Webb exposes complex atmosphere of starless super-Jupiter
An international team of researchers has discovered that previously observed variations in brightness of a free-floating planetary-mass object known as SIMP 0136 must be the result of a complex combination of atmospheric factors, and cannot be explained by clouds alone.
Using NASA’s James Webb Space Telescope to monitor a broad spectrum of infrared light emitted over two full rotation periods by SIMP 0136, the team was able to detect variations in cloud layers, temperature, and carbon chemistry that were previously hidden from view.
The results provide crucial insight into the three-dimensional complexity of gas giant atmospheres within and beyond our solar system. Detailed characterization of objects like these is essential preparation for direct imaging of exoplanets, planets outside our solar system, with NASA’s Nancy Grace Roman Space Telescope, which is scheduled to begin operations in 2027.
Rapidly rotating, free-floating
SIMP 0136 is a rapidly rotating, free-floating object roughly 13 times the mass of Jupiter, located in the Milky Way just 20 light-years from Earth. Although it is not classified as a gas giant exoplanet—it doesn’t orbit a star and may instead be a brown dwarf—SIMP 0136 is an ideal target for exo-meteorology: It is the brightest object of its kind in the northern sky. Because it is isolated, it can be observed with no fear of light contamination or variability caused by a host star. And its short rotation period of just 2.4 hours makes it possible to survey very efficiently.
Prior to the Webb observations, SIMP 0136 had been studied extensively using ground-based observatories and NASA’s Hubble and Spitzer space telescopes.
“We already knew that it varies in brightness, and we were confident that there are patchy cloud layers that rotate in and out of view and evolve over time,” explained Allison McCarthy, doctoral student at Boston University and lead author on a study published in The Astrophysical Journal Letters. “We also thought there could be temperature variations, chemical reactions, and possibly some effects of auroral activity affecting the brightness, but we weren’t sure.”
To figure it out, the team needed Webb’s ability to measure very precise changes in brightness over a broad range of wavelengths.
Charting thousands of infrared rainbows
Using NIRSpec (Near-Infrared Spectrograph), Webb captured thousands of individual 0.6- to 5.3-micron spectra—one every 1.8 seconds over more than three hours as the object completed one full rotation. This was immediately followed by an observation with MIRI (Mid-Infrared Instrument), which collected hundreds of spectroscopic measurements of 5- to 14-micron light—one every 19.2 seconds, over another rotation.
The result was hundreds of detailed light curves, each showing the change in brightness of a very precise wavelength (color) as different sides of the object rotated into view.
“To see the full spectrum of this object change over the course of minutes was incredible,” said principal investigator Johanna Vos, from Trinity College Dublin. “Until now, we only had a little slice of the near-infrared spectrum from Hubble, and a few brightness measurements from Spitzer.”
The team noticed almost immediately that there were several distinct light-curve shapes. At any given time, some wavelengths were growing brighter, while others were becoming dimmer or not changing much at all. A number of different factors must affect the brightness 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 Muirhead, also from Boston University. “Blue would increase as oceans rotate into view. Changes in brown and green would tell you something about soil and vegetation.”
