Six planets in coordinated orbits

by | December 13, 2023, 12:00 PM | Solar Systems

In my personal opinion, one of the mathematically coolest things in our solar system is the orbits of Jupiter’s three inner Galilean moons. It just happens, that these worlds orbit to a beat, with innermost Io circling four times for every two times Europa goes around to every one time Ganymede goes around. This 4:2:1 resonance is just cool to watch, and as a band kid, I find it pleasing.

We don’t currently know just how common orbital resonances are in our universe, but we know Jupiter’s moons do this. We think our own Solar System once had a resonance between Jupiter and Saturn, and now we know that a star not so far away has six planets in its own series of resonances. 

These six worlds orbit the star HD110067, a fairly bright star located about 100 light-years away in the constellation Coma Berenices.

This system is more complicated than Jupiter’s moons. The innermost moon orbits three times for every two times the next world out orbits, in what’s called a 3:2 resonance. That second planet is in a 3:2 resonance with the third, which is in a 3:2 with the fourth! The last two worlds mix things up a bit and planet four4 is in a 4:3 resonance with five, and five is in a 4:3 with six. 

Whew. It’s a lot of syncopated beats. 

Trying to understand exactly what was going on was less than easy. The innermost two planets were discovered by NASA’s TESS observer when they passed between us and the star they orbit. This transit caused the star’s light to fade just enough that software could spot the difference. Followup observations by ESA’s CHEOPS mission showed many more transits, and it was clear this was a multi-planet system. What was going on required some creative thinking. It had been noticed early on that the innermost planets – b and c –  orbit in a 3:2 resonance with orbits of about nine and fourteen days. The new data for the next planet showed another 3:2 resonance, with planet d having a 20.5-day period. This got the research team to wonder: could all the worlds be in some kind of resonance? 

Team member Adrien Leleui explains, “This led to dozens of possibilities for their orbital period, but combining existing observational data from TESS and CHEOPS, with our model of the gravitational interactions between the planets, we could exclude all solutions but one: the 3:2, 3:2, 3:2, 4:3, 4:3 chain.”

It’s unknown if this is a passing moment in time for this solar system or if the planets will be able to maintain their beat. Whatever this system’s future, it’s amazing to get to see what it is doing now. 

This work is published in the journal Nature and led by R Luque.