Unique star system with six planets in geometric formation

On Dec 3, 2023

Astronomers have discovered a rare star system in which six planets orbit around one star in an elaborate geometrical pattern due to a phenomenon called orbital resonance. Using both NASA’s Transiting Exoplanet Survey Satellite (TESS) and the European Space Agency’s (ESA) CHaracterising ExOPlanet Satellite (CHEOPS), the researchers have built up a picture of the beautiful, but complex HD110067 system, located 100 light-years away.

The six planets of the system orbit in a pattern whereby one planet completes three orbits while another does two, and one completes six orbits while another does one, and another does four orbits while another does three, and so one. The six planets form what is called a “resonant chain” where each is in resonance with the planets next to it.

A rare family of six exoplanets has been unlocked with the help of ESA’s Cheops mission. The planets in this family are all smaller than Neptune and revolve around their star HD110067 in a very precise waltz. When the closest planet to the star makes three full revolutions around it, the second one makes exactly two during the same time. This is called a 3:2 resonance. The six planets form a resonant chain in pairs of 3:2, 3:2, 3:2, 4:3, and 4:3, resulting in the closest planet completing six orbits while the outer-most planet does one. Cheops confirmed the orbital period of the third planet in the system, which was the key to unlocking the rhythm of the entire system. This is the second planetary system in orbital resonance that Cheops has helped reveal. The first one is called TOI-178. — A rare family of six exoplanets has been unlocked with the help of the European Space Agency’s CHEOPS mission. ESA, CC BY-SA 3.0 IGO

It is this chain of resonances that makes the system so unusual. “Amongst the over 5,000 exoplanets discovered orbiting other stars than our sun, resonances are not rare, nor are systems with several planets. What is extremely rare though, is to find systems where the resonances span such a long chain of six planets,” explained one of the researchers, Hugh Osborn of the University of Bern, in a statement.

The planets in this system are all of a type called sub-Neptunes, which are planets smaller than Neptune that are unlike any of the planets in our solar system, but are thought to be some of the most common exoplanets. Planets are thought to often form in resonance, due to the gravitational forces involved, however, this delicate balance is easily thrown out by perturbations such as a passing star or an impact from a large asteroid or comet.

Orbital geometry of HD110067: Tracing a link between two neighbour planets at regular time intervals along their orbits, creates a pattern unique to each couple. The six planets of the HD110067 system together create a mesmerising geometric pattern due to their resonance-chain. — Tracing a link between two neighbor planets at regular time intervals along their orbits, creates a pattern unique to each couple. The six planets of the HD110067 system together create a mesmerizing geometric pattern due to their resonance-chain. CC BY-NC-SA 4.0, Thibaut Roger/NCCR PlanetS

Researchers are keen to investigate systems like HD110067 because it can show what a system might look like if it does not experience any of these dramatic events.

“We think only about 1% of all systems stay in resonance,” said researcher Rafael Luque of the University of Chicago. “It shows us the pristine configuration of a planetary system that has survived untouched.”

The research is presented in the journal Nature.

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