A four-planet system observed several years ago by the Kepler spacecraft is actually a rarity: Its planets, all miniature Neptunes nestled close to the star, are orbiting in a unique resonance that has been locked in for billions of years. For every three orbits of the outermost planet, the second orbits four times, the third six times and the innermost eight times.

Such orbital resonances are not uncommon - our own dwarf planet Pluto orbits the sun twice during the same period that Neptune completes three orbits - but a four-planet resonance is.

Astronomers from the University of Chicago and University of California, Berkeley, who are reporting the discovery online May 11 in Nature, are particularly interested in this stellar system because our system's four giant planets - Jupiter, Saturn, Neptune and Uranus - are thought to have once been in resonant orbits that were disrupted sometime during their 4.5-billion-year history.

According to co-author Howard Isaacson, a UC Berkeley research astronomer, the Kepler-223 star system can help us understand how our solar system and other stellar systems discovered in the past few decades formed. In particular, it could help resolve the question of whether planets stay in the same place they formed, or whether they move closer to or farther from their star over the eons.

"Basically, this system is so peculiar in the way that it's locked into resonances that it strongly suggests that migration is the method by which the planets formed - that is, migrating inward toward the star after forming farther out," he said.

NASA's Kepler mission has unearthed many alternative scenarios for how planets form and migrate in a planetary system that is different from our own.

"Before we discovered exoplanets, we thought that every system must form like ours," Isaacson said. "Thanks to Kepler, we now have hot Jupiters, many planets that are closer to their star than Mercury or in between the size of the Earth and Neptune. Without the discovery of exoplanets, we would not have known that the Earth is something of an outlier."

California Planet Search
As part of the California Planet Search team, Isaacson obtained a spectrum of Kepler-223 in 2012 using the high-resolution echelle spectrometer (HIRES) spectrometer on the Keck-1 10-meter telescope atop Mauna Kea in Hawaii. The spectrum revealed a star very similar in size and mass to the sun but much older - more than 6 billion years old.

"You need to know the precise size of the star so you can do the dynamical and stability analysis, which involve estimates of the masses of the planets," he said. "The Keck telescope is absolutely critical in this regard."

Sean Mills, a graduate student at the University of Chicago, and his collaborators then used brightness data from the Kepler telescope to analyze how the four planets block the starlight and change each other's orbits, thus inferring the planets' sizes and masses. The team performed numerical simulations of planetary migration that could have generated the system's current architecture.

"Exactly how and where planets form is an outstanding question in planetary science," Mills said. "Our work essentially tests a model for planet formation for a type of planet we don't have in our solar system."

The resonance could have been set up within a few 100,000 years, as each planet in turn migrated close enough to the others to get captured. The astronomers suspect there were special circumstances that allowed the resonance to persist for 6 million years.

"These resonances are extremely fragile," said co-author Daniel Fabrycky of the University of Chicago. "If bodies were flying around and hitting each other, then they would have dislodged the planets from the resonance."

Scientists suspect that our solar system's massive planets may have been knocked out of resonances that once resembled those of Kepler-223, possibly after interacting with numerous asteroids and small planets or planetesimals. Other processes, including tidal forces that flex the planets, might also cause resonance separation.

"Many of the multi-planet systems may start out in a chain of resonances like this, fragile as it is, meaning that those chains usually break on long timescales similar to those inferred for the solar system," Fabrycky said.

The work was funded by the National Aeronautics and Space Administration, Alfred P. Sloan Foundation, and Polish National Science Centre.

Research paper: A Resonant Chain of Four Transiting, Sub-Neptune Planets