Scientists don’t go looking for coincidences, and in general, when we find them, we don’t trust them to be more than accident and chance. But sometimes, the things we find are actually related.

Enter astronomers Jason Curtis and Marcel Agüeros. Working in a larger, multi-institutional team, these scientists were doing follow-up work on the planet-having systems Kepler 52 and Kepler 968. Both these systems have three planets orbiting a K-dwarf star – a star cooler and smaller than our Sun that is capable of living for billions of billions of years without showing its age.

This last characteristic is actually a bit problematic. When we look at systems with K-dwarf stars, we can’t easily measure their age – by which I mean we can’t measure their age. Instead, we guess based on where they are and what they seem to be related to. And sometimes we guess wrong.

Going into this study, it was thought K-52 and K-968 were several billion years old and utterly unrelated – simply strangers passing in the night through the original Kepler observing field. This starfield, however, is located in the region of the sky being studied by the Gaia space telescope, a telescope that can very precisely measure stellar distances, brightness, and motion.

We know that stars form in clusters as massive molecular clouds of gas and dust collapse and fragment. Over time, these clusters fall apart as stars at slightly different distances from the galactic center orbit at different rates. After enough orbits, the cluster stars that formed together are thoroughly mixed in with all the other stars of the galaxy.

But older clusters that may not be obvious to the eye can still be found using Gaia data because all the stars will be moving together through the galaxy like a load of people released from a shuttle bus to zip toward an airport entryway. Some will race ahead, others will lag behind, but the members can be spotted by their similar motions. If it’s a sports team or tour group, they may also all be wearing identical clothes that help them stand out as a single group. While stars don’t wear jerseys, all the stars that formed together will share the same chemical composition, and that is something we can see with spectrographs.

It turns out that these two stars are moving together, share the same spectra, and are part of a newly discovered star cluster that was named Theia 520. Other stars in this cluster, ones that it is easy to measure the age of, revealed that these stars are actually just 350 million years old. These are young planetary systems where the worlds are still settling into their orbits. And now we have two systems that are nearly identical that we get to study. Essentially, we can do a twins study just with stars.

So next time someone says, “That’s just a coincidence!”, remind them that sometimes there is something that is the cause behind it.

More Information

PDF: How Two Old and Unrelated Exoplanet Systems Turned Out to Be 350-million-year-old Siblings (AAS)