Free Astronomy Magazine May-June 2025

MAY-JUNE 2025 T hese light curves show the change in brightness of three different sets of wavelengths (colors) of near-infrared light coming from the isolated plane- tary-mass object SIMP 0136 as it rotated. The light was captured by Webb’s NIRSpec (Near-Infrared Spectrograph), which collected a total of 5,726 spectra — one every 1.8 seconds — over the course of about 3 hours on July 23, 2023. (SIMP 0136 completes one rotation every 2.4 hours.) By comparing these light curves to models, researchers were able to show that each set of wavelengths probes different depths (pressures) in the atmosphere. The curve shown in red tracks the brightness of 0.9- to 1.4-micron light thought to originate deep in the atmosphere at a pressure of about 10 bars (about 10 times the air pressure at sea level on Earth), within clouds made of iron particles. The curve shown in yellow tracks the brightness of 1.4- to 2.3-micron light from a pressure of about 1 bar within higher clouds made of tiny grains of silicate minerals. The variations in brightness shown by these two curves is related to patchiness of the cloud layers, which emit some wavelengths of light and absorb others. The curve shown in blue tracks the brightness of 3.3- to 3.6-micron light that originates high above the clouds at a pressure of about 0.1 bars. Changes in brightness of these wavelengths are related to variations in temperature around the object. Bright “hot spots” could be related to auroras that have been detected at radio wavelengths, or to up- welling of hot gas from deeper in the atmosphere. The differences in shape of these three light curves show that there are complex variations in SIMP 0136’s atmosphere with depth as well as longitude. If the atmosphere varied around the object in the same way at all depths, the light curves would have similar patterns. If it varied with depth, but not longitude, the light curves would be straight, flat lines. Note that this graph shows the relative change in brightness for each given set of wavelengths over time, not the difference in absolute brightness between the different sets. At any given time, there is more light coming from the deep atmosphere (light curve in red) than from the upper atmosphere (light curve in blue). The diagram at the right illustrates the possible structure of SIMP 0136’s atmosphere, with the colored arrows representing the same wavelengths of light shown in the light curves. Thick arrows represent more (brighter) light; thin arrows represent less (dimmer) light. [NASA, ESA, CSA, Joseph Olmsted (STScI)] ness variations. “Imagine watching Earth from far away. If you were to look at each color separately, you would see different patterns that tell you something about its surface and atmosphere, even if you couldn’t make out the individual features,” explained co-author Philip Muir- head, also from Boston University. “Blue would increase as oceans ro- tate into view. Changes in brown and green would tell you something about soil and vegetation.” To figure out what could be causing the variability on SIMP 0136, the team used atmospheric models to show where in the atmosphere each wavelength of light was originating. “Different wavelengths provide in- formation about different depths in the atmosphere,” explained Mc- Carthy. “We started to realize that the wavelengths that had the most similar light-curve shapes also probed the same depths, which re- inforced this idea that they must be caused by the same mechanism.” One group of wavelengths, for ex- ample, originates deep in the atmos- phere where there could be patchy clouds made of iron particles. A sec- ond group comes from higher clouds thought to be made of tiny grains of silicate minerals. The vari- ations in both of these light curves are related to patchiness of the cloud layers. A third group of wavelengths origi- nates at very high altitude, far above the clouds, and seems to track temperature. Bright “hot spots” could be related to auroras that were previously detected at radio wavelengths, or to upwelling of hot gas from deeper in the atmosphere. Some of the light curves cannot be explained by either clouds or tem- perature, but instead show varia- tions related to atmospheric carbon chemistry. There could be pockets of carbon monoxide and carbon diox- ide rotating in and out of view, or chemical reactions causing the at- mosphere to change over time. “We haven’t really figured out the chemistry part of the puzzle yet,” said Vos. “But these results are re- ally exciting because they are show- ing us that the abundances of mol- ecules like methane and carbon dioxide could change from place to place and over time. If we are look- ing at an exoplanet and can get only one measurement, we need to consider that it might not be repre- sentative of the entire planet.” !