4 Mini-Neptunes Have Been Locked in a Fragile Orbit for Over 6 Billion Years

May 13, 2016 | Joanne Kennell

Artist's impression of Saturn and Uranus
Photo credit: pixabay.com

These synchronized worlds may reveal clues to how planets form.

A four-planet system observed several years ago by the Kepler spacecraft seems to have little in common with the planets in Earth’s own solar system. However, a new study published in the online edition of the journal Nature has shown that the system, called Kepler-223, is actually trapped in an orbit that Jupiter, Saturn, Uranus, and Neptune may have escaped from in the early history of the solar system.

All four of the planets are considered miniature Neptunes — puffy, gaseous planets far more massive than Earth — that orbit closer to their host star than Mercury is to the sun. What’s more, their orbits are in a precise pattern that has been locked in for billions of years, making the system incredibly rare.

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Why is it so unusual? Each planet is orbiting in a unique resonance: for every three orbits of the outermost planet, the second orbits four times, the third six times, and the innermost eight times.

"This is the most extreme example of this phenomenon," said study co-author Daniel Fabrycky, an assistant professor of astronomy and astrophysics at the University of Chicago (UC), in a UC news release.

Currently, there is a debate about how planets that orbit so close to their stars form, how they got there, and why our solar system doesn’t have one. Kepler-223 may help astronomers understand how our solar system, and others, materialized.

"Exactly how and where planets form is an outstanding question in planetary science," said the study’s lead author, Sean Mills, a graduate student in astronomy and astrophysics at UC, in the release. "Our work essentially tests a model for planet formation for a type of planet we don’t have in our solar system."

Using brightness data from NASA’s Kepler telescope, Mills and his collaborators performed numerical simulations of planetary migrations that could generate this type of system.

It turns out that the orbital configuration of our solar system continued to evolve since its birth 4.6 billion years ago, but the much older Kepler-233 system has maintained one orbital configuration for much longer  — it’s star, which is similar in size and mass to the sun, is over 6 billion years old, and the planets likely entered their arrangements within the first few 100,000 years.

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Planet formation happens in one of two ways: violently, through the collision of large bodies — how Earth, Mercury, Venus, and Mars formed — or gently evolving through gaseous disks, which is likely how the planets of Kepler-223 were created.

"We think that two planets migrate through this disk, get stuck and then keep migrating together; find a third planet, get stuck, migrate together; find a fourth planet and get stuck," Mills explained.

However, scientists are pretty sure that Jupiter, Saturn, Uranus, and Neptune moved around quite a bit during their formation. They may have been knocked out of resonances that once resembled Kepler-223 after interacting with asteroids and planetesimals (small planets), or because of tidal forces.

"These resonances are extremely fragile," Fabrycky said. "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."

But amazingly, Kepler-223 has managed to avoid other cosmic bodies, allowing the planets to keep their unique orbits.

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