This planetary system’s perfect rhythm tells us a lot about how it came to be

A planetary system of six exoplanets in near-perfect synchronization gives astronomers a hint at how such groupings could evolve.

Over a period of seven years, astronomers have observed the star HD 158259 — invisible to the naked eye — discovering six planets in orbit around it in near-perfect synchronization. The extrasolar planets (exoplanets) consist of one ‘Super-Earth’ and five ‘Mini-Neptunes’ with exceptionally regular spacing and timing. Their regimented order gives researchers a hint as to how the system may have formed.

The study, led by researchers from the University of Geneva (UNIGE), was conducted with the aid of the SOPHIE spectrograph and observations from the TESS space telescope and is published in the journal Astronomy & Astrophysics.

The team was able to determine that the planet closest to HD 158259 and the five outer planets have a mass ranging from two to six times the mass of the Earth respectively. The distance to the outermost planet from its parent star is just over two and a half times smaller than the distance between the Sun and its closest planet — Mercury — thus making the system incredibly compact.

The HD 158259 system is quite remarkable as of the hundreds of multi-planetary systems astronomers have discovered thus far, only a handful have consisted of six planets or more. Its most extraordinary feature, however, isn’t the number of incumbent planets, but rather the regularity and rhythm with which they occupy the system.

The Cosmic Rhythm of the HD 158259 System

The planets in the HD 158259 are synchronized in such a way that in almost the same time that it takes the innermost planet to complete three orbits, the next planet out completes two orbits. Continuing the pattern, as this second planet completes three orbits the third is, in turn, almost completing its second.