Astronomers Discover Rare Solar System Where Planets Orbit in Mathematical Harmony

The “resonant” planets could provide insight about how such systems form and evolve—and why our own solar system is not synced up

An illustration of different colored planets orbiting a star
For the inner four planets in this solar system, each planet orbits the sun three times for every two orbits of the planet immediately to its outside. For the fourth, fifth and sixth planets, they orbit four times for every three orbits of its outer neighbor. Thibaut Roger / NCCR PlanetS, CC BY-NC-SA 4.0

In a solar system 100 light years away, six planets are locked into a precise dance. The amount of time it takes each one to orbit the system’s sun forms a neat ratio with the orbits of neighboring planets—a trait that’s rare in outer space, researchers reported last week in the journal Nature.

While solar systems typically start out with their planets’ orbits having this type of relationship, called resonance, only about one percent of systems stay this way, according to a statement from the European Space Agency (ESA). The orbital periods of planets in our solar system, for example, are not so aligned, and the new research could provide insight into why that is, writes the Guardian’s Nicola Davis.

“It’s like looking at a fossil,” Rafael Luque, a co-author of the study and astronomer at the University of Chicago, says of the distant planetary system to the New York Times’ Katrina Miller. “The orbits of the planets today are the same as they were a billion years ago.”

Most planetary systems, however, get thrown out of these synced relationships over time—whether by a massive collision, the formation of a very large planet or a close pass by another star. Studying resonant orbits can help researchers understand how the systems formed and evolved, according to the ESA.

The solar system in the new study is located in the constellation Coma Berenices, and its star has a radius and mass about 80 percent that of our sun. The planets’ radii measure between 1.94 and 2.85 times that of Earth, making them between the size of Earth and Neptune. Researchers studied the system using data from NASA’s Transiting Exoplanet Survey Satellite and the ESA’s Characterizing Exoplanet Satellite.

But what really intrigued the team was the orbits: The first planet, for starters, orbits the sun almost exactly three times as its neighbor, the second from the star, orbits twice. They found the same relationship between the second and third planet.

infographic shows six planets, the first four planets in orbits with a 3:2 ratio and the outer ones in 4:3 ratio orbits
Six exoplanets orbit star HD110067 in precise ratios. ESA under CC BY-SA 3.0 IGO

The team then uncovered three more planets by predicting the relationships of their orbits and matching these predictions to the collected data. Even these bodies moved around the star in balanced ratios: The third planet orbits three times for every two orbits of the fourth planet, and the fourth and fifth planets orbit four times for every three orbits of the fifth and sixth planets, respectively.

“I was kind of shocked and delighted,” Hugh Osborn, a co-author of the study and an astrophysicist at the University of Bern in Switzerland, says to the Guardian.

“The period ratios are measured exquisitely, precisely, by the data,” Renu Malhotra, a planetary scientist at the University of Arizona who was not involved in the research, tells the New York Times.

All six planets have orbits between nine and 55 days, which are short periods compared to those in our solar system. (Mercury, the closest planet to our sun, completes an orbit every 88 days.) As a result, the newfound planets are located close to their star and are very hot—between 330 and 980 degrees Fahrenheit.

The researchers examined the masses of three of the planets, and their low densities suggest they have atmospheres with lots of hydrogen. They could also have rocky cores, according to Science’s Daniel Clery.

The findings could shed light on where resonant systems form and how they survive. Additionally, the discovery may help scientists better understand exoplanets between the size of Earth and Neptune. Planets of this size closely orbit more than half of all stars like our sun, according to the paper, but they remain mysterious to astronomers.

“Even though we have found so many planets like these ones outside of the solar system, we do not know much about them,” Luque tells the Guardian.

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