By this time, we have discovered hundreds of stars with several planets orbiting the galaxy. Each one is unique, but a system that revolves around the star HD 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 hunt for the exoplanet. It orbits six planets: a super-Earth and five mini-Neptunes.
After observing it for seven years, astronomers have discovered that all six of the planets revolve around HD 158259 in an almost perfect orbital resonance. This discovery may help us to better understand the mechanisms of planetary systems formation and how they end up in the configurations we see.
Orbital resonance is when the orbits of two bodies around their parent body are closely linked to each other, as the two orbiting bodies exert 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 what is described as a 2: 3 orbital resonance. For every two rounds that Pluto makes around the sun, Neptune makes three. It’s like measures played at the same time, but with different time signature – two beats for the first, three for the second.
Orbital resonances have also been identified in exoplanets. But each planet orbiting HD 158259 is in an almost 3: 2 resonance 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 one completes two.
Using measurements taken using the SOPHIE spectrograph and the TESS space telescope for exoplanet hunting, 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 stiff. Starting closest to the star – the super-Earth, revealed by TESS to be about twice the mass of the Earth – the orbits are 2.17, 3.4, 5.2, 7.9, 12 and 17.4 days.
These produce period ratios of 1.57, 1.51, 1.53, 1.51 and 1.44 between each pair of planets. It’s not quite perfect resonance – but it’s 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 multiple planets in or near resonances are known, such as TRAPPIST-1 or Kepler-80,” explains astronomer Stephane Udry of the University of Geneva.
“It is believed that such systems form far from the star before migrating there. In this scenario, the resonances play an important role.”
This is because these resonances presumably arise when planetary embryos grow in the protoplanetary disk and migrate inward, away from the outer edge of the disk. It produces a chain of orbital resonance throughout the system.
Once the remaining gas disappears from the disk, it can destabilize the orbital resonances – and this may be what we see with HD 158259. And the small differences in the orbital resonances can tell us more about how this destabilization takes place.
“The current deviation from the 3: 2 period ratios contains a lot of information,” Hara said.
“With these values on the one hand and tidal effect models on the other hand, we can limit 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 research was published in Astronomy & Astrophysics.
A version of this article was first published in April 2020.