A NASA telescope has discovered two planets sharing an identical orbit. It’s the first time that’s been found in reality, and gives a little more weight (or should that be mass?) to a theory about the formation of our Moon.

The Kepler telescope is housed on a spacecraft that trails our solar system in a manner that means its view isn’t obscured by sunlight. It’s designed to discover new planets outside the solar system and has so far found more than 1,000. Most notably it’s expanded the number of known planets that appear to be at the right temperature to support liquid water from two to more than fifty.

The latest discovery is in a system currently referred to as KOI-730. While the system features four planets circling the same star, two orbit at exactly the same speed, taking 9.8 days. That means one planet is always around 60 degrees ahead of the other in relation to the central star. New Scientist notes this means that from one planet, the other will always be visible in the night sky at a constant intensity, and vice versa.

That such a set-up is possible isn’t surprising news. It’s already known that the 60 degree gap is just right for two bodies to orbit in sync: the effects of gravity mean that gap produces two of the five Lagrangian points where such a sync is possible, a mathematical rule discovered by 18th century Joseph Louis Lagrange.

Until now, though, such a relationship had only been seen in practice in a setup involving a star, a planet and a smaller body such as asteroids. For example, the diagram above shows our Sun, the Earth (the blue and brown sphere) and the five Lagrangian points for the Earth. But in reality these points don’t appear to contain much more than space dust.

Kepler has now discovered that in KOI-730, one planet is in the same position as Earth in the diagram, with another bang on point L4 or L5 (depending on which of the pair you use as the reference point.)

The concept of a Lagrangian point is cited in one theory about the creation of the Moon. That says that there was once a planet named Theia, roughly the size of Mars, that was 60 degrees removed from Earth, with Theia and Earth orbiting in sync. The theory is that eventually Theia’s orbit destabilized until eventually it crashed into Earth, causing a chunk of Earth to break off and eventually become our Moon.

Unfortunately any astronomers hoping to see if that sequence of events could happen with the KOI-730 planets will be out of luck: the two planets are forecast to remain in stable orbit for at least the next two million years.