This morning, the discovery of Earth’s first Trojan companion was announced by a group of Canadian astronomers.

The object in question, 2010 TK7, is a lump of rock just a few hundred metres across, and is currently moving in lockstep with our planet, trapped in what is known as a “tadpole orbit”.

Discovered using data from NASA’s Wide-field Infrared Survey Explorer (WISE), 2010 TK7 represents the first confirmed member of a population of solar system bodies that has long been considered plausible, but hard to observe.

But what is a “Trojan object”, and why are they interesting?

Achilles

The first Trojan to be discovered was 588 Achilles, in 1906. While the great majority of known asteroids move on orbits that keep them well within the orbit of Jupiter (mainly within the famous Asteroid Belt), Achilles instead moves on an orbit almost identical to that of the giant planet, with essentially the same orbital period.

Even though it might seem such an orbit must be highly unstable, and that Achilles must surely soon experience a close encounter with Jupiter, either colliding with the planet or being flung by its immense gravity on to an entirely new orbit, a fluke of celestial dynamics means that Achilles is protected from such a fate.

Rather than approaching Jupiter, Achilles instead leads the planet in its orbit, lying some 60 degrees ahead of it.

Over a period of tens of years, Achilles pulls ahead of Jupiter, before distant gravitational tweaks modify its orbit such that it begins to fall back toward the planet.

Then Achilles slowly relinquishes its lead over Jupiter, moving ever so gradually back towards it.

Eventually, and long before Achilles gets at all close to the giant planet, further perturbations modify its orbit. Its motion, relative to Jupiter, is reversed, causing it to once again drift away from the planet.

Over the course of each of these “laps”, the average lead of Achilles over Jupiter is 60 degrees – and when the path it follows is plotted along with Jupiter’s orbit, it can be seen to be moving around a point on Jupiter’s orbit known as the leading Lagrange point, L4.

Such behaviour is called “libration”, and is common to all Trojan objects.

The more the merrier

In the century or so since Achilles’ discovery, many more Trojans have been found.

Jupiter’s Trojan family, by far the largest, contains almost 5,000 members to date, with objects both leading and trailing the planet in two clouds centred 60 degrees ahead (the “Greeks”) and behind it (the “Trojans”) in its orbit.

Jupiter’s family has been estimated to be at least as populous as the main Asteroid belt, hosting around one million members of one kilometre or more across.

That population, though, might well be dwarfed by the Neptune Trojans. Although the first of those distant objects was only found in 2001, and the known population at the current day numbers just seven, it is estimated there may be as many as 10 million of those objects, held in cold storage in the depths of the solar system.

It’s not just the solar system’s giant planets that host Trojans – Mars, too, is known to have four small Trojans, one leading the planet in its orbit, and three trailing it (librating around the trailing L5 Lagrange point).

Over the years, dynamical studies of the solar system have shown the giant planets Saturn and Uranus should not be expected to host significant Trojan populations – such objects would be so unstable as to have been lost by the current day.

Artist’s conception of the Sun, the Earth and asteroid 2010 TK7. Connors et. al. Nature

Our Trojan

It has long been suspected Earth could host Trojan asteroids, sharing our orbit, and shepherding us as we move around the Sun.

Unfortunately, the search for Trojan companions to Earth is remarkably difficult.

Since such objects tend to spend most of their time far away from their host planet, Earth’s Trojans will typically be quite far away from us.

Even worse, an object leading Earth in its orbit by 60 degrees will therefore be just 60 degrees from the Sun in the sky (looking down at the solar system from above, it would form an equilateral triangle with the Earth and Sun).

By the time the night sky is truly dark, such a Trojan would be low in the sky, and therefore very challenging to search for.

Although searches have been conducted for Trojan companions to Earth, none had been successful. Until now.

Like Achilles, our newly discovered companion moves on an orbit that librates around Earth’s leading Lagrange point.

But, unlike Achilles, the scale of its libration is much greater – and therefore 2010 TK7 can approach Earth far more closely than Achilles will ever approach Jupiter.

In fact, although 2010 TK7 currently shares the Earth’s orbit, it seems unlikely that it has been there since the birth of the solar system.

The greater the range over which a Trojan librates, typically, the less stable its orbit will be on astronomical timescales.

The great majority of the Jovian and Neptunian Trojans move on orbits that feature relatively small-scale libration, and have most likely been trapped within the Trojan clouds since the final stages of the solar system’s formation.

But 2010 TK7 appears to be far less dynamically stable. Indeed, the scientists responsible for it’s discovery state that “the asteroid’s precise behaviour cannot be predicted with certainty outside [a] 7,000 year span.”

Rather than being a long-term companion, then, it seems more likely that 2010 TK7 was captured to its current orbit relatively recently from the near-Earth object population , to which it will eventually return.

Proof in the pudding

Perhaps the most exciting thing about 2010 TK7 is the simple fact it proves our planet can have Trojans.

It has been suggested, in the past, that any Trojan companions to Earth would represent particularly easy asteroids to visit and study in depth, due to their extremely low relative velocities compared to our planet.

Unfortunately, 2010 TK7 moves on an orbit that is highly inclined to our own, meaning it would actually be far more challenging to visit it than to visit one of the many near-Earth asteroids that cross Earth’s orbit.

That said, it’s an exciting and enigmatic addition to the zoo of Solar system objects, and also brings with it great promise.

The history of astronomy has shown us that where one object is found, many more will soon follow.

Our new friend 2010 TK7 may well be the first of many, and we might well soon find its hidden companions, some of which will certainly be ideal targets for space exploration.