Neptune has about 17 times Earth’s mass. And mass creates gravity. So Neptune’s self-gravity is stronger than Earth’s: the planet squeezes itself more tightly. And thus Neptune’s radius is only 4 times that of Earth. So far, many known exoplanets fall between Earth and Neptune in terms of size and mass. But what to call them? Super-Earth, mini-Neptune, sub-Neptune? All 3? Image via NASA/ Astronomy.
Super-Earth, mini-Neptune or sub-Neptune
Some of the most commonly known exoplanets – worlds orbiting distant stars – are those larger than Earth, but smaller than Neptune. That sounds simple enough, but nature isn’t always so neat and tidy. Astronomers use a variety of terms – super-Earth, mini-Neptune and sub-Neptune – for worlds that fall between Earth and Neptune in size and/or mass. The terms get swapped around, and sometimes used interchangeably, by different astronomers and astronomy writers. What do these words mean exactly?
One caveat. Exoplanet research is new. It’s only been since the early 1990s that astronomers could see and study distant planets at all. That’s even though our Milky Way galaxy likely contains billions of exoplanets. So astronomers themselves don’t always use these terms to mean the same things. It’s as though they’re ...