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The closest planet to the Sun is Mercury. It’s the smallest planet in the solar system — a little ball of metal and rock that’s blasted by the Sun’s fury.
Many other star systems, though, have planets that are much closer in than Mercury is. And some of those worlds are mammoth — they can be many times bigger and heavier than Jupiter, the giant of the solar system.
Such worlds are known as hot Jupiters. They’re as little as a few million miles from their parent stars. At that range, such a planet’s upper atmosphere can be heated to hundreds or even thousands of degrees. And it’s generally locked so that one hemisphere of the planet always faces the star, just as one side of the Moon always faces Earth.
Hot Jupiters may form much farther out, then move closer to their stars. The heat and winds from a star should vaporize planet-making material that’s close in, and blow it far away. That wouldn’t leave enough gas and dust to make a giant world.
Some recent research, though, suggests that some hot Jupiters — and perhaps most of them — form right where they’re seen today. Such worlds would grow from massive “seeds” — big balls of rock and metal known as super-Earths. Such planets have been found in many star systems, and many of them are quite close to their stars. Once a super-Earth forms, it should quickly sweep up any leftover gas and dust, growing to gigantic proportions — making a hot Jupiter.