Six-Planet Star System Discovered Where Worlds Orbit in Near-Perfect Rhythm

Researchers have discovered a star system with six planets whose orbits are in almost perfect rhythm.

According to a study published in the journal Astronomy & Astrophysics, the star, dubbed HD 158259, is orbited by a "super-Earth" and "five mini-Neptunes."

This in itself is unusual, given that astronomers only know of around dozen star systems containing six or more planets. However, what makes the HD 158259 system so remarkable is the exceptionally regular spacing of its planets.

All pairs of subsequent planets in the system are said to be close to the "3:2 resonance." This essentially means that as the first planet—or the one closest to the star—completes three orbits, the second one completes roughly two orbits. And as the second planet completes three orbits, the third completes roughly two. This pattern continues for all the subsequent planets.

"This is comparable to several musicians beating distinct rhythms, yet who beat at the same time at the beginning of each bar," Nathan Hara, first author of the study from the University of Geneva (UNIGE), said in a statement.

These intriguing orbital periods could provide scientists with clues as to how the system formed, suggesting that they did not form close to their current positions.

star, exoplanets
Stock image: Artist's illustration of several exoplanets orbiting a star. iStock

The evidence indicates that the star system is compact, highlighted by the fact that the distance between the outermost planet and HD 158259 is 2.6 times smaller than the distance between the sun and Mercury in our own solar system.

"Several compact systems with several planets in, or close to resonances are known, such as TRAPPIST-1 or Kepler-80. Such systems are believed to form far from the star before migrating towards it. In this scenario, the resonances play a crucial part," Stephane Udry, another author of the study from UNIGE, said in the statement.

The planets of the system—which is located around 88 light-years away—appear to have masses between two and six times that of the Earth's.

Five of them are defined as super-Earths, meaning that they have a mass higher than that of our planet but significantly lower than Uranus or Neptune. The remaining planet is described as a "mini-Neptune"—planets that are less massive than Neptune while still resembling the gas giant in our own solar system in terms of atmosphere and composition.

The latest findings are the result of observations with the SOPHIE spectrograph instrument on the Haute-Provence Observatory in France.

"The discovery of this exceptional system has been made possible thanks to the acquisition of a great number of measurements, as well as a dramatic improvement of the instrument and of our signal processing techniques," François Bouchy, another author of the study from UNIGE, said in a statement.