The Earth is not alone on its orbit around the Sun: there are bodies in a particular state of equilibrium that share the orbit with our planet
The Earth is not alone in its orbit. Our planet's first Trojan asteroid is called 2010TK7 and has been detected for some time now. Then, in 2015, a second co-orbital "host" was observed and many speculate it may be a second Trojan.
The observations, made thanks to the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS1) located in Hawaii have recently given the expected confirmation: the object detected in 2015 is a second Trojan asteroid.
What are Trojan asteroids?
Astronomers define Trojans as those celestial bodies, mostly asteroids, that share an orbit with much larger objects: located near Lagrange points L4 and L5, these objects have a stable orbit and travel in front (leading) or behind (trailing) the main celestial body.
The Lagrange points are five in total, and can be observed in the presence of two large orbiting bodies: these are, very briefly, those particular positions where the gravitational force operated by celestial bodies balances with the centrifugal force that would tend to eject smaller bodies.
The result is that on the Lagrange points it is possible to maintain an incredibly stable orbit, so much so that today there are dozens of instruments located on Lagrange points, from probes to telescopes that are giving us the most beautiful images of the universe. And it is on the Lagrange points that we will place the great space stations of the future, it seems.
On the Lagrange points of the Earth, or rather of the Sun-Earth pair, there are also the "Trojan" asteroids that travel together with our planet on a stable orbit that anticipates that of the Earth around the Sun.
Trojans are a fairly common object in the Solar System: while most of the known Trojan bodies are co-orbital to Jupiter, which hosts more than a million of them, Mars, Neptune and Uranus also feature Trojans.
There are now 9 Martian Trojans, 28 on the orbit of Neptune, 2 Trojan bodies on the Lagrange points of Uranus and, thanks to the new research, also 2 Trojan asteroids that share orbit with Earth.
Earth's second Trojan
The new object has been named 2020 XL5, and although it has been officially recognized as a Trojan body its orbit does not coincide perfectly with that of Earth.
As Paul Wiegert of the University of Western Ontario, Canada, says, "Imagine it as a racetrack, the Earth is a car going sped along the track, while this object moves in a more confusing way." They are on the same "circuit", but not exactly on the same line: 2020 XL5 has in fact some small "oscillations".
The definition of the orbit of the second Earth Trojan also tells us about its near future: every 30 years, the celestial object goes to be less than 15 million kilometers from Venus. This means that in the next 6,000 years, there is a very high probability that it will be ejected from its current orbit, to become what Wiegert calls "a trivial near-Heart asteroid."
Is the Trojan a temporary host, then? It would appear so. According to Wiegert's team's calculations, 2020 XL5 should have acquired its current orbit only around 1444. It will continue to anticipate Earth's course around the Sun for many more years, and the scientists are convinced that the new study of Trojan objects can provide "many clues about how the Solar System was formed."