Each planet in this system makes three orbits for every two orbits of the next planet out. While such “orbital resonance” has been seen in other systems of planets beyond our own — that is, systems of exoplanets — this offers one of the most striking examples.
88 light-years away, five super-Earths, and at least one “mini-Neptune” orbit a Sun-like star. These five planets are locked in a rhythmic dance of gravitational harmony and have a probable sixth companion farther out. NASA reports the discovery of a solar system where six exoplanets orbit a Sun-like star in nearly perfect gravitational harmony. Five planets are believed to be super-Earths, and one is described as a mini-Neptune.
Locked in cosmic rhythmic harmony
The solar system is 88 light-years from Earth, and all the planets appear to be locked in cosmic rhythmic harmony, almost like a galactic waltz. Super-Earths are worlds with a mass higher than our planet but much lower than planets such as Neptune or Uranus. A mini-Neptune is a less massive planet than Neptune, although still resembling the gas giant in our solar system in composition and atmosphere. Each of the exoplanets of the star designated HD 158259 makes three orbits for every two orbits of the next planet located outwards.
The planets closely match the “3:2” resonance
Although such an orbital resonance has been observed in other solar systems, NASA reveals that this is one of the striking examples: the planets closely match the “3:2” resonance. The authors consider it “unlikely that the period ratios so close to 3: 2 come from mere randomness.” Therefore, they admit that at some point, the planets underwent a “migration in the protoplanetary disk, during which each consecutive pair of planets would fixate on this orbital resonance.”
The conjecture about forming such a compact and nearly perfect harmony system also includes the idea that the resonance itself, when perfect, could squeeze the orbits like the effect of the tide. The study’s first author, Nathan Hara of the University of Geneva, compares exoplanets from HD 158259 in a university statement to “several musicians each playing a different beat, but always coming together at the beginning of each bar.” Hara also notes that measurements made by NASA’s Exoplanet in Transit Survey Satellite (TESS) “allow us to estimate the radius of the planet and provide valuable information about its internal structure.”
SOPHIE spectrograph instrument
However, the SOPHIE spectrograph instrument was decisive in this observation at the Haute-Provence Observatory in France. SOPHIE measured all the system parameters, including eclipses, planetary masses, and the star’s radial velocity. “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 our signal processing techniques,” François Bouchy, another author of the study from UNIGE, said in a statement.
The solar system is compact
The study revealed that the solar system is compact, highlighted by the distance between the outermost exoplanet and HD 158259 is 2.6 times smaller compared to the distance between the sun and the planet Mercury in our star 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, the study’s co-author, revealed in the statement.