Astronomers race to capture image of exoplanet near star
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Planet AF Lep b is a world of firsts. In 2023, it was the lowest-mass planet outside our solar system to be directly observed and have its mass measured using astrometry. This is a technique that charts the subtle movements of a host star over many years to gain insights about orbiting companions, including planets.
Now, AF Lep b is the lowest-mass planet with the smallest angular separation—that is, how close it is to its host star as seen from Earth—that has been directly observed by the James Webb Space Telescope (JWST). Results were published recently in The Astrophysical Journal Letters.
At 23 million years old, AF Lep b is a relatively young gas giant. (For comparison, Jupiter is 4.6 billion years old.) This makes it bright, and therefore, a great candidate to observe. However, the team that wished to learn more about the planet—led by graduate students Kyle Franson at The University of Texas at Austin and William Balmer at Johns Hopkins University—had to race against the clock to capture it. That's because it's moving closer to its host star in its orbit. The closer it gets, the harder it will be 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 explained. "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."
What's more, JWST uses a coronagraph to observe planets near their stars. This is a tool that blocks the light of the star so nearby objects can be seen. At the angular separation of AF Lep b from its star, the coronagraph blocks out over 90% of the planet's light. As the planet nears its star, more and more of its light will be blocked.
"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 about 25 years to orbit its star. Though in theory, astronomers could capture an image of it on the other side of the star when it will be visible again, it could be more than a decade before that's a possibility.
To ensure they didn't miss this opportunity, the team applied for—and received— Director's Discretionary Time. This is time on the telescope that's held in reserve for critical, time-sensitive observations. It's competitive for proposals to receive observing time on JWST, and even more so under this special designation.
"This is the first Director's Discretionary Time program led by graduate students," said Bowler. "And it's one of only a few within the field of exoplanets."
"It's quite something that two graduate students are able to harness all these incredible technological innovations," added Laurent Pueyo, an astronomer at the Space Telescope Science Institute and co-author of the paper.
The team was eager to learn more about AF Lep b's atmosphere, as it's uncommon to directly observe planets of a similar mass to the gas giants in our own solar system. Based on their observations, it has a very active atmosphere, with convection currents mixing elements between its lower and upper levels.
"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."
While learning more about AF Lep b is exciting, the ability to make these observations at all is particularly noteworthy.
"In the big picture, these data were taken in JWST's second year of operations. There's a lot more to come," said Bowler. "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."
More information: Kyle Franson et al, JWST/NIRCam 4–5 μm Imaging of the Giant Planet AF Lep b, The Astrophysical Journal Letters (2024). DOI: 10.3847/2041-8213/ad736a
Journal information: Astrophysical Journal Letters
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