What is the most common type of planet?

The most common planets we have found so far are not Earth-like planets, and they are not Jupiter-like planets either.

They are mostly mini-Neptunes and super-Earths.

Which is a little strange, because our own solar system has neither of them.

We have small rocky worlds like Mercury, Venus, Earth, and Mars. Then we have the gas and ice giants farther out. But we do not have anything in between: no planet bigger than Earth but smaller than Neptune.

And yet, around other stars, that in-between range seems to be everywhere.

A rocky terrestrial planet can be roughly Earth-sized or somewhat larger. Once you go beyond that, you enter the loose category of super- Earths: planets more massive than Earth, but still smaller than Uranus or Neptune. Some may be rocky. Some may be ocean worlds. Some may have thick atmospheres. The name “super-Earth” does not necessarily mean “Earth, but better.” It mostly means “bigger than Earth.”

Mini-Neptunes are a step above that. They are smaller than Neptune, but wrapped in thick atmospheres, often with large rocky or icy cores underneath. They may be worlds with deep oceans, dense envelopes of gas, or interiors where the distinction between gas, liquid, and supercritical fluid becomes blurry under high pressure.

Some mini-Neptunes may not remain mini-Neptunes forever.

If they orbit too close to their star, radiation can strip away their outer atmosphere. Over time, a puffy mini-Neptune can lose enough gas to become something more like a super-Earth. NASA has reported examples of mini-Neptunes apparently losing their atmospheres in exactly this way.

There is one important catch, though.

Our instruments are still biased.

We are much better at finding large planets close to their stars than tiny planets far away from them. Small Mars-sized worlds are much harder to detect. So while mini-Neptunes and super-Earths are the most common kinds of exoplanets we have found so far, it is still possible that smaller rocky planets are even more common in the universe and are simply harder for us to see.

So the answer depends on what we mean by “most common.”

Most common among the planets we have detected? Mini-Neptunes and super-Earths.

Most common in the universe overall? We are still not completely sure.

What does seem clear is that our solar system is not the standard template we once imagined. Other systems often contain these intermediate worlds, while ours has a strange gap between Earth and Neptune.

Maybe Jupiter played a role. Its gravity may have disrupted the formation of planets in that size range. Or maybe the early Solar System once had super-Earths or mini-Neptunes, and they were later swallowed, scattered, or ejected by the giant planets.

We do not know for sure.

Mini-Neptunes themselves could also be very different from one another. The one thing they usually have in common is a thick atmosphere. But beneath that atmosphere, one might hide a rocky surface. Another might have deep global oceans. Another might gradually shift from gas to liquid to exotic high-pressure phases without any clean surface at all.

Super-Earths are often more interesting when we talk about life.

A super-Earth with the right temperature, the right atmosphere, and liquid water could be a very good place for biology. Its stronger gravity might help it hold on to a thicker atmosphere. That could protect the surface and make flying or floating life easier, especially if the air is dense.

But stronger gravity would also change life.

If complex animals evolved there, they probably would not look like giraffes or elephants. Large, tall bodies would be harder to support. Creatures might be shorter, sturdier, and closer to the ground. More legs would make sense. Wide bodies would make sense. Flying organisms might exist too, especially in a thick atmosphere, but they would be shaped by a very different balance of lift, weight, and gravity.

There could even be floating life in the air: something like aeroplankton, tiny organisms drifting through dense alien skies.

Ocean super-Earths are another possibility, but they may not be as perfect for life as they sound.

A planet covered entirely by ocean could have liquid water everywhere, which seems good at first. But on Earth, land matters. Rivers carry minerals and nutrients from continents into the sea. Those nutrients feed ecosystems. Without land, erosion, rivers, and exposed rock, a global ocean planet might have trouble supporting large, complex biospheres. It might still support microbial life, but complex life could be harder. Recent work on water-rich rocky planets also stresses how important land, oceans, and nutrient cycles may be for long-term habitability.

So the universe may be full of planets unlike anything in our solar system.

Not quite Earth. Not quite Neptune. Something in between.

And strangely enough, those missing middle worlds may be the most common planets we know of.