Discovery of Saturn's First Trojan Asteroid

Astronomers have finally found an asteroid keeping pace with Saturn in its orbit around the sun. Such objects, called Trojan asteroids, are already known for the other three giant planets.

“Saturn was sort of the odd man out, if I can call it that, because even though it’s the second most massive planet in the solar system, it didn’t have any Trojans,” says Paul Wiegert, an astronomer at the University of Western Ontario in London, Canada. Like Saturn, the new asteroid takes about 30 years to revolve but lies 60 degrees ahead of the planet in its orbit, Wiegert and colleagues report in work submitted September 29 to arXiv.org.

Most asteroids in the solar system revolve around the sun between the paths of Mars and Jupiter. In 1906, however, German astronomer Max Wolf discovered the first Trojan, which he named Achilles, orbiting the sun 60 degrees ahead of Jupiter. Since then, astronomers have found thousands of additional Trojan asteroids — some are 60 degrees ahead of Jupiter, others are 60 degrees behind. The NASA spacecraft Lucy will visit eight of them between 2027 and 2033 (SN: 10/15/21) .

Trojan asteroids also exist for Uranus and Neptune and even for Earth and Mars (SN: 2/1/22).

After a telescope image in Hawaii captured the new asteroid in 2019, an amateur astronomer in Australia, Andrew Walker, suggested that the object might be a Saturnian Trojan — if it had the right orbit around the sun.

“The key to getting a good orbit for something in our solar system is having a lot of observations of it through different telescopes over a long period of time,” Wiegert says. So astronomer Man-To Hui at Macau University of Science and Technology in China looked for previous images of the asteroid and planned new observations as well. Measurements of the asteroid’s position — from 2015 to 2024 — confirmed its Trojan nature. Named 2019 UO14, the asteroid is only about 13 kilometers across, the same size as Deimos, the smaller of the two moons of Mars.

Scientists have long predicted Saturnian Trojans, says astronomer Carlos de la Fuente Marcos of Complutense University of Madrid, who was not involved with the discovery. But all Saturnian Trojans should have unstable orbits, because Saturn has giant planets on either side of it.

“Jupiter seems to be the culprit,” de la Fuente Marcos says. Jupiter’s great gravity gradually pulls on a Saturnian Trojan, making its orbit around the sun more and more elliptical. The asteroid then wanders so close to Jupiter or Uranus that one of those giant planets yanks the small body out of its Trojan orbit.

In fact, the researchers estimate the asteroid has been a Trojan for only about 2,000 years and will remain so for only another 1,000 years. Prior to its affair with the ringed planet, the asteroid was probably a centaur, an asteroid moving around the sun among the orbits of the giant planets (SN: 11/12/77).

The asteroid probably isn’t Saturn’s sole Trojan. “I’m quite sure there are more — maybe only a few, but this can’t be the only one,” Wiegert says.