Although planets surround stars in the galaxy, how they form remains a subject of debate. Despite the wealth of worlds in our own solar system, scientists still aren't certain how planets are built. Currently, two theories are duking it out for the role of champion.
The first and most widely accepted, core accretion, works well with the formation of the terrestrial planets but has problems with giant planets such as Uranus. The second, the disk instability method, may account for the creation of giant planets.
"What separates the ice giants from the gas giants is their formation history: during core growth, the former never exceeded [critical mass] in a full gas disk," wrote researchers Renata Frelikh and Ruth Murray-Clay in a research paper.
The core accretion model
Approximately 4.6 billion years ago, the solar system was a cloud of dust and gas known as a solar nebula. Gravity collapsed the material in on itself as it began to spin, forming the sun in the center of the nebula.
With the rise of the sun, the remaining material began to clump together. Small particles drew together, bound by the force of gravity, into larger particles. The solar wind swept away lighter elements, such as hydrogen and helium, from the closer regions, leaving only heavy, rocky materials to create terrestrial worlds. But farther away, the solar winds had less impact on lighter elements, allowing them to coalesce into gas giants such as Uranus. In this way, asteroids, comets, planets, and moons were created.
Unlike most gas giants, Uranus has a core that is rocky rather than gaseous. The core likely formed first, and then gathered up the hydrogen, helium and methane that make up the planet's atmosphere. Heat from the core drives Uranus' temperature and weather, overpowering the heat coming from the distant sun, which is almost 2 billion miles away.
Some exoplanet observations seem to confirm core accretion as the dominant formation process. Stars with more "metals" — a term astronomers use for elements other than hydrogen and helium — in their cores have more giant planets than their metal-poor cousins. According to NASA, core accretion suggests that small, rocky worlds should be more common than the more massive gas giants.
The 2005 discovery of a giant planet with a massive core orbiting the sun-like star HD 149026 is an example of an exoplanet that helped strengthen the case for core accretion.
"This is a confirmation of the core accretion theory for planet formation and evidence that planets of this kind should exist in abundance," said Greg Henry in a press release. Henry, an astronomer at Tennessee State University, Nashville, detected the dimming of the star.
In 2017, the European Space Agency plans to launch the CHaracterising ExOPlanet Satellite (CHEOPS), which will study exoplanets ranging in sizes from super-Earths to Neptune. Studying these distant worlds may help determine how planets in the solar system formed.
"In the core accretion scenario, the core of a planet must reach a critical mass before it is able to accrete gas in a runaway fashion," said the CHEOPS team. "This critical mass depends upon many physical variables, among the most important of which is the rate of planetesimals accretion."
By studying how growing planets accrete material, CHEOPS will provide insight into how worlds grow.
The disk instability model
But the need for a rapid formation for the giant gas planets is one of the problems of core accretion. According to models, the process takes several million years, longer than the light gases were available in the early solar system. At the same time, the core accretion model faces a migration issue, as the baby planets are likely to spiral into the sun in a short amount of time.
"Giant planets form really fast, in a few million years," Kevin Walsh, a researcher at the Southwest Research Institute in Boulder, Colorado, told Space.com. "That creates a time limit because the gas disk around the sun only lasts 4 to 5 million years."
According to a relatively new theory, disk instability, clumps of dust and gas are bound together early in the life of the solar system. Over time, these clumps slowly compact into a giant planet. These planets can form faster than their core accretion rivals, sometimes in as little as a thousand years, allowing them to trap the rapidly-vanishing lighter gases. They also quickly reach an orbit-stabilizing mass that keeps them from death-marching into the sun.
As scientists continue to study planets inside of the solar system, as well as around other stars, they will better understand how Uranus and its siblings formed.
The biggest challenge to core accretion is time — building massive gas giants fast enough to grab the lighter components of their atmosphere. Recent research on how smaller, pebble-sized objects fused together to build giant planets up to 1000 times faster than earlier studies.
"This is the first model that we know about that you start out with a pretty simple structure for the solar nebula from which planets form, and end up with the giant-planet system that we see," study lead author Harold Levison, an astronomer at the Southwest Research Institute (SwRI) in Colorado, told Space.com in 2015.
In 2012, researchers Michiel Lambrechts and Anders Johansen from Lund University in Sweden proposed that tiny pebbles, once written off, held the key to rapidly building giant planets.
"They showed that the leftover pebbles from this formation process, which previously were thought to be unimportant, could actually be a huge solution to the planet-forming problem," Levison said.
Levison and his team built on that research to model more precisely how the tiny pebbles could form planets seen in the galaxy today. While previous simulations, both large and medium-sized objects consumed their pebble-sized cousins at a relatively constant rate, Levison's simulations suggest that the larger objects acted more like bullies, snatching away pebbles from the mid-sized masses to grow at a far faster rate.
"The larger objects now tend to scatter the smaller ones more than the smaller ones scatter them back, so the smaller ones end up getting scattered out of the pebble disk," study co-author Katherine Kretke, also from SwRI, told Space.com. "The bigger guy basically bullies the smaller one so they can eat all the pebbles themselves, and they can continue to grow up to form the cores of the giant planets."
Pebble accretion is more likely to work for the giant planets than the terrestrial worlds. According to Sean Raymond, of France's University of Bordeaux, that's because the "pebbles" are a little larger and much easier to hold onto past the snow line, the imaginary line where gas is cold enough to become ice.
"For pebbles, it's definitely a little better to be just past the snow line," Raymond told Space.com.
While pebble accretion works well for the gas giants, there are some challenges for the ice giants. That's because the millimeter- to centimeter-sized particles accrete extremely efficiently.
"They accrete so quickly that it is difficult for ice giant cores to exist at roughly their current core masses for a significant fraction of the disk lifetime while accreting a gas envelope," Frelikh and Murray-Clay wrote.
"To avoid runaway, they must therefore complete their growth at a specific time, when the gas disk is partially, but not entirely, depleted."
The pair proposed that the majority of gas accretion onto the cores of Uranus and Neptune coincided with their movement away from the sun. But what could make them change their home in the solar system?
A Nice model
Originally, scientists thought that planets formed in the same part of the solar system they live in today. The discovery of exoplanets shook things up, revealing that at least some of the most massive objects could migrate.
In 2005, a trio of papers published in the journal Nature proposed that Uranus and the other giant planets were bound in near-circular orbits much more compact than they are today. A large disk of rocks and ices surrounded them, stretching out to about 35 times the Earth-sun distance, just beyond Neptune's present orbit. They called this the Nice model, after the city in France where they first discussed it. (That's pronounced Neese.)
As the planets interacted with the smaller bodies, they scattered most of them toward the sun. The process caused them to trade energy with the objects, sending the Saturn, Neptune and Uranus farther out into the solar system. Eventually the small objects reached Jupiter, which sent them flying to the edge of the solar system or completely out of it.
Movement between Jupiter and Saturn drove Uranus and Neptune into even more eccentric orbits, sending the pair through the remaining disk of ices. Some of the material was flung inward, where it crashed into the terrestrial planets during the Late Heavy Bombardment. Other material was hurled outward, creating the Kuiper Belt.
As they moved slowly outward, Neptune and Uranus traded places. Eventually, interactions with the remaining debris caused the pair to settle into more circular paths as they reached their current distance from the sun.
Along the way, it's possible that one or even two other giant planets were kicked out of the system. Astronomer David Nesvorny of the Southwest Research Institute in Colorado has modeled the early solar system in search of clues that could lead to understanding its early history.
"In the early days, the solar system was very different, with many more planets, perhaps as massive as Neptune, forming and being scattered to different places," Nesvorny told Space.com.
A dangerous youth
The early solar system was a time of violent collisions, and Uranus was not exempt. While the surface of the moon and Mercury both show evidence of bombardment by smaller rocks and asteroids, Uranus apparently suffered a significant collision with an Earth-size protoplanet. As a result, Uranus is tipped on its side, with one pole pointing toward the sun for half the year.
Uranus is the largest of the ice giants, perhaps in part because it lost some of its mass during the impact.