University of Arizona scientists, led by Daniel Apai, associated professor of astronomy and planetary sciences, have found the reason behind the unusual fluctuation in brightness of brown dwarfs.

Brown dwarfs are celestial bodies smaller than planets but larger than stars. According to UA researcher and code designer Theodora Karalidi, brown dwarfs have approximately the same diameter as Jupiter, but three times the mass.

Karalidi said stars form because immense amounts of gas and dust floating in space collapse in together through gravity. With enough mass, the pressure on the core becomes great enough to fuse hydrogen into helium. Brown dwarfs are typically referred to as failed stars because they lack the mass to fuse hydrogen into helium.

The brown dwarfs have been observed to have atmospheric waves, which affect their thick, silicon clouds.

Using NASA’s Spitzer Space telescope and a complex algorithm, astronomers tracked cloud movements and oscillations in brightness, according to Karalidi. 

“The code randomly chooses spots on the atmosphere and makes a light curve to see how the light should look as it rotates around its axis,” Karalidi said. “Then it compares the simulated light to what is observed.” 

Depending on how well the light curve fits, Karalidi said the code will try more light curves until it can find the best map for the observations. 

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The code used to create the algorithm was fortran, a relatively old code that’s been used for atmospheric mapping for decades, Karalidi said.

“It’s more traditional,” Karalidi said. “Many of the codes were built in the 70s, and generation by generation, [we] put fixes on top of it.”

As a result, it’s a rather slow code, taking days or even weeks to gather data. To counter this, the UA provided powerful supercomputers. 

“If we were to run it on a standard computer, one run would take forever, but with the supercomputers, I can throw one hundred trials in one go and get a lot more models,” Karalidi said. 

Astronomers have observed the swift oscillations in the atmospheres of brown dwarfs but lacked the evidence to understand why. 

“Our data was the first to see just how much it changes in such a short time,” Karalidi said. “Daniel came up with the idea that it could be because of bands and waves like in Jupiter, so I incorporated these features into the mapping code and we started getting maps that fit the observations.” 

When the maps matched the observations with revisions accounting for atmospheric waves, they had the evidence to show what caused the rapid fluctuations in brightness.

Similar to Jupiter, the brown dwarf rotates on its axis, but its bands move at different rates, independent of the planet. This creates two very different waves and amplitudes, with the amplitude being the measurement of how high or low a wave can fluctuate.

In wave physics, if the high point, or peak, of a wave meets the low point, or trough, of another wave, they cancel out. Similarly, high points can combine to make even higher waves, and low points will go even lower. 

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“Depending on how you see them combining, you can have zero signals or high amplitudes, and that’s something we think we’re seeing in brown dwarfs,” Karalidi said. “For example, if the waves meet at the peak, you get a really bright signal.”

 Karalidi said because the clouds rotate at a different rate than the axis of the planet the light emitted from the brown dwarf fluctuates rapidly over time.

While similar to Jupiter in some ways, the brown dwarfs differ in terms of cloud composition and temperature, according to Karalidi. Jupiter is a cold planet, averaging around 165 kelvin, which allows for certain compounds, such as ammonia, to condense into clouds. 

“Some of our observation targets are around 1,300 kelvin, so there we have more exotic things, such as silicate and corundum clouds,” Karalidi said. 

For a sense of scale, the temperature of our sun is 5,778 kelvin.

“We always assumed that because these brown dwarfs are so much hotter than other objects, and because they’re isolated in space, that they would be really different than what we know,” Karalidi said. “But the more data we got, the more we understood how similar they are to other objects.” 

 Karalidi said the brightness from parent stars obscures the light from objects around them, so isolated brown dwarfs are easier to observe. 

With the James Webb Space Telescope launching next year, Karalidi hopes they’ll be able to make similar observations on exoplanets, which are planets that orbit a star outside the solar system. 

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