A study of the older Kepler-223 system offers clues into how Kepler-223’s planets stayed in resonance for so long, and takes us a step forward in understanding the role giant planets played in the formation of our system.
The origin and evolution of our Solar System has for long been the subject of debate, with new theories cropping up every few decades attempting to make sense of how the planets came to be and in their current positions around the Sun. Some four billion years ago, perhaps more, the thinking is that the planets were nothing but clumps of rock, metal and ice left over from the collapse of a giant cloud of gas swirling around the Sun. These planetesimals, through frequent interactions with other clumps of matter, gradually grew to become planets.
However, a simple explanation like this does not answer several questions about the current state of our stellar neighbourhood. For one: because Mars formed much farther out than Venus and Earth, it had more raw materials at its disposal during formation. So it should by all means be larger than Venus and Earth – but it isn’t. Why is this the case? For others: how did the Kuiper Belt beyond Neptune’s orbit, which holds millions of icy bodies and is the source of comets, form? How did the sphere of bodies that surrounds us even beyond the Kuiper Belt, called the Oort Cloud, form? And how does the asteroid belt contain both rocky objects and icy objects? These mysteries had us stumped for many years before a satisfactory answer was hit upon, called planetary migration.
As the planets were forming, Jupiter was located much closer to the Sun than where it is now. Since there were still lots of swirling gases around the star, Jupiter got caught in these currents and started moving closer to the Sun. While on this journey, it gobbled up a lot of the material that should have gone to Mars instead, stunting Mars’s growth. As the gas giant reached the point where Mars is today (Mars wasn’t there then), it started getting pulled back by the newly powerful Saturn, with which Jupiter developed a resonance. As a result, Jupiter got dragged all the way back to where it sits today. This region used to contain icy bodies that Jupiter finally flung inward, toward and into the asteroid belt.
What’s more, Jupiter’s and Saturn’s dance caused enough disturbance to fling the then-next planet, Neptune, out into icy regions beyond Uranus, sending it crashing through a mess of icy objects and reversing the locations of Uranus and Neptune. These icy objects were thrown inward, and upon coming in contact with Jupiter’s gravitation, were flung back outward in random directions, thus forming the Kuiper Belt and the Oort Cloud.
We see evidence of this kind of planetary formation and migration in other, extrasolar systems. Most systems seem to have giant gaseous planets much larger than Earth orbiting much closer to their host star. Indeed, we see so much of this around us that we were led to conclude that our Solar System was an ‘oddball’, and that the only explanation for the way planets exist today are models that explain planetary migration. However, we’ve never discovered exoplanetary systems that have more than two planets orbiting in resonance – until we discovered the Kepler 223 star system.
An example of planetary resonance goes like this: for every one orbit a planet makes, another makes two. Resonances are found everywhere we’ve looked in the universe. Jupiter’s three largest moons are in a 1:2:4 resonance; Earth and Venus are in an 8:13 resonance; and Pluto and Neptune are in a (famous) 2:3 resonance. As it happens, resonances are fragile and are susceptible to being distorted by interactions with planetesimals. The four planets found orbiting closer to Kepler-223 than Mercury is to the Sun does are in an 8:6:4:3 resonance. This system resembles our own Solar System at a time when the four outer planets were in resonance with each other.
Astronomers suggest that planetary migration is a vital clue to answer this question. The resonance could have come into existence in less than a hundred thousand years, as one planet migrated close enough to another to capture it in resonance, and both of them then migrating towards the third, and so on. There were probably unique circumstances that allowed the resonance to persist for 6 billion years, while the Solar System’s massive planets may have been knocked out of resonances after interacting with asteroids, planetesimals, and other icy bodies.
A study of the much older Kepler-223 system could offer clues into how Kepler-223’s planets stayed in resonance for so long. It also would take us a big step forward in understanding the role giant planets played in the formation of our system.
Sandhya Ramesh is a science writer focusing on astronomy and earth science.