In the planetary system HD 158259, all pairs of subsequent planets are close to the 3:2 resonance! The inner one completes about three orbits as the outer completes two. UNIGE/NASA

The star HD 158259 is almost visible to the naked eye in the Draco constellation 88 light-years from Earth. Nothing special so far. What makes the star intriguing is that it is surrounded by six planets. Only 12 other systems have this many or more orbiting a star. Not only that but the orbit of the planets have a specific rhythm.

The planets are said to be in almost 3:2 resonance. This means that for every three orbits of the innermost planet, the second innermost one completes two orbits. And for every three orbits of the second, the third does two, and so on.

“This is comparable to several musicians beating distinct rhythms, yet who beat at the same time at the beginning of each bar,” first author Nathan C. Hara, from the University of Geneva, said in a statement. The findings are published in the journal Astronomy & Astrophysics.

Resonance, which has been seen in other multi-planetary systems such as TRAPPIST-1, may be key to keeping certain systems stable. In these systems, planets form far away from the star and then move closer and closer due to their interaction with the disk of material from which they formed. Where multiple planets are concerned, these migrations may end up knocking a planet into a star or another planet, or kick-it outside the system. But if the planets get locked in a resonance, it's possible their migration may remain stable.

The team believe the six planets must have moved closer to HD 158259 by staying in resonance and only afterward did they start deviating from perfect harmony. The planets are only in about a 3:2 resonance. The team are intrigued by what this information can tell us.

“Here, 'about' is important. Besides the ubiquity of the 3:2 period ratio, this constitutes the originality of the system,” explained Hara. “Furthermore, the current departure of the period ratios from 3:2 contains a wealth of information. With these values on the one hand and tidal effect models on the other, we could constrain the internal structure of the planets in a future study. In summary, the current state of the system gives us a window on its formation.”

Using the SOPHIE spectrograph installed at the Haute-Provence Observatory in the South of France, the team measured the properties of the system. Five of the planets are mini-Neptunes with a mass of about six times our planet. The innermost was measured with planet-hunting telescope TESS and has a mass of about two Earths.