To date, astronomers have confirmed the existence of 4,152 extrasolar planets in 3,077 star systems. While the majority of these discoveries involved a single planet, several hundred star systems were found to be multi-planetary. Systems that contain six planets or more, however, appear to be rarer, with only a dozen or so cases discovered so far.

This is what astronomers found after observing HD 158259, a Sun-like star located about 88 light-years from Earth, for the past seven years using the SOPHIE spectrograph. Combined with new data from the Transiting Exoplanet Space Satellite (TESS), an international team reported the discovery of a six planet system where all were in near-perfect rhythm with each other.

The international team responsible for this discovery was led by Dr. Nathan Hara, a postdoctoral researcher at the University of Geneva (UNIGE), a member of the Swiss PlanetS institute, and a Fellow with the European Space Agency’s (ESA) CHaracterising ExOPlanets Satellite (CHEOPS) mission. The study that describes their findings recently appeared in the journal Astronomy & Astrophysics.

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. Credit and ©: UNIGE/NASA

Using SOPHIE, astronomers have been conducting velocity measurements of many stars in the northern hemisphere to determine if they have exoplanets orbiting them. This method, known as the Radial Velocity Method (or Doppler Spectroscopy), consists of measuring the spectra a star to see if it is moving in place – which is an indication that the gravitational force of one or more planets is working on it.

Interestingly enough, it was SOPHIE’s predecessor (the ELODIE spectrograph) that led to one of the earliest exoplanet discoveries in 1995 – the “hot Jupiter” 51 Peg b (Dimidium). After examining HD 158259 for seven years, SOPHIE succeeded in obtaining high-precision radial velocity measurements that revealed the presence of a six planet system.

This system consists of an innermost large rocky planet (a “super-Earth”) and five small gas giants (“mini-Neptunes”) that have exceptionally regular spacing between them. As François Bouchy, a professor of astronomy and science at UNIGE and the coordinator of the observation program, explained in a UNIGE press release:

“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.”

Artist’s impression of 51 Pegasi b (Bellerophon), a hot Jupiter discovered by SOPHIE that orbits a star about 50 light-years from Earth. Credit: ESO/M. Kornmesser/Nick Risinger (skysurvey.org)

These planets range from being 2 (the innermost “super-Earth”) to 6 times (the “mini-Neptunes”) as massive as Earth. The system is also very compact, with all of six planets orbit closely to the star and the outermost being just 0.38 times as distant as Mercury is from the Sun. This places the planets well inside the star’s habitable zone (HZ), which means none are likely to have water on the surfaces or dense enough atmospheres to support life.

Meanwhile, TESS monitored HD 158259 for signs of transits (aka. the Transit Method) and observed a decrease in the star’s brightness as the innermost planet passed in front of the star. According to Isabelle Boisse, a researcher at the Marseille Astrophysics Laboratory and co-author of the study, the TESS readings (combined with the radial velocity data) allowed them to constrain the properties of this planet (HD 158259 b) further.

“The TESS measurements strongly support the detection of the planet and allow to estimate its radius, which brings very valuable information on the planet’s internal structure,” she said. But as noted earlier, the most impressive feature of this system is its regularity. Basically, the planets in the system have an almost exact 3:2 orbital resonance

This means that for every three orbits the innermost planet makes, the second one will complete about two. In the time it takes the second planet to complete three orbits, the third will complete about two. This ratio applies to all six planets in the system and came as quite a surprise to Hara and his colleagues.

When describing the planets’ orbits, Hara compared it to an orchestra playing music, though the arrangement is not quite perfect:

“This is comparable to several musicians beating distinct rhythms, yet who beat at the same time at the beginning of each bar. Here, “about” is important. Besides the ubiquity of the 3:2 period ratio, this constitutes the originality of the system.”

Resonances, even imperfect ones, are of interest to astronomers because of how they provide hints to a star system’s formation and evolution. In astronomical circles, there is still considerable debate about how star systems come together and change over time. A particularly contentious point is whether planets form close to their final position in the system, or if they change their orbits after forming.

This latter scenario (known as planetary migration) has been gaining traction in recent years thanks to the discovery of exoplanets like “Hot-Jupiters”, leading many astronomers to question if planetary “shake-ups” occur. This theory would appear to explain the formation of the six planets in the HD 158259 system. Said Stephane Udry, a professor of astronomy and science at UNIGE:

“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.”

This artist’s concept shows what each of the TRAPPIST-1 planets may look like, based on available data about their sizes, masses and orbital distances. Credits: NASA/JPL-Caltech

The fact that HD 158259’s planets are close to a 3:2 resonance, but not exactly within one, suggests that they were trapped in one in the past. However, they would have subsequently undergone synchronous migration and moved away from the resonance. According to Hara, that’s not all that this system can tell us.

“Furthermore, the current departure of the period ratios from 3:2 contains a wealth of information,” he said. “With these values on the one hand, and tidal effect models on the other hand, 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.”

The more we learn about this multi-planet system and others like it, the more we can learn about how star systems like our own came to be. The resolution of these and other questions about the formation and evolution of planetary systems will put us one step closer to knowing how life can emerge (and perhaps where to look for it!)