We have now discovered hundreds of stars with multiple planets orbiting around them, spread across the galaxy. Each one is unique, but a system orbiting 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 our exoplanet hunt. It revolves around six planets: a super Earth and five mini-Neptunes.
After seven years of tracking, astronomers have discovered that all six planets orbit HD 158259 in near-perfect orbital resonance. This discovery could help us better understand the mechanisms of planetary system formation and how they end up in the configurations we see.
Orbital resonance is when the orbits of two bodies around their mother body are closely connected, as the two orbiting bodies exert gravitational influences 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’s like bars of music played simultaneously, but with different time signatures: two beats for the first, three for the second.
Orbital resonances have also been identified in exoplanets. But every planet orbiting HD 158259 is in a resonance of nearly 3: 2 with the next planet away from the star, also described as a period ratio of 1.5. That means that for every three orbits each planet makes, the next one completes two.
Using measurements made using the SOPHIE spectrograph and the TESS space telescope that hunts exoplanets, 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, which, according to TESS, is about twice the mass of Earth – its 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. That’s not quite a 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 did not form where they are today.
“Several compact systems are known with different planets in or near resonances, such as TRAPPIST-1 or Kepler-80,” explains astronomer Stephane Udry from the University of Geneva.
“Such systems are believed to have formed far from the star before migrating to it. In this scenario, the resonances play a crucial role.”
That’s because these resonances are thought to arise when planetary embryos in the protoplanetary disk grow and migrate inward, away from the outer edge of the disk. This produces a chain of orbital resonance throughout the system.
Once the remaining gas of the disk has dissipated, it can destabilize the orbital resonances – and this could be what we see with HD 158259. And those small differences in the orbital resonances could tell us more about how this destabilization happens.
“The current departure from 3: 2 period ratios contains a wealth of information,” said Hara.
“With these values on the one hand and tidal effect models on the other, we could limit the internal structure of the planets in a future study. In summary, the current state of the system gives us a view of its formation.”
The research is published in Astronomy and Astrophysics.
A version of this article was first published in April 2020.