Researchers discover a six-planet system with near 3:2 resonance

With orbit periods ranging from only 2-12 days, this must be one of the most compact multi-planet systems found so far.

Almost visible to the naked eye in the Draco constellation, the star HD 158259 has been observed for the last seven years by astronomers using the SOPHIE spectrograph, reports Phys.org.

This instrument, installed at the Haute-Provence Observatory in the South of France, acquired 300 measurements of the star.

The analysis of the data which was done by an international team led by researchers from the University of Geneva (UNIGE), has resulted in the discovery that HD 158259 has six planetary companions: a “super-Earth” and five “mini-Neptunes.”

These planets display an exceptionally regular spacing, which hints at how the system may have formed.

This study, published in the journal Astronomy & Astrophysics, also includes TESS space telescope observations of the star, which unveil the density of the innermost planet.

The SOPHIE measurements of HD 158259 allow the radial velocity of the star to be measured with great precision—that is its velocity of the star in the direction of the line of sight. It is the same type of measurement that led to the discovery of 51 Peg b by the 2019 Nobel Prize winners Michel Mayor and Didier Queloz in 1995. What’s more, the detection of 51 Peg b and of the six planets of HD 158259 was done using the same telescope.

Unveiling a system with such small planets would not have been feasible back in 1995, nor at the installation of SOPHIE back in 2006. “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,” says François Bouchy, professor in the Department of Astronomy of the Faculty of science of UNIGE and coordinator of the observational program.

“This is great work and shows the important role smaller telescopes play in furthering the advances of astronomy through high-quality research using old, but well funded observatories,” adds Paul A. Wilson, researcher at the University of Warwick and co-author of the study.

SOPHIE observations showed that the planet closest to HD 158259 and the five outer planets present masses of two and six times that of the Earth, respectively. The system has been found to be compact, in the sense that the distance of the outermost planet to its star is 2.6 times smaller than the distance between Mercury and the Sun.

Furthermore, NASA’s TESS space telescope observed a decrease of the star’s brightness as the innermost planet transited between the observer and the star. “The TESS measurements strongly support the detection of the planet and allow us to estimate its radius, which brings very valuable information on the planet’s internal structure,” says Isabelle Boisse, researcher at the Marseille Astrophysics Laboratory and co-author of the study.

Planets in rhythm

Hundreds of multi-planetary systems are known, but only a dozen contain six planets or more. The presence of six planets orbiting HD 158259 makes this system remarkable, but its most interesting feature is its regularity.

Indeed, the period ratio of any two subsequent planets is close to 3:2. This means that as the first planet—the one closest to the star—completes three orbits, the second one completes about two. As this second planet completes three orbits, the third completes about two, and so on.

This situation is better understood in the general framework of “resonance,” which plays a key role in the architecture of planetary systems. Several planets are said to be in resonance when they find themselves in the same configuration periodically after completing a potentially different number of orbits.

“This is comparable to several musicians beating distinct rhythms, yet who beat at the same time at the beginning of each bar,” says Nathan C. Hara, researcher at the Department of astronomy of the Faculty of science of UNIGE, member of the PlanetS institute and first author of the study.

Planets can also be close to the resonance, but not exactly within. This is the case of the planets of HD 158259.

Full report here.