Astronomers discovered a beautiful 6-planet system in almost perfect orbital harmony

So far, we have found hundreds of stars with multiple planets scattered throughout the galaxy. Each one is unique, but 88 light-years away, the HD 158259, the star-orbiting system is truly special.

The star itself is about 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-Neptune.

After observing it for seven years, astronomers have discovered that all six of these planets revolve around HD 158259 in almost full orbit. This discovery can help us understand the construction methods of the planetary system and how they end up in the configurations we look for.

Orbital echoes occur when the orbits of two bodies around their parent body are closely connected, as the two orbiting bodies exert a gravitational influence on each other. In the solar system, it is very rare in the body of planets; Perhaps the best examples are Pluto and Neptune.

These two bodies are what are described as 2: 3 orbital echoes. For every two hairs formed around Pluto the Sun, Neptune makes three. It’s like a bar of music playing together, but with different time signatures – two beats for the first, three for the second.

Orbital resonance is also identified in the exoplanet. But H.D. Each planet orbiting around 158259 is about 3: 2 with the planet farther away from the star, which is also described as a period ratio of 1.5. This means that for every three orbits created by each planet, the next one completes two.

An international team of researchers led by Nathan Hara, an astronomer from the University of Geneva in Switzerland, has determined the orbit of each planet, using measurements taken using the Sophie Spectrograph and the TCS Explanate-Hunting Space Telescope.

They are all very tight. The closest start to the star – the super-Earth, has been revealed by TSS to be twice the mass of Earth – orbits are 2.17, 3.4, 5.2, 7.9, 12 and 17.4 days.

Each pair of these planets produces periods of 1.57, 1.51, 1.53, 1.51 and 1.44. It’s not quite the perfect echo – but it’s close enough to classify the HD 158259 as an exceptional system.

And this, the researchers believe, is a sign that the planets orbiting the star are no longer where they used to be.

Stephen Udry, an astronomer at the University of Geneva, explained, “Some compact systems with or near many planets, such as Trappist-1 or Kepler-800, are known. “

“Such systems are thought to form far away from the star before migrating to it. In this case, the echo will play a crucial role.”

This is because the result of these echoes is considered to be when the planetary embryo grows in this prosoplanetary disc and migrates inwards away from the outer edge of the disc. This produces a chain of orbital echoes throughout the system.

Then, once the rest of the disc’s gas is dissipated, this can destabilize the orbital echoes – and this is what we’re seeing with the HD 158259. And those small differences in orbit can tell us more about how this instability occurs.

“The current departure from the current period ratio 3: 2 contains a lot of information,” Hara said.

“With these values ​​on the one hand, and tidal effect patterns on the other, we can limit the internal formation of planets in future studies. In summary, the current state of the system gives us a window into its formation.”

Research has been published in Astronomy and Astrophysics.

A version of this article was first published in April 2020.