A star with six planets orbiting exactly in sync

Astronomers have discovered six planets orbiting a bright star in perfect resonance. The star system, located 100 light-years from Earth, was described on Wednesday in a book paper Published in Nature magazine.

The discovery of the system could give astronomers a unique opportunity to trace the evolution of these worlds back to when they first formed, and perhaps provide insight into how our solar system got to where it is today.

“It’s like looking at a fossil,” said Raphael Locke, the University of Chicago astronomer who led the study. “The orbits of the planets today are the same as they were a billion years ago.”

Researchers believe that when planets first form, their orbits around the star are in sync. This means that the time it takes for one planet to orbit its host star may be the same amount of time as it takes for another planet to orbit exactly twice, or exactly three times.

Systems that line up in this way are known as orbital resonances. But, despite this theory, finding echoes in the Milky Way is rare. Only 1% of planetary systems still maintain this symmetry.

Most often, planetary orbits fall out of sync due to an event that upsets the system’s gravitational balance. That could be a close encounter with another star, the formation of a massive planet like Jupiter, or a giant collision from space on one planet causing a ripple effect in other orbits. When this happens, planetary orbits become so chaotic that they cannot be described mathematically, and knowledge of their evolution cannot be deciphered, Dr. Locke said.

Astronomers are lucky to find just one pair of resonant exoplanets. But in the newly discovered star system, there are five super pairs, because all six planets have orbits synchronous with each other. Dr. Locke described it as “1% of 1%.”

To characterize this system, the team used data from 12 telescopes, including NASA’s Transiting Exoplanet Survey Satellite, which first observed the planets passing in front of their host star in 2020. Subsequent observations using the exoplanet characterization satellite have helped European Space Agency researchers. Know the relationships between orbits.

The innermost planet completes a full revolution every nine days. It makes three revolutions around its sun in the same time it takes the second planet to make exactly two revolutions. There is the same ratio between the cycles of the second and third planets in the system, in addition to the cycles of the third and fourth planets.

The last pair are related by a different ratio: it takes the outer planet four complete orbits around the inner planet to make exactly three orbits.

“Period ratios are measured beautifully and accurately from the data,” said Renu Malhotra, a planetary scientist at the University of Arizona who was not involved in the work. While the three inner planets have been unambiguously discovered, researchers have “done some really cool detective work” to identify and characterize the outer part of the system, she added.

Although orbital resonance is a rare discovery, the planets themselves — all larger than Earth, and smaller than Neptune — are some of the most common types in the galaxy. Because the host star is bright enough to be seen from telescopes on Earth, continuous monitoring of the system will be possible in the future.

With more data, astronomers can better determine the masses and sizes of planets, and even learn about the composition of their interiors and atmospheres, which differ from Earth’s. Dr. Malhotra said this knowledge may “expand our imagination about the conditions on planets that could harbor life.”

It may also shed light on the structure of our solar system, and the chaos that has moved its planetary orbits away from the harmonious equilibrium in which they are likely to be.

“Even in our solar system, these echoes do not seem to have survived,” Dr. Luckey said. By studying the untouched system, he added, “we can learn a lot about why the majority don’t.”