Now, astronomers using the James Webb Space Telescope (JWST) have made AF Lep b the lowest-mass planet with the smallest angular separation from its host star ever directly observed. The results of this observation were recently published in *The Astrophysical Journal Letters*.
At just 23 million years old, AF Lep b is relatively young compared to planets like Jupiter, which is 4.6 billion years old. Its youth makes it bright, a key factor for observation. However, a team led by graduate students Kyle Franson from The University of Texas at Austin and William Balmer from Johns Hopkins University had to act fast to study the planet before it moved too close to its star, making it more difficult to observe.
"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," Franson said. "And we're observing at longer wavelengths, which has the effect of making objects look fuzzier. It becomes difficult to separate one source out from the other source when they appear so close together."
The JWST uses a coronagraph to block the light from stars and reveal planets nearby. However, AF Lep b's proximity to its host star means the coronagraph blocks over 90% of the planet's light. As the planet moves closer, this percentage will only increase, further complicating future observations.
"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," said Brendan Bowler, an astronomer at UT and co-author of the study. "We really are pushing the instrumentation to its limits here."
AF Lep b takes roughly 25 years to complete its orbit around its star. While it may be possible to capture images of it when it moves to the opposite side of the star, that opportunity is likely more than a decade away.
To ensure this critical observation could take place, the team secured Director's Discretionary Time on the JWST, a special allocation for time-sensitive studies. This was the first such time awarded for a program led by graduate students, according to Bowler, and one of the few in the exoplanet field. "It's quite something that two graduate students are able to harness all these incredible technological innovations," said Laurent Pueyo, an astronomer at the Space Telescope Science Institute and co-author of the paper.
The team's observations of AF Lep b also revealed insights into its atmosphere. "We observed much more carbon monoxide than we initially expected," explained Balmer. "The only way to get gas of that type into the planet's upper atmosphere is with strong updrafts."
Bowler emphasized the importance of these observations for the future of astronomy: "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 soon discover. This is foreshadowing some of the exciting work that we will see in the coming years."
The research was supported by the National Science Foundation, NASA, Alfred P. Sloan Foundation, and UKRI/EPSRC. The work is based on observations from the NASA/ESA/CSA James Webb Space Telescope and the European Southern Observatory's La Silla Paranal Observatory. Data was gathered from the Mikulski Archive for Space Telescopes and the W.M. Keck Observatory, with additional resources from the Oschin Schmidt Telescope and the UK Schmidt Telescope.
Research Report:JWST/NIRCam 4 - 5 um Imaging of the Giant Planet AF Lep b
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