The scientists used a technique called astrometry to obtain a direct image of AF Lep b, one of the lowest-mass planets orbiting a young Sun-like star. This method could revolutionize the detection of extrasolar planets, especially those that are difficult to detect due to their distance, mass or orientation relative to Earth.

Astronomers using the WM Keck Observatory in Maunakea, Hawaii, have discovered one of the lowest-mass planets ever directly imaged. They managed not only to measure its mass, but also to determine that its orbit is similar to that of the giant planets in our solar system.

The planet AF Lep b was one of the first planets discovered by a method called astrometry; this method measures the subtle movements of the host star over many years to help astronomers determine whether it is gravitationally attracted by elusive orbital companions, including planets.

A direct image of an extrasolar planet with a mass and orbit similar to Jupiter taken by the Keck II AF Lep b telescope. Credit: University of Texas Austin/WM Keck Observatory

The study, led by Kyle Franson, a graduate student in astronomy at the University of Texas at Austin (UT Austin), Astrophysical Journal Letters.

“When we processed the observations in real time with the Keck II telescope to carefully remove the glare from the star, the planet appeared immediately and became more visible as we watched it,” said Franson.

Direct images taken by Franson’s team showed that AF Lep b is about three times the mass of Jupiter and orbits a young Sun-like star, AF Leporis, about 87.5 light-years away. They took a series of deep images of the planet starting in December 2021; since then, two other teams have taken pictures of the same planet.

“This is the first time this method has been used to find a giant planet orbiting a young sun counterpart,” said Brendan Bowler, associate professor of astronomy at UT Austin and senior author of the study. “This opens the door to using this approach as a new tool for exploring exoplanets.”

The movement of exoplanet AF Lep b (white dot at 10 o’clock) around the parent star (center) can be seen in these two images taken in December 2021 and February 2023. Images were collected using WM Keck. 10 meter observatory telescope in Hawaii. Credit: Kyle Franson, University of Texas Austin/WM Keck Observatory

Although an orbiting planet is much less massive than its host star, the star’s position oscillates slightly around the planetary system’s center of mass. Astrometry uses this shift in a star’s position in the sky relative to other stars to detect the presence of orbiting planets. Based on over 25 years of observations from the moons Hipparcos and Gaia, Franson and Bowler determined that the star AF Leporis potentially contains a planet.

The UT Austin team used the Keck Observatory’s adaptive optics system, which corrects for fluctuations caused by turbulence in the Earth’s atmosphere, together with the Keck II telescope’s Near Infrared Camera 2 (NIRC2) Vector Vortex Coronograph to obtain a direct view of the planet. suppresses light from the host star to see the planet more clearly. AF Lep b is about 10,000 times fainter than its host star and is about 8 times the Earth-Sun distance.

“Imagining planets is a challenge,” said Franson. “We only have 15 examples, and we believe this new ‘dynamically informed’ approach, made possible by the Keck II telescope and NIRC2’s adaptive optics, will be far more productive than blind studies that have been conducted over the past two decades.”

The two most common ways to look for exoplanets are to watch for a slight, periodic dimming of the starlight if the planet regularly passes in front of the star, such as a moth orbiting a patio light, and to measure small changes in the frequency of the starlight. It is caused by the planet pulling the star back and forth in the direction of the Earth. Both methods tend to work best on large planets orbiting close to their stars, and both methods are indirect: we don’t see the planet, we only see how it affects the star. Source