Although the probability is one in a million, there could still be many Earth-like planets with horseshoe orbits among the hundreds of billions of intergalactic stars.
Earth is the only planet that moves in an almost circular orbit around the Sun. But what if Earth shared an orbit with another planet? One of the most unusual orbits that two planets can share is called a horseshoe orbit. Instead of moving in a circle around the star, each planet will move along a horseshoe-shaped orbit, like the two halves of a broken ring.
“I think horseshoe orbits are among the most interesting orbital shapes for other planets,” said researcher Sean Raymond from the Bordeaux Astrophysics Laboratory in France. “Since the two planets form on the same planar disc around the star, studying their evolution is like studying the lives of twins separated at birth.”
A horseshoe trajectory seems very unlikely. However, Saturn’s moons Janus and Epimetheus move in a horseshoe-shaped orbit about 150,000 km from the planet, outside of its main ring. The closest distance between them is 15,000 km. Imagine a pair of Earth-sized planets in the habitable zone around the Sun, an area where the temperature is just right for liquid water to exist on the planet’s surface. We call them Terra and Tellus, which both mean “Earth” in Latin.
At their closest distance, Terra and Tellus are only about 4-5% of one astronomical unit (AU) apart, the average distance between Earth and the Sun (150 million km). Then, when viewed from one of the planets, the other is 1/4 to 1/5 the diameter of the full moon. Then they will slowly pull away from each other until they are completely out of sight. “It will be impressive to see the companion planet rise on the horizon, become a major source of light,” commented Raymond.
The approach and departure cycle times depend on the width of the horseshoe orbit. For Terra and Tellus, the horseshoe orbits will vary from around 0.995 to 1,005 astronomical units, so the approaches will be 33 years apart, according to Raymond. The small difference in distance from the Sun means that the climates of Terra and Tellus won’t change much as they change sides in horseshoe orbits. Planets with horseshoe orbits certainly evolved into the protoplanetary stage, both colliding and “migrating” from orbit to orbit.
