Exo-Saturns and Exo-Jupiters Are Within JWST’s Reach

Of the nearly 6,000 currently known exoplanets, few closely resemble any of the planets in our solar system. New research suggests that JWST is capable of directly imaging exoplanets with temperatures and orbital distances similar to Jupiter and Saturn, placing truly familiar exoplanets within our observational grasp.

Increasingly Cold Discoveries

JWST has already proven itself to be a powerful tool to directly image exoplanet systems. The telescope has imaged increasingly cold planets, but the gas giants in our solar system are substantially colder than the coldest planet imaged by JWST so far. This raises the question of whether JWST is capable of directly imaging Jupiter and Saturn if they orbited another star.

Answering this question requires a deep dive into the abilities of JWST’s instruments. The current go-to method for directly imaging planets with JWST is coronagraphy with its Near-Infrared Camera (NIRCam). In this observing mode, the instrument blocks the light from the star, allowing the fainter thermal glow of the planet to shine through.

But as Rachel Bowens-Rubin (University of Michigan and Eureka Scientific) and collaborators note in a recent research article, this may not be the best way to detect cold giant planets. Models suggest that these planets have cloudy atmospheres, which means that they wouldn’t be bright at NIRCam’s preferred near-infrared wavelengths, and would instead be detected more easily in the mid-infrared, where JWST’s Mid-Infrared Instrument (MIRI) reigns.

Combining Data and Models

To examine the capabilities of both of these instruments, Bowens-Rubin’s team analyzed JWST observations from the Cool Kids on the Block program, which targets cold, low-mass giant planets around nearby low-mass stars with NIRCam coronagraphy and MIRI imaging. The team used observations of nearby M-dwarf stars Wolf 359 and EV Lac to construct constrast curves: the level of planet–star flux contrast that is detectable by each instrument as a function of distance from each star. These curves depend on the flux of the star and the planet as well as the limitations of the instrument — the detector noise and background noise.

Bowens-Rubin and coauthors converted the contrast curves into information about the coldest planet each instrument can detect. To do this, the team modeled the atmospheres of planets with temperatures down to 50K and generated thermal emission spectra, which allowed them to relate the temperature of their modeled planets to the level of contrast.

NIRCam vs. MIRI

This analysis showed that MIRI is the best choice for directly imaging cold planets around nearby stars (within 65 light-years). MIRI should be able to detect giant planets with temperatures down to 94K around Wolf 359 and 114K around EV Lac — about the temperature of Saturn and slightly colder than Jupiter, respectively. For Wolf 359, sub-100K planets are detectable at orbital distances of at least 4.8 au, meaning these planets could also have similar orbital separations to Jupiter and Saturn.

NIRCam coronagraphy can match MIRI’s performance only for the unlikely case of cloud-free giant planets; for cloudy planets around nearby stars, MIRI can spot planets 90–130K colder than NIRCam can. NIRCam has the advantage for more distant stars — beyond about 200 light-years — but only planets significantly warmer than Jupiter and Saturn are detectable at these distances.

As impressive as these results are already, Bowens-Rubin and coauthors noted that future work, such as developing strategies to mitigate MIRI’s “brighter-fatter effect” that limits sensitivity at small angular separations from the host star, could enhance the search for exo-Saturns and exo-Jupiters even further.

Citation

“NIRCam Yells at Cloud: JWST MIRI Imaging Can Directly Detect Exoplanets of the Same Temperature, Mass, Age, and Orbital Separation as Saturn and Jupiter,” Rachel Bowens-Rubin et al 2025 ApJL 986 L26. doi:10.3847/2041-8213/addbde