The dividing line between gas giant planets and failed stars is blurry at best. The isolated planetary-mass object SIMP J013656.5+093347.3 could be either one. The distinction is largely semantic. However we choose to label and define it, the object displays a surprisingly complex atmosphere for an isolated object without any stellar energy.

On most planets with atmospheres, including ours, a star’s output affects the atmosphere in fundamental ways. Light and heat not only affect the temperature and structure of atmospheres, but they can also affect the overall chemistry. The Sun’s heat creates convection in Earth’s atmosphere, circulating warmth around our planet. The UV from the Sun also creates Earth’s ozone layer.

How do planetary atmospheres behave when there’s no star nearby? We might expect a relatively simple atmosphere on a free-floating planetary-mass object like SIMP J013656.5+093347.3 (hereafter referred to as SIMP). But that’s not the case. Instead, SIMP has a deep, layered atmosphere replete with aurorae and even clouds made of iron particles.

These details and more were discovered by the JWST and presented in new research in The Astrophysical Journal Letters. The article is titled “The JWST Weather Report from the Isolated Exoplanet Analog SIMP 0136+0933: Pressure-dependent Variability Driven by Multiple Mechanisms,” and the lead author is Allison McCarthy, a postdoc in Astronomy at Boston University.

SIMP is a free-floating object with about 13 Jupiter masses, meaning it could be classified as a brown dwarf. It’s about 20 light-years away and rotates rapidly, completing one revolution in about 2.4 hours. Even though astronomers don’t classify it as an exoplanet, it’s still a great target in the study of exo-meteorology. It’s bright, and it’s isolated, meaning there’s no light pollution to foul observations.

SIMP may not orbit a star, and it may be isolated from the influence of one, but it still rotates. According to the research, its rotation helps drive its unusually complex atmosphere. “Many isolated planetary-mass objects show variations in their infrared brightness consistent with nonuniform atmospheric features modulated by their rotation. SIMP J013656.5+093347.3 is a rapidly rotating isolated planetary-mass object, and previous infrared monitoring suggests complex atmospheric features rotating in and out of view,” the authors write.

There’s more to SIMP’s atmospheric complexity than just rotation, though. “The physical nature of these features is not well understood, with clouds, temperature variations, thermochemical instabilities, and infrared-emitting aurora all proposed as contributing mechanisms,” the researchers explain.

The researchers observed SIMP with the JWST with two instruments: NIRSpec (Near-Infrared Spectrograph) and MIRI (Mid-Infrared Instrument). NIRSPec captured thousands of individual spectra in near-infrared for more than three hours, long enough for SIMP to complete a single rotation. Then, MIRI captured hundreds of mid-infrared spectroscopic measurements as SIMP completed another rotation.

McCarthy and her colleagues found very distinct patterns in SIMP’s light curves. As some wavelengths brightened, others dimmed, and others didn’t change much. This implies that different factors are influencing the changes.

The data also showed how SIMP changes in brightness as it rotates.

These light curves show the change in brightness of three different sets of wavelengths (colours) of near-infrared light coming from the isolated planetary-mass object SIMP 0136 as it rotated. Researchers think the brightness variations are related to the features in SIMP’s atmosphere. The features include low-altitude clouds of iron particles, high altitude clouds composed of tiny grains of silicate minerals (forsterite), and hot and cold spots also at high altitudes. These features come in and out of view as SIMP rotates. The diagram at the right illustrates the possible structure of SIMP 0136’s atmosphere, with the coloured arrows representing the same wavelengths of light shown in the light curves. Thick arrows represent more (brighter) light; thin arrows represent less (dimmer) light. Image Credit: NASA, ESA, CSA, and Joseph Olmsted (STScI)

“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 in a press release. “Until now, we only had a little slice of the near-infrared spectrum from Hubble and a few brightness measurements from Spitzer.”

Planetary atmospheres are seldom featureless, and by observing a planet during its rotation, scientists can learn something about its nature. Co-author Philip Muirhead from Boston University likens observing SIMP to observing our own planet.

“Imagine watching Earth from far away. If you were to look at each colour 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 Muirhead. “Blue would increase as oceans rotate into view. Changes in brown and green would tell you something about soil and vegetation.”

There’s a lot of interplay among the different aspects of SIMP’s complex atmosphere, and it’s unlikely that any single mechanism can account for the complexity. Think of Earth and all its complexity. Our atmosphere is affected by multiple factors, including ocean temperature, mountains, ocean currents, pollution, CO2 content, and energy from the Sun. If we had only one day’s worth of observations, we would throw our hands in the air trying to understand how it all works.

However, the researchers explain that some of SIMP’s wavelengths have similarly shaped light curves, and that can be a key to understanding its atmosphere from 20 light-years away. “Since physical mechanisms like clouds, aurora, hot spots, or chemical instabilities may all cause light-curve variability, we infer that similar shapes indicate shared physical mechanism(s),” the authors write in their research letter.

“Different wavelengths provide information about different depths in the atmosphere,” explained McCarthy. “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.”

As part of their research, the team grouped lights wave into clusters according to shape, which helped them extract meaningful conclusions from the data. “Wavelengths with similar light curves are likely impacted by the same mechanism(s), so by grouping together similar-shaped light curves, we can investigate these mechanism(s),” they explain.

This figure from the research shows the JWST NIRSpec spectra placed into nine clusters on the left. Gray vertical lines mark the object’s 2.4 hr rotation period. The image on the right shows the variance for the pressure of each cluster. The green-shaded region represents the pressures where forsterite clouds exist, and the pink-shaded region represents the pressures where iron clouds might exist. Image Credit: McCarthy et al. 2025.

The authors say that clouds at different altitudes can block some of the flux in the atmosphere, helping create some of the temperature variability. They also say that molecular abundances create some of the light-curve shapes. However, some of the bright spots are at high altitudes, far above the clouds. These could be related to aurorae or to upwelling hot gas from deeper in the atmosphere.

When it comes to chemistry, there’s even more complexity. Some light curves can’t be explained by clouds or temperature. Instead, they could be related to carbon chemistry. Pockets of CO and CO2 could be rotating in and out of view, or chemical reactions could be causing atmospheric changes over time.

“We haven’t really figured out the chemistry part of the puzzle yet,” said Vos. “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.”

SIMP has been studied many times prior to this work, and this is another instance of the JWST expanding and deepening astronomers’ understanding of puzzling objects. “This study, the first JWST spectroscopic variability analysis of a planetary-mass object, demonstrates JWST’s unique power to probe extrasolar atmospheres,” the authors write. “When combined with previous studies of this object indicating patchy clouds and aurorae, these measurements reveal the rich complexity of the atmosphere of SIMP J013656.5+093347.3.”

Even though the JWST has given us our first day-long look at SIMP, it’s still really only a glimpse. If we based our understanding of the planets in our Solar System on one day’s worth of observations, we would misunderstand them. The same thing is true for SIMP.

“Longer observations spanning multiple rotations are crucial for deeper insights into evolving atmospheric mechanisms and distinguishing variability timescales. Monitoring changes in light-curve shapes over time will help disentangle these mechanisms and assess their correlations,” the authors conclude.