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Still confused as to whether the Moon is a star or a planet?
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I can fix that.
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Hey guys, Amy here to talk space on DNews.
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There has been a lot of talk recently about dwarf planets vs. “regular” planets, comets,
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asteroids… not to mention some questions what what exactly a natural satellite is.
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Well, let's break down what exactly is kicking around in our Solar System.
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At the centre is our Sun, which is a star.
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A star is a luminous ball of gas, mostly hydrogen and helium, that is held together by its own gravity
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Nuclear fusion in its core produces photons, the light particles we see, as well as heat and trace amounts of other heavy elements.
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But not all stars are the same.
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Stars can be different colours depending on their temperature, hotter stars looking slightly
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bluer while cooler stars look slightly more red.
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And stars are typically different sizes, the cooler stars being smaller.
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As stars evolve and age, depending on their mass, they expand and eventually puff up as
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red giants and rip themselves to shreds (that's what happens to stars about the mass of our Sun)
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and turn into a white dwarf or, if they are really massive, they explode as supernovas
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before turning into neutron stars or black holes.
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Then there are brown dwarfs, which are sometimes called failed stars because they just aren't
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big enough to start the fusion reaction that makes a star bright.
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Brown dwarfs also have planet-like qualities, making them a kind of bridge between stars and planets.
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Orbiting all kinds of stars are the planets, but for a body to be classified as a planet
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it has to meet three additional criteria laid out by the International Astronomical Union:
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It has to be large enough to be round, it cannot be a satellite of another body,
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and it must have cleared its orbit of all nearby debris.
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This all basically puts a minimum size limit on planets, but as long and they're big
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enough planets can come in different types and sizes.
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Earth is a rocky, terrestrial planet, as are Mercury, Venus, and Mars.
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The outer planets Jupiter, Saturn, Uranus, and Neptune, are called gas giants, though those last two are sometimes called ice giants as well.
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When we talk about planets around other stars, we have slightly different descriptors.
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There are super Earths, which aren't exactly what they sound like.
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These are planets more massive than Earth but still far less massive than Uranus or
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Neptune, which are 15 and 17 times as massive as the Earth respectively.
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On the other end of the size spectrum are dwarf planets, planets that are too small
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to have cleared their orbital path of debris, like Ceres, the dwarf planet we've talked about before, in the asteroid belt.
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Pluto is a dwarf planet as well, though in recognition of everyone's favourite little body,
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the IAU recently designated all dwarf planets that orbit beyond Neptune as plutoids.
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But regardless of size, type, or distance from its host star, any planet can have a moon.
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A moon is a natural satellite, a body that makes an orbit around a planet.
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Moons can have water like Europa, can have thick atmospheres like Titan, or be, for all
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intents and purposes, a dead world like our own Moon.
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As long as it's orbiting a planet, it's a moon.
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Of course, there are more subcategories and intricacies in individual bodies, but those are sort of the basics.
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Does that clear things up?
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If you have any more questions on celestial bodies, let us know in the comments below or you can ask me on Twitter as @astVintageSpace.
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And don't forget to subscribe for more DNews every day of the week.