May 17, 2016
Jet Propulsion Laboratory, Pasadena, Calif.
NASA Ames Research Center, Moffett Field, Calif.
Written by Steve Koppes
University of Chicago
The Kepler-223 planetary system. Image credit: W. Rebel.
The four planets of the Kepler-223 star system appeared to have little in common with the planets of our own solar system today. But a new study using data from NASA’s Kepler space telescope suggests a possible commonality in the distant past. The Kepler-223 planets orbit their star in the same configuration that Jupiter, Saturn, Uranus and Neptune may have had in the early history of our solar system, before migrating to their current locations.
“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 the University of Chicago in Illinois. “Our work essentially tests a model for planet formation for a type of planet we don’t have in our solar system.”
Sean Mills (left) and Daniel Fabrycky (right), researchers at the University of Chicago, describe the complex orbital structure of the Kepler-223 system in a new study. Credits: Nancy Wong/University of Chicago
Mills and his collaborators used data from Kepler — its mission is now known as K2 — to analyze how the four planets block their stars’ light and change each other’s orbits. This information also gave researchers the planets’ sizes and masses. The team performed numerical simulations of planetary migration that generate this system’s current architecture, similar to the migration suspected for the solar system’s gas giants. These calculations are described* in the May 11 Advance Online edition of Nature.
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These animations show approximately 200,000 years of orbital evolution in the Kepler-223 planetary system. The planets’ interactions with the disk of gas and dust in which they formed caused their orbits to shrink toward their star over time at differing rates.
The orbital configuration of our own solar system seems to have evolved since its birth 4.6 billion years ago. The four known planets of the much older Kepler-223 system, however, have maintained a single orbital configuration for far longer.
Astronomers call the planets of Kepler-223 “sub-Neptunes.” They likely consist of a solid core and an envelope of gas, and they orbit their star in periods ranging from only seven to 19 days. They are the most common type of planets known in the galaxy, even though there is nothing quite like them around our sun.
Kepler-223’s planets also are in resonance, meaning their gravitational influence on each other creates a periodic relationship between their orbits. Planets are in resonance when, for example, every time one of them orbits its sun once, the next one goes around twice. Three of Jupiter’s largest moons, where the phenomenon was discovered, display resonances. Kepler-223 is the first time that four planets in an extrasolar system have been confirmed to be in resonance.
“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.
The Kepler-223 system provides alternative scenarios for how planets form and migrate in a planetary system that is different from our own, said study co-author Howard Isaacson, a research astronomer at the University of California, Berkeley, and member of the California Planet Search Team.
“Data from Kepler and the Keck Telescope were absolutely critical in this regard,” Isaacson said.
Thanks to observations of Kepler-223 and other exoplanetary systems, “We now know of systems that are unlike our sun’s solar system, with hot Jupiters, planets closer than Mercury or in between the size of Earth and Neptune, none of which we see in our solar system. Other types of planets are very common.”
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,” Mills explained.
That process differs completely from the one that scientists believe led to the formation of Mercury, Venus, Earth and Mars, which likely formed in their current orbital locations.
Earth formed from Mars-sized or moon-sized bodies smacking together, Mills said, in a violent and chaotic process. When planets form this way, their final orbital periods are not near a resonance.
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 (planetesimals).
“These resonances are extremely fragile,” Fabrycky said. “If bodies were flying around and hitting each other, then they would have dislodged the planets from the resonance.” But Kepler-223’s planets somehow managed to dodge this scattering of cosmic bodies.
A resonant chain of four transiting, sub-Neptune planets
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The Kepler Mission, NASA Discovery mission #10, is specifically designed to survey our region of the Milky Way galaxy to discover hundreds of Earth-size and smaller planets in or near the habitable zone→ and determine the fraction of the hundreds of billions of stars in our galaxy that might have such planets.
The operations phase of the Kepler mission is managed for NASA by the Ames Research Center, Moffett Field, CA. NASA’s Jet Propulsion Laboratory (JPL), Pasadena, CA, managed the mission through development, launch and the start of science operations. Dr. William Borucki of NASA Ames is the mission’s Science Principal Investigator. Ball Aerospace and Technologies Corp., Boulder, CO, developed the Kepler flight system.
In October 2009, oversight of the Kepler project was transferred from the Discovery Program at NASA’s Marshall Space Flight Center, Huntsville, AL, to the Exoplanet Exploration Program at JPL
Extending Kepler’s power to the ecliptic
The loss of a second of the four reaction wheels on board the Kepler spacecraft in May 2013 brought an end to Kepler’s four plus year science mission to continuously monitor more than 150,000 stars to search for transiting exoplanets. Developed over the months following this failure, the K2 mission represents a new concept for spacecraft operations that enables continued scientific observations with the Kepler space telescope. K2 became fully operational in June 2014 and is expected to continue operating until 2017 or 2018.