A planetary system with an almost perfect orbital “harmony” found

To date, astronomers have discovered hundreds of planetary systems scattered throughout the galaxy. Each of them is unique, but the system that revolves around the star HD 158259 88 light years from us can be called especially unusual.

The mass of HD 158259 is comparable to that of the sun and slightly larger than the sun in diameter. The planet closest to the star is super-earth with a mass of about twice that of the earth and a radius of 1.2 earth. The rest of the celestial bodies are heavier than the Earth about six times and belong to the class of mini-neptunes.

After seven years of observing the system, astronomers discovered that all six planets revolve around their star in an almost perfect orbital resonance. This discovery can help us better understand the mechanisms of formation of planetary systems and how they appear in the configurations that we see.

Orbital resonance in celestial mechanics is a phenomenon when the orbits of two bodies around a parent body are closely connected, since both objects exert a gravitational effect on each other. So, in the solar system at orbital resonance 3: 2 are Neptune and Pluto. This means that for every two circles Pluto makes around the Sun, Neptune makes two. It resembles musical measures performed simultaneously, but with different time signatures – two beats for the first and three for the second.

The researchers found that in the HD 158259 system, all the planets are as close as possible to the 3: 2 orbital resonance, which can also be described as a ratio of periods of 1.5. Using measurements made using the SOPHIE spectrograph and the TESS space 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.

All of them are located compactly: even the outermost of the six exoplanets of the system is 2.6 times closer to the star than Mercury to the Sun. These planets make a complete revolution around HD 158259 for 2.7, 3.4, 5.2, 7.9, 12, and 17.4 Earth days, respectively.

Therefore, the ratio of the periods for each pair of planets is equal to 1.57; 1.51; 1.53; 1.51 and 1.44. This is not quite an ideal resonance, but it is close enough to classify HD 158259 as an extraordinary system.

It is believed that planets in resonance form at a relatively large distance from the star. Probably the HD 158259 system was once the same, but later became compact.

“There are several compact systems with several planets in resonance or near them, for example, TRAPPIST-1 or Kepler-80. It is believed that such systems are formed far from the star before migrating to it. In this scenario, resonances play a decisive role, ”said astronomer Stefan Udry of the University of Geneva.

This is because these resonances are thought to occur when protoplanets (planetary embryos) in a protoplanetary disk grow and migrate inward, away from the outer edge of the disk. This creates a chain of orbital resonance throughout the system. Then, when the remaining disk gas is scattered, it can destabilize orbital resonances, as in the example with HD 158259. These tiny differences in orbital resonances can tell us more about how such destabilization occurs.

“The current deviation of period ratios from 3: 2 contains a huge amount of information. With these values, on the one hand, and models of tidal effects, on the other hand, we could find out the internal structure of the planets in future studies. Thus, the current state of the system opens a window for us at the time of its formation, ”- Nathan Hara.

The study was published in the journal Astronomy & Astrophysics.

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