Rogue super-Jupiter: Brownish planet in space with parallel bands in its atmosphere and many stars in the background.
View larger. | Artist’s concept of SIMP 0136. It is a rogue super-Jupiter floating freely in space, with no star. New observations by NASA’s Webb space telescope show that variations in the object’s brightness can be explained by patchy clouds, hot spots and carbon chemistry. Image via NASA/ ESA/ CSA/ Joseph Olmsted (STScI).
SIMP 0136 is an isolated super-Jupiter drifting freely in space with no host star. It is one of many known so-called “rogue planets” in our Milky Way galaxy.
Astronomers had previously detected variations in brightness of this intriguing object. What was causing them?
New observations by NASA’s Webb space telescope suggest that patchy clouds, hot spots and carbon chemistry are responsible.
Rogue super-Jupiter is surprisingly complex
SIMP 0136 is an unusual object, what astronomers call a rogue planet, about 20 light-years away. It’s similar to other gas giant planets but is freely floating in space, not orbiting any star. Previously, astronomers had observed variations in its brightness that might be caused by clouds. But on March 3, 2025, an international team of researchers said new observations by NASA’s Webb space telescope show clouds alone can’t explain the changes. It said the variations must be the result of a complex combination of atmospheric factors.
The researchers published their peer-reviewed findings in The Astrophysical Journal Letters on March 3, 2025.
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Meet SIMP 0136, a rogue super-Jupiter
SIMP 0136 – also known as SIMP J01365663+0933473 – is big, about 13 times more massive than Jupiter, and 1.2 times the radius. In 2018, astronomers reported evidence for auroras on SIMP 0136. Its magnetic field is also about 4 million times stronger than Earth’s.
It’s one of many so-called rogue planets that astronomers have discovered in recent years. Those are planets, or planet-like objects, that don’t orbit any stars. Instead, they drift alone in space. In this case, it’s also possible that SIMP 0136 is actually a brown dwarf, but it’s still not gravitationally bound with any other stars. Brown dwarfs are objects that are between the largest planets and the smallest stars in size. They can also have planet-like qualities, with deep atmospheres like those of the gas giant planets in our solar system.
Although rogue planets are generally difficult to study because there are no stars nearby to illuminate them, SIMP 0136 is one of the easier ones. It’s the brightest of its kind in the northern skies as seen from Earth. It also rotates in only 2.4 hours, so astronomers can easily observe all hemispheres.
Before Webb, astronomers had observed the mysterious world with the Hubble Space Telescope, the Spitzer Space Telescope and observatories on the ground. Lead author Allison McCarthy at Boston University in Massachusetts said:
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. 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.
Webb Exposes Complex Atmosphere of Free-Floating Super-Jupiter SIMP 0136astrobiology.com/2025/03/webb… #astrobiology #exoplanet #astrochemistry #interstellar #astronomy
— Astrobiology (@astrobiology.bsky.social) 2025-03-04T16:28:07.692Z
Hundreds of colors
As SIMP 0136 completed one rotation, Webb used its NIRSpec (Near-Infrared Spectrograph) instrument to study the electromagnetic spectrum – composed of separate bands of frequencies or wavelengths – of the object. Altogether, Webb collected thousands of individual 0.6- to 5.3-micron spectra, one every 1.8 seconds over three hours. Then, during another rotation, Webb collected hundreds of measurements of 5- to 14-micron light, one every 19.2 seconds.
Overall, Webb produced hundreds of light curves, or graphs, of the object’s brightness. Each light curve represented a distinct wavelength with its own color. This allowed the researchers to monitor the changing brightness of SIMP 0136 as it rotated. This was much more than Hubble or Spitzer had done previously. Principal investigator Johanna Vos at Trinity College Dublin in Ireland, said:
To see the full spectrum of this object change over the course of minutes was incredible. Until now, we only had a little slice of the near-infrared spectrum from Hubble, and a few brightness measurements from Spitzer.
Intriguing brightness variations
The new Webb observations supported the earlier ones from Hubble and Spitzer. They also showed the variations in the brightness of SIMP 0136. But they also added a level of detail not possible before. Webb detected several distinct light curves. And while some grew brighter, other would become dimmer at the same time. This hinted at a previously unknown complexity of the object’s atmosphere. Clouds alone were unlikely to explain them. Co-author Philip Muirhead, also at Boston University, compared this to Earth, saying:
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. Blue would increase as oceans rotate into view. Changes in brown and green would tell you something about soil and vegetation.
Chart showing red, yellow and blue curving lines above 3 similar planets on the left. A smaller graphic on the right shows cloud layers with vertical arrows. White text labels on both graphics and title text at the top.
View larger. | This chart depicts the variations in brightness of SIMP 0136 as it rotates. Image via NASA/ ESA/ CSA/ Joseph Olmsted (STScI).
Patchy clouds, hot spots and carbon on rogue super-Jupiter
So if clouds alone weren’t causing the brightness variations, then what was? The wavelengths of light that Webb measured provided the clues. As McCarthy explained:
Different wavelengths provide information about different depths in the atmosphere. We started to realize that the wavelengths that had the most similar light-curve shapes also probed the same depths, which reinforced this idea that they must be caused by the same mechanism.
Some of the wavelengths suggested that clouds deeper down are patchy and composed of iron particles. Other wavelengths hinted at clouds higher up that are made of silicate minerals. Yet other wavelengths seem to come from “hot spots” in the atmosphere, far above the clouds themselves. These may be related to auroras. In addition, there is evidence for interesting carbon chemistry in the atmosphere. These might involve pockets of carbon dioxide or carbon monoxide. Vos said:
We haven’t really figured out the chemistry part of the puzzle yet. But these results are really exciting because they are showing us that the abundances of molecules like methane and carbon dioxide could change from place to place and over time. If we are looking at an exoplanet and can get only one measurement, we need to consider that it might not be representative of the entire planet.
Bottom line: New observations by Webb of rogue super-Jupiter SIMP 0136 show that its atmosphere is more complex than thought, explaining its variations in brightness.
Source: The JWST Weather Report from the Isolated Exoplanet Analog SIMP 0136+0933: Pressure-dependent Variability Driven by Multiple Mechanisms
Via NASA
Read more: Astronomers find weird rogue world with wild auroras
Read more: Are there more rogue planets than stars in our galaxy?
Paul Scott Anderson
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About the Author:
Paul Scott Anderson has had a passion for space exploration that began when he was a child when he watched Carl Sagan’s Cosmos. He studied English, writing, art and computer/publication design in high school and college. He later started his blog The Meridiani Journal in 2005, which was later renamed Planetaria. He also later started the blog Fermi Paradoxica, about the search for life elsewhere in the universe. While interested in all aspects of space exploration, his primary passion is planetary science and SETI. In 2011, he started writing about space on a freelance basis with Universe Today. He has also written for SpaceFlight Insider and AmericaSpace and has also been published in The Mars Quarterly. He also did some supplementary writing for the iOS app Exoplanet. He has been writing for EarthSky since 2018, and also assists with proofing and social media.