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Two planets glide down the southwestern sky this evening. Bright orange Mars is low in the south-southwest as night falls, with fainter Saturn to the right by about the width of your fist held at arm’s length.
These worlds represent different categories of planets. Mars is a small and rocky planet, like Earth, while Saturn is a big ball of gas.
Many of the thousands of planets found in other star systems fall into discrete categories as well. There are super-Earths, mini-Neptunes, super-Neptunes, super-Jupiters, and hot Jupiters.
Super-Earths are up to a few times the mass of Earth. They’re still balls of rock and metal, but they may be much denser than Earth.
Neptune is the most-distant planet in the solar system, and consists of layers of rock, ice, and gas. Mini-Neptunes are smaller than Neptune, but they have the same basic structure. Super-Neptunes are also put together like our Neptune, but they’re up to a few times more massive.
Super-Jupiters are more massive than our Jupiter, the giant of the solar system. They probably have dense cores enveloped by deep layers of gas. They’re about the same size as Jupiter, but up to several dozen times its mass.
Hot Jupiters were some of the first planets ever discovered. Such a world is similar to Jupiter, but much closer to its star. That can heat its outer atmosphere to thousands of degrees — creating a category of planets unlike anything in our own solar system.
More about exoplanets tomorrow.