Title: A measurement of the wind speed on a brown dwarf
Authors: Katelyn. N. Allers, Johanna M. Vos, Beth A. Biller, Peter K. G. Williams
First Author’s Institution: Department of Physics and Astronomy, Bucknell University, Lewisburg, PA 17837-2029, USA
Status: Published in Science [closed access]
If someone tells you to imagine Jupiter, I’d bet the image that comes to mind is a circle with orange-ish yellow stripes and a big red spot. Those stripes that we’ve seen since elementary school science classes are latitudinally-banded clouds, all rotating around the planet’s gaseous atmosphere.
Jupiter, and other gas giant planets, have these banded clouds flowing around the planet in zonal winds (that is, along latitudinal lines), and astronomers think that brown dwarfs should have them, too. Brown dwarfs are objects between the size of a giant planet and a star; they’re very cool since they aren’t massive enough to start typical fusion, but they sometimes can fuse deuterium or lithium. They’re particularly interesting for atmosphere studies since they’re like a “bridge” between stellar physics and planetary physics! Atmospheric models suggest these clouds arise based on differences in how regions of the atmosphere cool and how the atmosphere couples to the interior of the gas giant or brown dwarf. So far, astronomers have already measured wind speeds for some gas giant planets using Doppler shifts in transit spectroscopy. The catch here is that this has only been done for tidally locked planets, where one side of the planet is always facing the star; high-speed winds in this scenario come from heat redistribution between the hot star-facing side to the cooler night side. This is only one specific version of a planetary atmosphere, and isn’t representative of a “typical” planet-mass object. Today’s paper introduces a new way of measuring wind speeds on exoplanets and brown dwarfs and displays the results of testing it out on a real target!
The big idea here is to measure the rotation period of the interior of the planet AND the rotation period of the upper atmosphere. As observed on Jupiter in our solar system, there’s a relationship between these rotation periods and the observed wind speed at the surface. The interior, where the magnetosphere originates and rotates as a rigid body, can be measured in the radio; the upper atmosphere can be measured in the optical/infrared. Since brown dwarfs have similar magnetic field origins, this technique should be applicable to them, too. Radio and optical periods have already been measured to better than 1 minute precision using photometric variability, so the technology to do this already exists as well!
The authors decided to test out this technique on the brown dwarf known as 2MASS J10475385+2124234 (abbreviated as 2MASS J1047+21). It’s a T dwarf, a cool brown dwarf characterized by its strong methane absorption, about 10 parsecs (~30 light-years) away from us. 2MASS J1047+21 is 16 to 68 times the mass of Jupiter, and has an estimated temperature of 880 ± 76 K. To measure the infrared period (e.g. the rotation period of the upper atmosphere), they used the Infrared Array Camera on the Spitzer Space Telescope (RIP Spitzer), shown in Figure 1. By combining the results of a Lomb-Scargle periodogram and the model fit shown in Figure 1, they determined the infrared period to be 1.741 ± 0.007 hours.
To measure the radio period, they used the Karl G. Jansky Very Large Array (VLA), observing radio pulses coming from the atmosphere over multiple nights, shown in Figure 2. This data revealed a radio period of 1.751 to 1.765 hours.
Using these two period measurements, they calculated the wind speed to be 650 ± 310 m/s. This measurement is higher than we generally see on solar system gas giants and serves as a proof of concept that this technique works! It should be applicable to exoplanets as well — they just will likely have lower frequency radio emissions. Looks like we’re one step closer to observing weather on other planets!