Uranus

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Object: A 60-kilometre-wide asteroid

Location: 3 billion kilometres ahead of Uranus, in the planet’s L4 Lagrange point

Uranus has a forbidden friend. The first asteroid to share the planet’s orbit has been found, despite claims that Jupiter’s mighty gravity should steal such companions away.

The finding hints that more of these asteroids, called Trojans, lurk around unexpected worlds. Since Trojans don’t always stay in place, finding new ones improves our picture of how space rocks migrate around the solar system. It also means there may be super-sized Trojans sharing orbits with massive exoplanets.

Mike Alexandersen of the University of British Columbia in Vancouver and colleagues discovered the 60-kilometre-wide Trojan, named 2011 QF99, from an observatory in Hawaii. It lives in a Lagrange point, where the gravitational tugs from the sun and Uranus balance out.

There are five possible Lagrange points around two massive bodies, such as the sun and a planet. The ones called L4 and L5 sit ahead of and behind the planet, respectively, in its orbital path. These points have long been known to act as dust-gathering niches. Jupiter’s L4 and L5 points host more than 3000 asteroids of various sizes. They are the original cosmic Trojans, named after the legendary fighters of ancient Troy.

Exo-Trojans

Since the Jovian discoveries, Trojan asteroids have also been spotted near Neptune, Mars and even Earth. But they were thought to be extremely unlikely for Saturn and Uranus, because massive Jupiter would have sucked in any space rocks leftover as the solar system formed.

Uranus’s Trojan is probably a temporary companion captured later in the planet’s history. The asteroid should jitter around the L4 region for about 70,000 years before becoming unstable enough to be ejected.

The discovery means that many more Trojans may lurk near other unexpected worlds, says Rudolf Dvorak of the University of Vienna in Austria. Such an array of Trojans in our solar neighbourhood strengthens the case that other star systems host larger versions of these orbital companions. “There could be planets in this Trojan configuration like our Earth,” he says, perhaps even in the habitable zone, the region around a star in which a planet could support life.

Reference: The first known Uranian Trojan and the frequency of temporary giant-planet co-orbitals