Temporary Companion Leads Uranus in Its Race Around the Sun

A small asteroid that orbits ahead of the seventh planet offers a clearer picture of the ongoing celestial pinball game in the solar system’s outer reaches

2011 QF99
Astronomers traced 2011 QF99, circled in green, across the sky to find that it shared an orbit with Uranus. Image via Science/Alexandersen et al. 2013

A rocky, icy body the size of Rhode Island is playing follow the leader with the seventh planet from the Sun, whizzing along Uranus’ orbit one-sixth of a revolution ahead of the planet. The body, temporarily dubbed 2011 QF99, is the first of its type found to circle with Uranus. Researchers reporting in the journal Science document its detection and show that it is probably not alone, promising a clearer picture of the ongoing celestial pinball game in the solar system’s outer reaches.

Thousands of similarly positioned bodies are known to exist around Jupiter; they are called Trojans because each is named for a mythological character in the Trojan War. But scientists had believed that gravitational tug around Uranus and Saturn, particularly the pull of Jupiter, made similar companions there unlikely.

What exactly are Trojans? Their story dates back to the 18th century, when a famous mathematician named Joseph-Louis Lagrange wrote an essay on the problem of three bodies, identifying five positions where the gravitational effects of a body orbiting another body (think of the Earth-Moon system as a single body circling the Sun) would allow a third smaller body to stay balanced. When located at any of these five Lagrange Points, the third body would appear stationary relative to the other two. Three of these five positions, called L1, L3 and L3, would be unstable–if the third body drifted just a bit off course from any of these positions, it could never recover from the misstep. L1 and L2 are ideal locations for placing artificial satellites that study the Sun and space, although the spacecrafts’ trajectories have to be constantly tweaked so that they stay at these points.

Lagrange Points

The Sun-Earth system has five Lagrange Points where a third smaller mass can remain stationary relative to the Sun and Earth. L2 is the future home of the James Webb Space Telescope, which will look out into the universe. Image via NASA/WMAP Science Team

But at two Lagrange Points, dubbed L4 and L5, the body would be pulled right back regardless of which way it drifted, causing it to swing around the point like a gymnast on a high bar. In fact, multiple bodies–many thousands–could dance around each point within an elongated region of stability that contours to the orbital path of the planet. One of these points sits 60 degrees ahead on that orbital path and another 60 degrees behind.

Other three-body systems have these same balance points, and in 1906 astronomers found an asteroid in the L4 region of Jupiter’s orbit around the Sun, naming the body Achilles. In the following years, more Trojan asteroids were discovered around Jupiter’s L4 and L5  and, more recently, Trojans have been found along other planets’ orbits, including Mars’, Neptune’s and even Earth’s.

But none had turned up for Uranus or Saturn–until now. As part of a Canada-France-Hawaii Telescope survey designed to search for small bodies orbiting beyond the most distant planet, Neptune, a team of astronomers spotted 2011 QF99 in three images taken an hour apart on the same patch of sky. The object’s brightness suggested it was 60 kilometers across and its orbit pinned it as distant as Uranus, but further observations in 2011 and 2012 distinguished it from a Centaur, an unstable icy body that orbits the Sun and occasionally crosses, but doesn’t follow or lead, planetary orbits. The team’s study showed 2011 QF99 running out ahead of Uranus like a dog on a leash: It was an L4 Trojan.

“A Uranian Trojan was not the focus of our survey,” says Mike Alexandersen, an astronomer at the University of British Columbia. “When we realized what it was, we were like ‘Whoa, wow.’”

Unlike most other known Trojans, which adopted their current positions early during the solar system’s formation, 2011 QF99 was probably first a Centaur and was captured at L4 later on, caught as it leaked inward from more distant reaches. Numerical analyses of the details of the orbit of 2011 QF99 suggest it will remain as a Trojan for 70,000 years before, after a million years or so, it moves beyond the L4 region of stability and rejoins the Centaurs.

2011 QF99, then, is a temporary Trojan. And simulations by Alexandersen and his team, reported for the first time in the new paper, find that 2011 QF99 is not alone. About 3 percent of the small bodies in the outer solar system share an orbit with Neptune or Uranus at any given time. “There are a lot of asteroids and comets flying around the solar system, and a lot of them cross the orbits of planets and only a tiny fraction get captured,” he says. Capture is “a low probability event. Intuitively, we thought it had an even lower probability.” 

While the more permanent Trojans have quite a lot to to say about primordial jostling, the temporary Trojans–including others discovered orbiting with Neptune and Earth–could reveal information on the amount of Centaurs populating the nether reaches, how exactly they got there and what paths they follow.

“Those unstable objects, the Centaurs, often go on to become Jupiter-family comets, many of which approach the Earth and could, eventually, pose an impact threat,” says Jonti Horner, an astronomer at the University of New South Wales who wasn’t involved in the study. “Being able to study those objects when they’re far from the Sun, and therefore not hidden by a cometary coma, can tell us a lot about comets and other objects that can threaten Earth.”

“It’s a really exciting discovery for me, and for other people who look at the solar system’s small bodies,” he added.

Alexandersen, who notes that the risk of impact is extremely low, says the results speak to how much is still left to know about our solar system. He predicts that more will be revealed as astronomers continue to detect smaller and smaller objects. “If there is one 60-kilometer Trojan, then there are probably dozens of one-kilometer Trojans,” he says. “We just can’t see them yet.”