So far, we have discovered hundreds of stars of several planets that scattered them throughout the galaxy. Each one is unique, but a system orbiting the Star HD 158259 88 light-years away is truly special.
The star itself is the same mass and slightly larger than the Sun – a minority in our exoplanet prey. It is orbited by six planets: one Super-Earth and five Mini-Neptunes.
After monitoring it for seven years, astronomers discovered that all six of those planets orbited HD 158259 in an almost perfect orbital resonance. This discovery can help us better understand the mechanisms that make up planetary systems, and how they end up in the configurations we see.
Orbital resonance occurs when the two orbiting orbits around their parent body are closely connected, as the two orbiting bodies have a gravitational effect on each other. In the solar system, it is quite rare among planetary bodies; Perhaps the best example is Pluto and Neptune.
These two bodies are described as 2: 3 orbital resonances. For every two laps Pluto builds around the Sun, Neptune makes three. It is played together like bars of music, but with different time signatures – two beats for the first, three for the second.
Orbital resonance has also been identified in exoplanets. But each planet orbiting the HD planet 158259 is in about 3: 2 resonance, far away from the star of the next planet, which is also described as a ratio of 1.5 in duration. This means that for each planet’s three orbits, the next one meets the outside two.
Using measurements using the SOPHIE spectrograph and the TESS Exoplanet-Hunting Space Telescope, an international team of researchers led by astronomer Nathan Hara of the University of Geneva in Switzerland were able to accurately calculate the orbits of each planet.
They are all very tight. Starting from the closest to the star – Super-Earth, detected by TESS to be about twice the mass of Earth – the orbits are 2.17, 3.4, 5.2, 7.9, 12 and 17.4 days.
These produce duration ratios of 1.57, 1.51, 1.53, 1.51, and 1.44 between each pair of planets. This is not exactly the right resonance – but it is enough to classify the HD 158259 as an exceptional system.
And this, the researchers believe, is a sign that the planets orbiting the planet did not form where they are now.
Astronomer Stephen Udieri of the University of Geneva explained, “Many compact systems with multiple planets, with or near them, resonance are known as TRAPPIST-1 or Kepler-80.”
Such systems are believed to be built far from the star before migrating to it. In this scenario, resonance plays an important role.
This is because the result of these resonances is thought to occur when planetary embryos grow in the protoplanetary disk and migrate away from the outer edge of the disk. This produces a series of orbital resonances throughout the system.
Then, once the remaining gas of the disc expands, it can destroy the orbital resonance – and this may be what we are seeing with HD 158259. And the small differences in orbital resonances can tell us more about where this instability is occurring.
“The current departure from 3: 2 to period ratio carries a wealth of information,” Hara said.
“With these values on the one hand, and the tidal impact model on the other, we can constrain the internal structure of the planets in future studies. In summary, the current state of the system gives us a window on its formation.”
The research has been published in Astronomy and Astrophysics.
A version of this article was first published in April 2020.