter) and Enceladus (Saturn).
For an isolated brown dwarf like W1935, there is no stellar wind to contribute to the auroral process and no explanation for the extra energy required for methane emissions in the upper atmosphere, which is a mystery. The team speculates that either unexplained internal processes like the atmospheric phenomena of Jupiter and Saturn, or external interactions with interstellar plasma or nearby active moons, could help explain the methane emissions.
The discovery of the aurora is like a detective story. A team led by astronomer Jackie Faherty of the American Museum of Natural History in New York was given time with the Webb telescope to survey 12 cold brown dwarfs. These include W1935, an object discovered by citizen scientist Dan Caselden, who worked on the Backyard Worlds Zooniverse project, and W2220, an object discovered using NASA's WideField Infrared Survey Explorer. The Webb Telescope revealed in exquisite detail that W1935 and W2220 are near-clones in composition. They also have similar brightness, temperature, and spectral characteristics of water, ammonia, carbon monoxide, and carbon dioxide. One notable exception is that W1935 exhibits methane emission, while W2220 does not exhibit the expected absorption signature. This is seen at a unique infrared wavelength, to which Webb has a unique sensitivity.
"We expected to see methane, because methane is everywhere in these brown dwarfs. But what we saw was the opposite, the methane was not absorbing light: the methane was glowing. My first thought was, what the hell is going on? Why is this object emitting methane?" The team used computer models to infer the reasons behind the methane emission. Modeling work shows that W2220's energy is distributed as expected throughout the atmosphere, getting cooler with altitude. On the other hand, the results of W1935 were unexpected. The best models favor a temperature inversion, where the atmosphere warms with increasing altitude.
"This temperature inversion is really puzzling," said Ben Burningham, lead modeler on the study and co-author of the University of Hertfordshire in the UK. "We've seen this on planets with nearby stars that can heat the stratosphere, but it's crazy to see it on a body with no obvious external heat source."
Looking for clues, the team looked to our own backyard - the planets of our solar system. The gas giant planet could serve as a proxy for conditions in the atmosphere of W1935, more than 40 light-years away.
The team realized that temperature inversions were prominent in planets such as Jupiter and Saturn. People are still trying to understand the cause of their stratospheric heating, but the leading theories for the solar system involve external heating of the aurora and internal energy transfer from deep in the atmosphere (the former being the leading explanation).
This isn't the first time auroras have been used to explain observations of brown dwarfs. Astronomers have detected radio emissions from several warmer brown dwarfs and cited auroras as the most likely explanation. Ground-based telescopes such as the Keck Observatory have searched for the infrared signatures of these radio-emitting brown dwarfs to further characterize the phenomenon, but no conclusions have been drawn.
W1935 is the first auroral candidate object with methane emission characteristics outside the solar system. It's also the coldest aurora candidate outside the solar system, with an effective temperature of about 400 degrees Fahrenheit (200 degrees Celsius), about 600 degrees Fahrenheit hotter than Jupiter.
In the solar system, the solar wind is a major contributor to auroral processes, with active moons such as Io and Enceladus playing a role in planets such as Jupiter and Saturn, respectively. W1935 has no companion star at all, so stellar winds cannot contribute to this phenomenon. Whether an active satellite may have played a role in W1935's methane emissions is currently unknown.
"With W1935, we now have a spectacular extension of a solar system phenomenon without any stellar radiation to help explain it," Faherty noted. "With the Webb telescope, we can really 'open the hood' of chemical reactions and understand how similar or different auroral processes are outside the solar system," she added.
Compiled source: ScitechDaily