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A star with six planets that orbit perfectly in sync

In a diagram provided by Thibaut Roger/NCCR Planets, six planets discovered by astronomers, with their orbits locked in sync with one another. The star system, 100 light-years from Earth, was described on Wednesday, Nov. 29, 2023 in a paper published in the journal Nature.

By Katrina Miller

Astronomers have discovered six planets orbiting a bright star in perfect resonance. The star system, 100 light-years from Earth, was described late last month in a paper published in the journal Nature.

The discovery of the system could give astronomers a unique opportunity to trace the evolution of these worlds to when they first formed, and potentially offer insights into how our solar system got to be the way it is today.

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

Researchers think that when planets first form, their orbits around a star are in sync. That is, the time it takes for one planet to waltz around its host star might be the same amount of time it takes for a second planet to circle exactly twice, or exactly three times.

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

Most of the time, planetary orbits get knocked out of sync by an event that upsets the gravitational balance of the system. That could be a close encounter with another star, the formation of a massive planet like Jupiter, or a giant impact from space on one planet that causes a ripple effect in other orbits. When this happens, Luque said, planetary orbits become too chaotic to mathematically describe, and knowledge of their evolution is indecipherable.

Astronomers are lucky to find even one pair of exoplanets in resonance. But in the newly discovered star system, there are a whopping five pairs, because all six planets have orbits that are in resonance with their neighbors. Luque described it as “the 1% of the 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. Follow-up observations with the European Space Agency’s Characterizing Exoplanet Satellite helped the researchers figure out the relationships between the orbits.

The innermost planet completes one full orbit every nine days. It makes three revolutions around its sun in the same amount of time it takes for the second planet to make exactly two. The same ratio exists between the periods of the second and third planets in the system, along with the third and the fourth.

The final two pairs are related by a different ratio: It takes four full orbits of the inner planet for the outer planet to make exactly three.

“The period ratios are measured exquisitely, precisely, by the data,” said Renu Malhotra, a planetary scientist at the University of Arizona who was not involved in the work. While the inner three planets were detected unambiguously, the researchers “did some really awesome detective work” to identify and characterize the outer part of the system, she added.

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

With more data, astronomers can better nail down the masses and sizes of the planets, and even learn about the composition of their interiors and atmospheres, which are different from Earth. This knowledge might “expand our imagination about conditions on planets that could potentially harbor life,” Malhotra said.

It may also shed light on the architecture of our solar system and the chaos that moved its planetary orbits away from the harmonious balance they most likely formed in.

“Even in our solar system, these resonances do not appear to have survived,” Luque said. By studying a system left untouched, he added, “we can learn so much about why the majority didn’t.”

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