A star with six planets that rotate in perfect synchronization
Astronomers have discovered six planets orbiting a bright star in perfect resonance. The star system, 100 light-years from Earth, was described Wednesday 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 back to when they first formed, potentially offering insight into how our solar system came to be the way it is today.
“It’s like looking at a fossil,” says Rafael Luque, an astronomer at the University of Chicago who led the study. “The orbits of today’s planets are the same as they were a billion years ago.”
Researchers think that when planets first form, their orbits around a star are synchronized. That is, the time it takes for a planet to orbit its parent star can be as long as the time it takes for a second planet to orbit exactly two or exactly three times.
Systems that align in this way are known as orbital resonances. But despite the theory, finding resonances in the Milky Way is rare. Only 1 percent of planetary systems still maintain this symmetry.
Usually, the orbits of planets become out of sync due to 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 gigantic impact from space on one planet that causes a ripple effect in other orbits. When this happens, said Dr. Luque, the orbits of planets become too chaotic to describe mathematically, and the knowledge of their evolution is indecipherable.
Astronomers are lucky to find even a few exoplanets in resonance. But in the newly discovered galaxy there are as many as five pairs, because all six planets have orbits that are in sync with each other. Dr. Luque described it as “the 1 percent of the 1 percent.”
The innermost planet completes a full orbit every nine days. It makes three revolutions around its sun in the same amount of time it takes the second planet to make exactly two. The same relationship exists between the periods of the second and third planets in the system, together with the third and fourth.
The last two pairs are linked by a different ratio: it takes four complete orbits of the inner planet for the outer planet to make exactly three.
“The period ratios are measured excellently 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 inner three planets were unequivocally detected, the researchers did “really great detective work” to identify and characterize the outer part of the system, she added.
Although orbital resonance is a rare find, the planets themselves – all larger than Earth and smaller than Neptune – are among the most common types in the Milky Way. And because the host star is bright enough to be visible from ground-based telescopes, continuous monitoring of the system will be possible in the future.
With more data, astronomers can better determine the masses and sizes of the planets, and even learn about the composition of their interiors and atmospheres, which differ from Earth’s. This knowledge could “expand our imagination about conditions on planets that could potentially harbor life,” said Dr. Malhotra.
It could also shed light on the architecture of our solar system, and on the chaos that pulled the planetary orbits away from the harmonious equilibrium in which they most likely formed.
“Even in our solar system, these resonances don’t seem to have survived,” said Dr. Luke. By studying a system that has gone untouched, he added, “we can learn so much about why the majority didn’t.”