Chapter 4: Moving Around

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Based on just an image of baby planets in a protoplanetary disk, it is impossible to determine what the system will look like as it matures.

"Is disk destiny? Probably not," said Heather Knutson, professor at Caltech in Pasadena. "Growing planets don't stay put. They move around. They interact with each other, and with the gas disk, in a way that can be quite random and messy."

The tail end of the formation of the solar system, between about 50 million and 500 million years after the Sun formed, was particularly turbulent for our solar system. This was when the giant planets of the outer solar system reorganized themselves. Based on careful study of the distribution of bodies in the asteroid belt between Mars and Jupiter and the Kuiper Belt beyond Neptune, we now know there must have been a period when the orbital positions of Jupiter and Saturn led to a tug-of-war. This shuffling may have scattered Uranus and Neptune out to their current orbits and bombarded Earth with debris.

"The late phases of planet formation were a really gradual process as planets slowly shifted in their orbits," said Avi Mandell, researcher at NASA's Goddard Space Flight Center. "We believe those small changes may have eventually led to a huge event of scattering between planets."

But what exactly caused it? A hotly debated question in planetary science is how a special alignment, or "resonance," between the orbits of Jupiter and Saturn could have changed their positions, as well as those of Uranus and Neptune.

We still have a few groups of small objects that were likely flung around as Jupiter and Saturn tumbled through their youth. The asteroid belt between Mars and Jupiter contains small rocky "planet wannabes" that never got to reach their potential because of interference from Jupiter’s gravity, Knutson said. The Kuiper Belt, which contains Pluto, was probably shaped by this process of planetary shuffling, too, especially because of Neptune’s scattering. There’s a whole other outpost even farther afield: the Oort Cloud, a sparse but vast sphere of comets circling the Sun. The Oort Cloud begins roughly 200 billion miles (300 billion kilometers) away from the Sun, and may extend as far as 9 trillion miles (10 trillion kilometers) from our star.

Scientists are looking at other star systems at various ages to search for evidence of similar processes happening elsewhere.

"By combining the archaeology of our own solar system with the formation of planets in other systems, we can create a holistic picture of how we came to have the planetary architecture we see today," Mandell said.

After the gas of the disk has been grabbed up by rocky planets or dissipated into the vastness of space, the planets don’t sit still. In such a young system, planets or planetary building blocks called "protoplanets" may be orbiting on collision courses with each other, and the absence of gas makes moving around easier than ever.

"Once you get rid of the disk, then you have the potential for the planets to interact with each other, rather than with the disk, and that can cause things to change," said Jang-Condell, the astronomer at the University of Wyoming.

Big planets may kick smaller bodies into the star or out of the system entirely. This is one possible explanation for large, lonely bodies called "rogue planets" that have been found without stars. The recent mysterious visitor to our own solar system, a small comet named 'Oumuamua, is thought to have been ejected from a different star system.

One revelation that made scientists rethink their ideas about a planet’s life story is that our galaxy has many "hot Jupiters." These are planets found in other star systems, not our own, that are about the same size as Jupiter or larger but orbit much closer to their host stars -- worlds where one "year" can be less than two days.

In 2017, scientists announced the discovery of KELT-9b, a planet hotter than most stars, where temperatures can reach 7,800 degrees Fahrenheit. It was discovered using one of the two ground-based telescopes called KELT, or Kilodegree Extremely Little Telescope. By 2018, evidence had emerged that the star’s gravity is actually ripping the planet apart.

KELT-9b and other hot Jupiters could not have formed so close to their stars, or else their building blocks would have been ripped apart long ago. Only in colder, farther regions of a protoplanetary disk could such immense planets take shape. These gas giants must have migrated toward the star only after reaching their immense sizes.

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But nothing stays young forever -- every planetary system eventually hits middle age, just like our own. Read on to Chapter 5 to find out more.

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