So far, we have discovered hundreds of stars with several planets orbiting them scattered across the galaxy. Each is unique, but a system that orbits the HD star 158259, 88 light-years away, is truly special.
The star itself is about the same mass and slightly larger than the Sun – a minority in our exoplanet hunts. It is orbited by six planets: a super-Earth and five mini-Neptunes.
After monitoring it for seven years, astronomers discovered that all six planets orbit HD 158259 in almost perfect orbital resonance. This discovery could help us better understand the mechanisms of formation of the planetary system and how they reach the configurations we see.
Orbital resonance is when the orbits of two bodies around their parent body are closely related, as the two orbiting bodies exert a gravitational influence on each other. In the solar system, it is quite rare in planetary bodies; probably the best example is Pluto and Neptune.
These two bodies are in what is described as a 2: 3 orbital resonance. For every two rounds Pluto makes around the Sun, Neptune makes three. It is as if the music bars are played simultaneously, but with different time signatures – two rhythms for the first, three for the second.
Orbital resonances have also been identified in exoplanets. But each planet orbiting HD 158259 is in a resonance of almost 3: 2 with the next planet outside the star, also described as a period ratio of 1.5. This means that for every three orbits that each planet makes, the next exit completes two.
Using measurements taken with the help of the SOPHIE spectrograph and the TESS exoplanet space telescope, an international team of researchers led by astronomer Nathan Hara of the University of Geneva in Switzerland was able to accurately calculate the orbits of each planet.
They are all very tight. Starting closest to the star – the super-Earth, revealed by TESS to be around twice the mass of Earth – the orbits are 2.17, 3.4, 5.2, 7.9, 12 and 17 ,4 days.
They produce period ratios of 1.57, 1.51, 1.53, 1.51 and 1.44 between each pair of planets. This is not a perfect resonance – but it is close enough to classify HD 158259 as an extraordinary system.
And this, the researchers believe, is a sign that the planets orbiting the star have not formed where they are now.
“Several compact systems with or without resonant planets are known, such as TRAPPIST-1 or Kepler-80,” said astronomer Stephane Udry of the University of Geneva.
“Such systems are thought to form far from the star before migrating to it. In this scenario, resonances play a crucial role.”
This is because these resonances are thought to result when the planetary embryos in the protoplanetary disk grow and migrate inward, away from the outer edge of the disk. This produces an orbital resonance chain throughout the system.
Then, once the remaining gas in the disk dissipates, this can destabilize the orbital resonances – and this could be what we see with HD 158259. And those small differences in orbital resonances could tell us more about how produces this destabilization.
“The current departure of the 3: 2 period ratio contains a lot of information,” Hara said.
“With these values on the one hand and tidal patterns on the other, we could constrain the internal structure of the planets in a future study. In short, the current state of the system gives us a window into its formation. “
The research was published in Astronomy and astrophysics.
A version of this article was first published in April 2020.