What Size Is It?

Astronomers have been finding more and more planets outside our home system lately. I find that extremely exciting. But when I'm reading about these discoveries, I'm sometimes left wondering, just how big is this newly-discovered planet?

First, let's understand that because of the distances involved, and the methods currently in place for discovering planets, it takes a big planet to be noticed. I don't find that disappointing, because within our own system, we have 4 big planets (Jupiter, Saturn, Neptune and Uranus), an equal number of smaller planets, and a scattering of dwarf planets. So just because they haven't found lots of Mars- or Venus-sized planets circling other stars, doesn't mean they aren't there.

So, they are finding big planets, but they've developed a short of short-hand lingo to indicate a few tidbits of information just in what they call a new discovery. I recently read an article in Astronomy magazine that briefly explained what this lingo meant.

A Super-Earth sounds exciting, doesn't it? A planet like Earth! Well, up to a point. They are rocky planets like Earth, but remember that they are mostly finding big planets. A Super-Earth can range in size up to 10 times the Earth's mass. That means a lot of gravity, and I don't think we'd be colonizing that size a planet. But they are a rocky planet! Our system is not the only place where rocky planets exist. In fact, these may be the most common type of planet in our galaxy.

Then there are Mini-Neptunes. These ice giants range in size from 14.5 to 17 times the mass of Earth. (Neptune weighs in at 17 times Earth's mass.) These are also pretty common around other stars.

Which brings us to the 'Jupiter' class of planet, which come in 2 types.

A Hot Jupiter is a gas giant that orbits their star in under 10 days. (I can't name them, but I've heard of some that circle their star in a matter of hours.) A Hot Jupiter usually orbits at around 1/10 the distance between the Earth and the sun, or less. These are not common around stars like ours, occurring in only about 1% of systems having a star similar to ours.

A Cold Jupiter is a gas giant that—like our own Jupiter—lies beyond the ice line, where it's too cold for water to remain a liquid. There was no comment given about how common these might be. I assume Saturn is also a Cold Jupiter.

And then there are Brown Dwarfs, which is a type of star, not a planet. But they don't last long as a star, and then they cool off and they're just a big mass. It's not clear where the dividing line is between massive planets and dead brown dwarfs, but it seems to be somewhere around 10 times the mass of Jupiter.

Got that? Good. The next time you read about a newly-discovered planet, you can figure out a little something about it just by what they call it.