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