By now we have discovered hundreds of stars with various planets orbiting them scattered throughout the galaxy. Each is unique, but a system that revolves around 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: one super-Earth and five mini-Neptunes.
After monitoring it for seven years, astronomers have discovered that the six planets orbit around HD 158259 in an almost perfect orbital resonance. This discovery could help us better understand the mechanisms of formation of the planetary system and how they end up in the configurations we see.
Orbital resonance is when the orbits of two bodies around the parent body are closely linked, as the two orbiting bodies exert a gravitational influence between them. In the solar system, it is quite rare in planetary bodies; probably the best example is Pluto and Neptune.
These two bodies are found in what is described as 2: 3 orbital resonance. For every two turns Pluto makes around the Sun, Neptune makes three. It’s like playing music bars simultaneously, but with different time signatures: two beats for the first and three for the second.
Orbital resonances have also been identified in exoplanets. But each planet orbiting HD 158259 has 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 exit completes two.
Using measurements made with the SOPHIE spectrograph and the TESS exoplanet hunting 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 closer to the star, the super-Earth, revealed by TESS which is 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 periods ratios of 1.57, 1.51, 1.53, 1.51 and 1.44 between each pair of planets. This isn’t a perfectly perfect resonance, but it’s close enough to classify the HD 158259 as an extraordinary system.
And that, the researchers believe, is a sign that the planets orbiting the star did not form where they are now.
“Several compact systems with several planets on or near resonances are known, such as TRAPPIST-1 or Kepler-80,” explained astronomer Stephane Udry of the University of Geneva.
“These systems are believed to form far from the star before migrating to it. In this scenario, resonances play a crucial role.”
This is because it is believed that these resonances will result when the planetary embryos of 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 from the disk dissipates, this can destabilize the orbital resonances, and this could be what we are seeing with HD 158259. And these small differences in orbital resonances could tell us more about how this destabilization occurs. .
“The current 3: 2 period ratio output contains a lot of information,” Hara said.
“With these values, on the one hand, and tidal models, on the other, we could limit the internal structure of the planets in a future study. In short, the current state of the system gives us a window on their training “.
The research has been published in Astronomy and astrophysics.
A version of this article was first published in April 2020.