So far, we have discovered hundreds of stars with multiple orbiting planets spread across the entire galaxy. Each one is unique, but the system orbiting the star HD 158259, 88 light-years away, is truly special.

The star itself is roughly the same mass and slightly larger than the Sun – a minority in the hunt for the outer planets. Six planets orbit around it: Earth and five little Neptune.

After observing it for seven years, astronomers discovered that all six of these planets orbit HD 158259 in an almost perfect orbital resonance. This discovery could help us better understand the mechanisms of planetary system formation, and how they end up in the formations we see.

Orbital resonance occurs when the orbits of two objects around their body are closely related, as the two orbital bodies exert the effect of gravity on each other. In the solar system, it is very rare in planetary bodies; Perhaps the best example of this is Pluto and Neptune.

These two bodies lie in what has been described as the 2: 3 orbital resonance. For every two revolutions Pluto travels around the sun, Neptune makes three revolutions. It’s like cassettes of music playing simultaneously, but with different time signatures – two for the first, three for the second.

Its orbital resonance It has also been identified in exoplanets. But each planet orbits HD 158259 in an approximate 3: 2 resonance with the next planet further away from the star, which is also described as a period ratio of 1.5. This means that for every three orbits each planet makes, the next planet completes two.

Using the measurements taken using SOPHIE Spectrometer And the TESS Space Telescope to search for exoplanets, an international team of researchers led by astronomer Nathan Hara from the University of Geneva in Switzerland was able to accurately calculate the orbits of each planet.

They are all very narrow. Starting with the nearest star – the super-Earth, which TESS revealed to be twice the mass of the Earth – the orbits are 2.17, 3.4, 5.2, 7.9, 12, and 17.4 days.

These yield ratios of 1.57, 1.51, 1.53, 1.51 and 1.44 between each pair of planets. That’s not an entirely perfect echo – but it’s close enough to classify HD 158259 as an exceptional system.

The researchers believe this is a sign that planets orbiting the star have not formed in their current location.

„Many embedded systems with many planets at or near resonances are known, such as TRAPPIST-1 or Kepler-80,“ Astronomer Stefan Audrey explained From the University of Geneva.

„Such systems are thought to form far from the star before migrating towards it. In this scenario, resonances play an important role.“

That’s because this resonance is thought to result when the planetary embryos are in the protoplanetary disk They grow and migrate inward, Away from the outer edge of the disc. This results in a series of orbital resonances throughout the system.

Then, once the residual gas has dissipated from the disk, this can destabilize the orbital resonance – and this may be what we see with HD 158259. These small differences in orbital resonance can tell us more about how this disturbance occurred.

„The current departure from 3: 2 ratios contains a wealth of information,“ Hara said.

„With these values ​​on the one hand, and tidal effect models on the other hand, we can restrict the internal structure of planets in a future study. In short, the current state of the system gives us a window into their formation.“

The research has been published in Astronomy and Astrophysics.

A version of this article was first published in April 2020.