From Nola Taylor Redd at Space.com:
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.
Now, as for Mercury:
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.
But disk instability works better for giant planets, researchers say.
Meanwhile, Mercury continues to surprise:
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. … More.
Surprisingly enough, we’re not sure how the moon was formed either.
See also: Another moon origin theory: epic crash
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