“Astrometry uses the shift in a star’s position on the sky relative to other stars to infer the existence of orbiting planets.
A team led by astronomers at The University of Texas at Austin has captured images of the lowest-mass extrasolar planet ever discovered that has both a direct mass measurement and an orbit similar to the giant planets in our own solar system. It’s also among the first ever discovered using a technique called astrometry, which relies on subtle movements of a host star over many years to provide insights about orbiting companions, including planets.
“When we processed the observations in real time at the telescope to carefully remove the glare of the star, the planet immediately popped out and became increasingly apparent the longer we observed,” said Kyle Franson, a UT Austin astronomy graduate student and lead author of the paper describing the team’s findings in the journal Astrophysical Journal Letters.
The UT Austin-led group captured direct images of AF Lep b, a planet about three times the mass of Jupiter orbiting AF Leporis, a young sun-like star about 87.5 light years away. They took a series of deep images of the planet starting in December 2021, and two other teams also captured images of the same planet since then.
“This is the first time this method has been used to find a giant planet orbiting a young analog of the sun,” said Brendan Bowler, an assistant professor of astronomy at UT Austin and senior author on the study. “This opens the door to using this approach as a new tool for exoplanet discovery.”
Despite having a much smaller mass than its host star, an orbiting planet causes a star’s position to wobble slightly around the center of mass of the planetary system. Astrometry uses this shift in a star’s position on the sky relative to other stars to infer the existence of orbiting planets. Franson and Bowler identified the star AF Leporis as one that might harbor a planet, given the way it had moved during 25 years of observations from the Hipparcos and Gaia satellites.
To directly image the planet, the UT Austin team used the W.M. Keck Observatory’s 10-meter telescope in Hawaii equipped with adaptive optics, which corrects for fluctuations caused by turbulence in the atmosphere, and a special instrument called a vector vortex coronagraph, which suppresses light from the host star. The planet AF Lep b is about 10,000 times fainter than its host star and is located at a distance of about 8 times the Earth-sun distance.
“Imaging planets is challenging,” Franson said. “We only have about 15 examples, and we think this new ‘dynamically informed’ approach will be much more efficient compared to blind surveys which have been carried out for the past two decades.”
The two most common ways to find extrasolar planets involve observing slight, periodic dimming of the starlight if a planet happens to regularly pass in front of the star — like a moth spiraling around a porchlight — and measuring minute changes in the frequencies of starlight that result from the planet tugging the star back and forth along the direction to Earth. Both methods tend to work best with large planets orbiting close to their host stars, and both methods are indirect: we don’t see the planet, we only see how it influences the star.
The method of combining direct imaging with astrometry could help astronomers find extrasolar planets that were hard to find before with other methods because they were too far from their host star, were too low mass, or didn’t have orbits that were edge on as seen from Earth. Another benefit of this technique is that it allows astronomers to directly measure a planet’s mass, which is difficult with other methods at wide orbital distances.
Bowler said the team plans to continue studying AF Lep b.
“This will be an excellent target to further characterize with the James Webb Space Telescope and the next generation of large ground-based telescopes like the Giant Magellan Telescope,” Bowler said. UT Austin is a founding partner of GMT, which is expected to regularly discover planets like AF Lep b. “We’re already planning more sensitive follow-up efforts at longer wavelengths to study the physical properties and atmospheric chemistry of this planet.”
Two other teams used the European Southern Observatory’s Very Large Telescope in Chile to take images of the same planet in fall 2022 and published their results in the journal Astronomy & Astrophysics (Mesa, et al. and De Rosa, et al.).
Other authors from UT Austin are Yifan Zhou, Lauren Biddle, Marvin Morgan, Aniket Sanghi, Quang Tran and Trevor Wolf. Authors from other institutions are Tim Pearce, Daniella Bardalez Gagliuffi, Timothy Brandt, Justin Crepp, Trent Dupuy, Jacqueline Faherty, Rebecca Jensen-Clem and Christopher Theissen.
This research was supported by the National Science Foundation, NASA, the Alfred P. Sloan Foundation, the Heising-Simons Foundation and Deutsche Forschungsgemeinschaft.”