Exoplanets' complex orbital structure points to planetary migration in solar systems

May 11, 2016, University of Chicago
This animation illustrates the Kepler-223 planetary system, which has long-term stability because its four planets interact gravitationally to keep the beat of a carefully choreographed dance as they orbit their host star. For example, each time the innermost planet (Kepler-223b) orbits the system's star 3 times, the second-closest planet (Kepler-223c) orbits precisely 4 times, and these two planets return to the same positions relative to each other and their host star. The orbital periods of the four planets of the Kepler-233 system have ratios of exactly 3 to 4, 4 to 6, and 6 to 8. The ratio of these orbits is so precise that they provide a stabilizing influence for the planetary system. Credit: W. Rebel Note to reporters: A license is granted for free worldwide public-domain use.

The four planets of the Kepler-223 star system seem to have little in common with the planets of Earth's own solar system. And yet a new study shows that the Kepler-223 system is trapped in an orbital configuration that Jupiter, Saturn, Uranus, and Neptune may have broken from in the early history of the solar system.

"Exactly how and where form is an outstanding question in ," said the study's lead author, Sean Mills, a in astronomy & astrophysics at the University of Chicago. "Our work essentially tests a model for planet formation for a type of planet we don't have in our solar system."

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," Mills said.

Mills and his collaborators used brightness data from NASA's 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 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.

The orbital configuration of the 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 one orbital configuration for far longer.

The planets of Kepler-223 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. Astronomers call these planets sub-Neptunes. They are the most common type of planets known in the galaxy.

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. Once two planets reach a resonant state (for example, one planet orbits its star three times every time the next planet orbits two times), the planets strongly interact with each other. The interactions become apparent in the animations as the orbits shrink (left and top right) when the orbital period ratios of neighboring planets (bottom right) get stuck at constant values. Even as the planets continue moving inwards (upper right) they do so in concert, migrating together locked in this configuration. They also cause each other's orbits to change from nearly circular to elliptical. This is represented by the varying orbits on the left panel and the spread of the orbital distances for each individual planet in the upper right. Credit: Daniel Fabrycky and Cezary Migazewski

Kepler-223's planets also are in resonance. Planets are in resonance when, for example, every time one of them orbits its sun once, the next one goes around twice. Jupiter's moons, where the phenomenon was discovered, display resonance.

Kepler-223's two innermost planets are in a 4:3 resonance. The second and third are in a 3:2 resonance. And the third and fourth are in a 4:3 resonance. Astronomers had seen extrasolar systems containing two or three planets in resonance, but not four.

"This is the most extreme example of this phenomenon," said study co-author Daniel Fabrycky, an assistant professor of astronomy & astrophysics at UChicago.

The arrangement and relative sizes of the four planets around Kepler-223, though not to scale. One AU (astronomical unit) is 93 million miles, the distance between Earth and sun in our solar system.

Formation scenarios

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 the 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."

Supercomputer simulation of the evolving orbits of four Neptune-size planets around Kepler-223. As they migrated inward toward the star, they got locked into synchronized orbits, a rare four-planet resonance. Simulations by Daniel Fabrycky and Cezary Migazewski, video by Stephen McNally and Roxanne Makasdjian, UC 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," Mills explained.

That process differs completely from the one that scientists 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, Mills said, a violent and chaotic process. When planets form this way their final orbital periods are not near a resonance.

Substantial movement

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).

The University of Chicago's Sean Mills (left) and Daniel Fabrycky describe the complex orbital structure of the Kepler-223 expolanetary system in the May 11, 2016 Advance Online edition of Nature. Credit: Nancy Wong

"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.

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 ," Fabrycky said.

Mills and Fabrycky's UC Berkeley collaborators were able to determine the size and mass of the star by making precise measurements of its light using the high resolution Echelle spectrometer on the 10-meter Keck I 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 six billion years old," UC Berkeley's Isaacson said. "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."

Explore further: Our sun may have eaten a super-Earth for breakfast

More information: Sean M. Mills et al, A resonant chain of four transiting, sub-Neptune planets, Nature (2016). DOI: 10.1038/nature17445

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5 / 5 (8) May 11, 2016
I tried to find out how many ly they are away but can't find it. Anyone know?
5 / 5 (9) May 11, 2016
4458 ly
4.3 / 5 (6) May 11, 2016
These four exoplanets are a system of periodic oscillators. Their orbits are periodic oscillations. Here, they have synchronized their oscillations so that for any given point in one system cycle, they will be at the same point in the next system cycle.

See Huygens' synchronized clocks, circa 1660. Or read "Coupled oscillators and biological synchronization," by Strogatz and Stewart, Scientific American, December 1993. Copy available free online.

Orbital resonance is just one of many examples of systems of coupled periodic oscillators. Or consider our moon: its orbit and spin (both of which are periodic oscillations) are coupled 1:1. Or the 3:2 spin-orbit coupling of Mercury.

Or consider the electron. The spin and orbit of an electron are coupled 2:1.

Coupling arrangements among systems of periodic oscillators are the glue that binds coherent systems.
5 / 5 (6) May 12, 2016
It is nice how other systems support the Nice types (resonant Jupiter/Saturn - at the very least - interaction) of our system development. [ https://en.wikipe...ce_model ]
5 / 5 (4) May 12, 2016
There appears to be an error in the caption of the first animation. The sentence reads
"For example, each time the innermost planet (Kepler-223b) orbits the system's star 3 times, the second-closest planet (Kepler-223c) orbits precisely 4 times, and these two planets return to the same positions relative to each other and their host star."
The information is reversed. The innermost planet orbits faster, so each time the innermost planet orbits the system's star 4 times, the second-closest planet orbits 3 times. You have the numbers flipped.
1 / 5 (11) May 13, 2016
No one ever observed planet migrations from one orbit to another in the same solar system.
So this idea remains in the scope of wishful thinking. Even there is no reason gravity to cause the rotation of matter far away from it center of gravity of solar system.
The reaosn is more simple as usual. The will of the Creator who have full control on the physical world and program its behavior according to HIs ideas and moral principles.
1 / 5 (11) May 13, 2016
All laws - the moral and physycal are created by intellinent beings with the idea to support established by them order. Physical or moral laws does not create order. Only supports already established order according to ideas, will and moral priciples of their creator.
2.3 / 5 (3) May 13, 2016
The 4 orbital patterns of the planets arise from two constraints. As a 4 oscillator system, they balance their mutual gravitational tugs on each other.

And they are a 5 oscillator system also, including their star at the center. Similar constraint there: balance the gravitational tugs of the 4 planets on the star, and,vice versa--the tugs of the star on the 4 planets. Given the incredible proximity of all 5 dance partners, gravity is very strong. So the choreography of all 5 oscillators must be precise--as it is.
3 / 5 (4) May 13, 2016
Starting with the innermost star, the orbits seem to be 8-6-4-3.

Why these numbers? Because the inner pair is in a 4:3 ratio as to each other. And so is the outer pair. In addition, the two pairs are cross linked in 2:1 patterns: 8 to 4 and 6:3 are both 2:1 ratios. These are the simplest integers (1,2,3,4) that will fulfill the goals I set forth above.

The numerical symmetries (2:1 twice and 4:3 internally for both pairs) speak volumes also.
2.3 / 5 (3) May 13, 2016
Compare this 4 way planet configuration to the 4-2-1 configuration of Jupiter's 3 nearest moons. These 3 Jupiter moons are also the nearest objects to Jupiter. They also use a 2:1 pattern (1 doubles to 2 and 2 doubles to 4).

The difference is that the 3 Jupiter moons are singlets, while the 4 exoplanets in the article are two pairs--doublets. The doublet system is probably a stronger structure.

Stable coupled oscillator systems come in various patterns, but the concept is simple.

All stable coupled oscillator patterns are akin to quantum forms: this level or that level will work, but nothing in between.
May 14, 2016
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May 14, 2016
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5 / 5 (4) May 14, 2016
No one ever observed planet migrations from one orbit to another in the same solar system.

Planetary migration observed in our own system:

"We find a pattern of excess depletion of asteroids, particularly just outward of the Kirkwood gaps associated with the 5:2, the 7:3 and the 2:1 Jovian resonances. These features are not accounted for by planetary perturbations in the current structure of the Solar System, but are consistent with dynamical ejection of asteroids by the sweeping of gravitational resonances during the migration of Jupiter and Saturn ~4 Gyr ago."


In other systems:

"Planetesimal-driven migration as an explanation for observations of high
levels of warm, exozodiacal dust"

[ http://www.ast.ca.../pdm.pdf ]

So, yes, we have seen that many times during one of our own orbits (or maybe two, depending on how fast they worked and published).

5 / 5 (4) May 14, 2016
For a faster migration, look at the Moon, migrating by tidal mechanisms from one orbit to the next, monthly.
1.7 / 5 (3) May 15, 2016
The full Nature article (of which this Physorg article is a summary) uses this 4 exoplanet system to support the "inward migration" theory of planet formation, versus the "in situ" theory of planet formation.

Kepler's third law arises from the observed orbital period distribution of the planets in our solar system. As we learned in high school, Kepler said "orbital period squared (power of 2) equals distance of planet from sun cubed (power of 3).

Think about that--the precise 3:2 ratio in the third law. Perhaps the simple integers of orbital resonance (of which this 4 exoplanet system is an example) are related to the simple integers in Kepler's third law. Deeply related--not just superficially. They both underpin systems of orbital order.

Please excuse my simplification of the distance concept in Kepler's third law.
5 / 5 (4) May 15, 2016
Well, maybe, maybe not. It sounds like numerology, and doesn't seem to predict the remaining resonances.
4.2 / 5 (5) May 15, 2016
Haaay guyzz Jazus did it!
3 / 5 (3) May 15, 2016
Good comment Torbjorn. Strong numerology flavor, yes. And the resonances require explanation.

If I were to offer a theory that might underlie and predict the "remaining resonances," I would go to Art Winfree's law of coupled oscillators, which he set forth circa 1968. He was a self-described bio-mathematician. The 1993 Scientific American article I mentioned above in my first post is the best exposition of Winfree's theory. The math is simple and well-vetted. Mother Nature embraces Winfree's law in biological systems. Why not physics as well?

Resonance--coupling--periodic oscillations--self-organizing systems. I'm just musing about a deep Kepler connection at this point. Leave it at numerology.

Side note about Kepler: I stopped by Tyco Brahe's tomb on a visit to Prague last month. Any discussion of Kepler should tip the hat to Brahe.

1 / 5 (4) May 16, 2016
My hero is Valentina Zharkova. http://phys.org/n...amo.html
May 17, 2016
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5 / 5 (2) May 18, 2016
Is there not some kind of internet mental institution to have you committed to?

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