The Kepler-223 multi-planet system is trapped in an orbital configuration that the Solar System’s four gas giants may have broken from in its early history, according to a team of scientists led by University of Chicago astronomer Sean Mills.
This artist’s concept shows a multi-planet system. Image credit: NASA / JPL-Caltech.
“Exactly how and where planets form is an outstanding question in planetary science. Our work essentially tests a model for planet formation for a type of planet we don’t have in our Solar System,” Mills said.
“These puffy, gaseous planets, far more massive than Earth, orbit close to their stars. That’s why there’s a big debate about how they form, how they got there, and why don’t we have one.”
The astronomers used brightness data from NASA’s Kepler Space Telescope to analyze how the four planets in the Kepler-223 system — Kepler-223 b, c, d and e — block the starlight and change each other’s orbits, thus inferring the planets’ sizes and masses.
They performed numerical simulations of planetary migration that generate this system’s current architecture, similar to the migration suspected for the Solar System’s gaseous planets. These calculations are described in a paper published online today in the journal Nature.
The orbital configuration of the Solar System seems to have evolved since its birth 4.6 billion years ago.
Kepler-223’s planets, however, have maintained one orbital configuration for far longer.
The planets of Kepler-223 are so-called sub-Neptunes – they are much larger than Earth, likely consisting of a solid core and an envelope of gas, and they orbit their star in periods ranging from only seven to 19 days. The planets also are in resonance.
According to astronomers, planets are in resonance when, for example, every time one of them orbits its Sun once, the next one goes around twice.
Kepler-223 b and c – the system’s two innermost planets – are in a 4:3 resonance. Kepler-223 c and d are in a 3:2 resonance. And Kepler-223 d and e are in a 4:3 resonance.
Astronomers had seen extrasolar systems containing two or three planets in resonance, but not four.
The Kepler-223 planetary system. Image credit: W. Rebel.
“The Kepler-223 system provides alternative scenarios for how planets form and migrate in a planetary system that is different from our own, “said co-author Dr. Howard Isaacson, from the University of California, Berkeley.
“Some stages of planet formation can involve violent processes, but during other stages, planets can evolve from gaseous disks in a smooth, gentle way, which is probably what the sub-Neptune planets of Kepler-223 did,” Mills said.
“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.”
That process differs completely from the one that astronomers believe led to the formation of Earth, Mercury, Venus, and Mars, which likely formed in their current orbital locations.
“Earth formed from Mars- or Moon-sized bodies smacking together, a violent and chaotic process. When planets form this way their final orbital periods are not near a resonance,” Mills said.
But scientists suspect that the Solar System’s larger, more distant planets of today — Jupiter, Saturn, Uranus, and Neptune — moved around substantially during their formation.
They may have been knocked out of resonances that once resembled those of Kepler-223, possibly after interacting with numerous asteroids and small planets.
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Sean M. Mills et al. A resonant chain of four transiting, sub-Neptune planets. Nature, published online May 11, 2016; doi: 10.1038/nature17445
