![]() He’s lead author of a recent paper about what his team called sub-Neptunes. Planetary scientist Renyu Hu at NASA’s Jet Propulsion Laboratory has expertise in modeling the physical and chemical processes that shape the atmospheres of planets and exoplanets. Can we list some similarities between super-Earths, mini-Neptune and sub-Neptunes? And can we list some differences? Each island-world is different … as different as snowflakes. It’s as though they’re explorers, standing on the shore of a vast ocean, with a billion uncharted islands before them. So astronomers themselves don’t always use these terms to mean the same things. That’s even though our Milky Way galaxy likely contains billions of exoplanets. ![]() It’s only been since the early 1990s that astronomers could see and study distant planets at all. The terms get swapped around, and sometimes used interchangeably, by different astronomers and astronomy writers. 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. That sounds simple enough, but nature isn’t always so neat and tidy. ![]() Some of the most commonly known exoplanets – worlds orbiting distant stars – are those larger than Earth, but smaller than Neptune. But what to call them? Super-Earth, mini-Neptune, sub-Neptune? All 3? Image via NASA/ Astronomy. So far, many known exoplanets fall between Earth and Neptune in terms of size and mass. And thus Neptune’s radius is only 4 times that of Earth. So Neptune’s self-gravity is stronger than Earth’s: the planet squeezes itself more tightly.
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