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 theory, core accretion, works well with the formation of the terrestrial planets like Mercury but has problems with giant planets. The second, the disk instability method, may account for the creation of these giant planets. 

Scientists are continuing to study planets in and out of the solar system in an effort to better understand which of these methods is most accurate.

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 up. Small particles drew together, bound by the force of gravity, into larger particles. The solar wind swept away lighter elements from the closer regions, leaving only heavy, rocky materials to create smaller terrestrial worlds like Mercury. But farther away, the solar winds had less impact on lighter elements, allowing them to coalesce into gas giants. In this way, asteroids, comets, planets, and moons were created.

Like Earth, the metallic core of Mercury formed first, and then gathered lighter elements around it to form its crust and mantle. Mercury, like other planets, likely collected the more nebulous pieces that would form its atmosphere. Unlike its siblings, however, the planet's small mass (Mercury is the smallest of the planets) and close proximity to the sun kept it from keeping a firm hold on the gases. Interactions with the solar wind constantly strip the planet of its thin atmosphere, even as it provides an influx.

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.

Although the core accretion model works fine for terrestrial planets, gas giants would have needed to evolve rapidly to grab hold of the significant mass of lighter gases they contain. But simulations have not been able to account for this rapid formation. 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.

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.

According to exoplanetary astronomer Paul Wilson, if disk instability dominates the formation of planets, it should produce a wide number of worlds at large orders. The four giant planets orbiting at significant distances around the star HD 9799 provides observational evidence for disk instability. Fomalhaut b, an exoplanet with a 2,000-year orbit around its star, could also be an example of a world formed through disk instability, though the planet could also have been ejected due to interactions with its neighbors.

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, which were 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."

This Messenger photo of Mercury shows wrinkle ridges around a network of troughs that formed when the volcanic plains were stretched apart. The wrinkle-ridge ring, about 100 km in diameter, is formed over the rim of a so-called ghost crater.
This Messenger photo of Mercury shows wrinkle ridges around a network of troughs that formed when the volcanic plains were stretched apart. The wrinkle-ridge ring, about 100 km in diameter, is formed over the rim of a so-called ghost crater.
Credit: NASA/The Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/Smithsonian Institution

Studies of Mercury reveal that its core is significantly more massive than expected in relation to the rest of the planet. With a radius of between 1,100 to 1,200 miles (1,800 to 1,900 kilometers), the mostly-iron core stretches through 75 percent of the planet's diameter and makes up a significant amount of its volume. The crust, on the other hand, is only 300 to 400 miles (500 to 600 km) thick. 

After Mariner 10 made three flybys of Mercury in the 1970s, its strange composition, including the bulky iron core, led to a wealth of theories about how the planet may have formed. One suggested that, if a larger Mercury formed quickly enough, it could have consolidated before the sun reached its peak. Elevated temperatures from the young star could have then cooked away much of the light crust, leaving only a small shell around the planet.

When NASA's MESSENGER mission observed Mercury, however, it studied the surface composition. It found that ratios of thorium to potassium were similar to other terrestrial planets. While thorium is a stable element, potassium is more volatile, easily baked away by higher temperatures. MESSENGER's findings suggest that the planet was not subjected to extreme heating or early evolution but formed much like other terrestrial worlds.

Instead, Mercury most likely suffered a violent event early in its life. Scientists theorize that the original planet, more massive and thicker crusted, could easily have been struck by a large body in the violent early solar system. Such a collision would have blown much of its crust into space, leaving behind a massive core enclosed by only a thin shell.

Collisions would have been frequent in the early solar system. One recent theory suggests that Mercury may have been the "last man standing." According to planetary scientist Kathryn Volk, ten at the University of British Columbus, several baby planets could have once orbited near the sun, but a series of collisions destroyed all but Mercury. 

"In a highly destructive regime, we're left with one survivor," Volk told Astrobiology magazine.

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