Although the probability is one part per million, there could still be many Earth-like planets with horseshoe orbits in the hundreds of billions of intergalactic stars.
Earth is the only planet to move 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 the two planets can share is called the horseshoe orbit. Instead of moving in a circle around the star, each planet would move along a horseshoe-shaped orbit, like two halves of a broken ring.
“I think the horseshoe shaped orbits are among the most interesting orbital shapes for other planets,” said researcher Sean Raymond at the Bordeaux Astrophysics Laboratory in France. “Since 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-shaped trajectory seems highly improbable. However, Saturn’s moons Janus and Epimetheus follow a horseshoe-shaped orbit about 150,000 km from the planet, outside its main ring. The closest distance between them is 15,000 km. Imagine that there are a pair of planets as large as Earth in habitable areas around the Sun, areas at the right temperature for liquid water to exist on the planet’s surface. We call them Terra and Tellus, which both mean “Earth” in Latin.
At the closest distance, Terra and Tellus are only about 4-5% of an astronomical unit (AU), the average distance between Earth and the Sun (150 million km). Then when viewed from one of the two planets, the other is 1/4 to 1/5 the diameter of the full moon. Then they will slowly pull away until they are completely out of sight. “It would be very impressive to see the companion planet rise on the horizon, become a major source of light,” commented Raymond.
The length of the back and forth cycle depends on the width of the horseshoe’s orbit. For Terra and Tellus, the horseshoe orbits will range 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 meant that the climates of Terra and Tellus would not change much when they rocked above the horseshoe orbit. Planets with horseshoe orbits inevitably evolved into the protoplanet, both colliding and “migrating” from one orbit to another.
