Astronomers find a system of six planets with near-perfect orbital harmony

(ORDO NEWS) — To date, we have discovered hundreds of stars with many orbiting planets scattered throughout the galaxy. Each is unique, but the system orbiting HD 158259, 88 light years away, is truly special.

The star itself is about the same mass and slightly larger than the Sun. Six planets revolve around it: a super-Earth and five mini-Neptunes.

After seven years of observing the system, astronomers have found that all six of these planets orbit HD 158259 in near-perfect orbital resonance. This discovery could help us better understand how planetary systems form and how they fit into the configurations we see.

Orbital resonance is when the orbits of two bodies around their parent body are closely related, as the two rotating bodies exert a gravitational influence on each other. In the solar system, it is quite rare in planetary bodies; probably the best examples are Pluto and Neptune.

These two bodies are in the so-called 2: 3 orbital resonance. For every two revolutions Pluto makes around the Sun, Neptune makes three. It is like musical measures played simultaneously, but with different timing: two beats for the first, three for the second.

Orbital resonances have also been found on exoplanets. But each planet orbiting HD 158259 is in almost 3: 2 resonance with the next planet further from the star, also described as a period ratio of 1.5. This means that for every three orbits each planet completes, the next one completes two.

Using measurements made with the SOPHIE spectrograph and the TESS space telescope to search for exoplanets, an international team of researchers led by astronomer Nathan Hare of the University of Geneva in Switzerland was able to accurately calculate the orbits of each planet.

Beginning with the closest super-Earth to the star, which TESS estimates is roughly twice the mass of Earth, the orbits are 2.17, 3.4, 5.2, 7.9, 12 and 17.4 days.

This gives the ratios of periods of 1.57, 1.51, 1.53, 1.51 and 1.44 between each pair of planets. It’s not exactly perfect resonance, but it’s close enough to classify the HD 158259 as an outstanding system.

And this, according to the researchers, is a sign that the planets orbiting the star did not form where they are now.

“There are several known compact systems with several planets in or near resonances, such as TRAPPIST-1 or Kepler-80,” explained astronomer Stephane Oudry of the University of Geneva.

“It is believed that such systems form far from the star before moving to it. In this scenario, resonances play a decisive role.”

Because it is believed that these resonances arise when planetary embryos in a protoplanetary disk grow and migrate inward, from the outer edge of the disk. This creates a chain of orbital resonance throughout the system.

Then, once the remaining gas in the disk dissipates, it can destabilize the orbital resonances – and this could be what we see with HD 158259. And these tiny differences in orbital resonances can tell us more about how this destabilization occurs.

“The current deviation of period ratios from 3: 2 contains a lot of information,” said Hara.

“With these values, on the one hand, and tidal models, on the other, we could limit the internal structure of planets in future exploration. Thus, the current state of the system gives us a window for its formation.”

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