Astronomers have successfully captured a direct image of AF Leporis b (AF Lep b), a young exoplanet orbiting a star 88 light-years from Earth. This gas giant is notable for being the lowest-mass exoplanet to be directly imaged by the James Webb Space Telescope (JWST) and for its proximity to its host star. The planet, which has a mass about 3.2 times that of Jupiter, was observed during a “race against time” before it moved too close to its star, rendering it unobservable for another decade.
A Young Planet with Groundbreaking Observations
AF Lep b is unique not only for its direct imaging but also because it is a relatively young planet at just 23 million years old. In comparison, Jupiter, the largest planet in our solar system, is about 4.6 billion years old. The youth of AF Lep b makes it brighter than older planets, which typically cool and fade over time. Its brightness allowed astronomers to observe it using JWST, despite the technical challenges posed by its closeness to its star.
What made this observation so challenging was the planet’s small angular separation from its host star as seen from Earth. As Kyle Franson, a researcher at the University of Texas at Austin, explained, “AF Lep b is right at the inner edge of being detectable. Even though it is extraordinarily sensitive, JWST is smaller than our largest telescopes on the ground. And we’re observing at longer wavelengths, which has the effect of making objects look fuzzier. It becomes difficult to separate one source from the other when they appear so close together.”
To overcome this, the JWST team used a coronagraph, a device that blocks the overwhelming light from the star so that faint objects like planets can be detected. Despite blocking more than 90% of the planet’s light, the team was able to observe AF Lep b at a crucial moment. The planet is currently moving closer to its star in its orbit, and in the coming years, it will be undetectable even with JWST’s advanced capabilities. Given that AF Lep b takes about 25 Earth years to complete one orbit, it could be more than a decade before the planet reappears on the other side of the star where it can be observed again.
The Race Against Time
Recognizing the urgency of capturing images of AF Lep b before it became too close to its star, the team applied for Director’s Discretionary Time—a special allocation of observation time reserved for critical and time-sensitive projects. It was a competitive process, but the team was able to secure this highly valuable time to make their observations. Brendan Bowler, an astronomer at the University of Texas and a co-author of the study, emphasized the significance of this achievement, saying, “The conventional wisdom has been that JWST is more sensitive to lower-mass planets on wide orbits than ground-based facilities, but before it launched, it wasn’t clear if it would be competitive at small separations. We really are pushing the instrumentation to its limits here.”
This was no easy task. Even with JWST’s powerful instruments, the proximity of AF Lep b to its host star meant that the coronagraph blocked a substantial portion of the planet’s light, making it difficult to see. However, the team succeeded in imaging the planet and analyzing its atmosphere. These images, taken between October 2023 and January 2024, reveal not only the planet’s position but also important details about its atmospheric composition.
Discoveries about AF Lep b’s Atmosphere
One of the most intriguing findings from this observation was the detection of carbon monoxide in the planet’s upper atmosphere. According to William Balmer, a graduate student at Johns Hopkins University and a co-author of the study, “We observed much more carbon monoxide than we initially expected. The only way to get gas of that type into the planet’s upper atmosphere is with strong updrafts.” This suggests that the planet has an active atmosphere with convection currents that are mixing materials between its lower and upper layers.
Such a dynamic atmosphere is uncommon in exoplanets that have been directly imaged, especially those with masses similar to the gas giants in our own solar system. The ability to detect and study these atmospheric processes on a planet outside our solar system marks a significant achievement in the field of exoplanetary science. These findings offer astronomers new insights into how gas giant planets evolve and the atmospheric conditions that prevail on such young worlds.
Pushing the Boundaries of Exoplanet Research
The successful imaging of AF Lep b not only sheds light on the characteristics of this particular exoplanet but also demonstrates the capabilities of the James Webb Space Telescope in advancing exoplanetary research. While JWST was designed primarily to study distant galaxies, its ability to directly image exoplanets near their stars showcases its versatility. Since the first exoplanets were discovered in the 1990s, most have been detected indirectly—through the gravitational tug they exert on their stars or by blocking part of the star’s light as they transit in front of it. Direct imaging, however, remains rare because it requires exceptional sensitivity and the ability to block out the star’s light without losing sight of the planet.
In this case, AF Lep b’s brightness and relatively close proximity to Earth—at 88 light-years—made it an ideal candidate for JWST’s coronagraph. Still, capturing its image was a challenge, as Franson pointed out: “Even though JWST is one of the most powerful telescopes we have, the small angular separation between the planet and its star means we had to push the limits of what JWST could do.”
The team’s findings also foreshadow future discoveries that could be made using JWST. As Bowler noted, “In the big picture, these data were taken in JWST’s second year of operations. There’s a lot more to come. It’s not just about the planets that we know about now. It’s also about the planets that we will soon discover.”
This study is an important milestone in exoplanetary science, highlighting both the power of JWST and the collaborative efforts of scientists to push the boundaries of what we can learn about planets beyond our solar system. With more observations planned in the coming years, astronomers are hopeful that JWST will continue to provide new insights into the diversity of planets orbiting distant stars.