Looking at the Solar System, there seem to be two basic types of planets.
The smaller planets, including Earth, are dense, lower mass, and composed of rock. The larger worlds—Jupiter and the other giants—are massive, made of compressed gas, and possess no surface to speak of. As we learn about exoplanets orbiting distant stars, those two basic categories seem to hold. However, as astronomers map the landscape of planets, they are discovering worlds that don’t fit what we once thought, and which suggest a richer galaxy of possibilities.
The most dramatic of these so far is a mega-Earth: a world about 17 times more massive than Earth. That’s approximately the same mass as the giant Neptune, yet this planet is denser than any other yet discovered, meaning it must be made of rock. That’s puzzling: based on the theory of planet formation, no rocky world should get that huge.
The same group of astronomers has also inferred a type of exoplanet that fits in between the rocky planets and the gas giants. These “gas dwarf” worlds seem to have rocky interiors, but thick atmospheres like Neptune’s. While this proposition is less head scratching than the mega-Earth, it fills in the gap between the largest rocky planets and the smallest gas giants. This type of planet may be fairly common, even though we have nothing in the Solar System like it.
Both the mega-Earth and the mini-Neptunes are examples of how exoplanets are helping us understand planets in general. As we are learning, complex processes in a star system as it begins determine what types of planets are born, where they form, and where they end up once things calm down.
Location in a star system matters. Kepler-10c, which is the proper name for the mega-Earth, orbits its star much closer than our planet does. Its year is about 45 days long, and its surface is certainly too hot for any liquid water, especially if it has an atmosphere containing any greenhouse gases. In no way is the mega-Earth much like home.
But that’s not what makes it puzzling.
Most of the atoms in a newborn star system are hydrogen, which is the lightest chemical element. Earth effectively has no hydrogen in its atmosphere: our planet’s low mass isn’t enough to keep any around over time. Jupiter and its cousins, by contrast, are mostly made of hydrogen and hydrogen compounds. The seeds for those planets were more massive, and the larger amount of raw materials let them grow huge. Since it’s as massive as Neptune, Kepler-10c could technically have kept a thick atmosphere of hydrogen compounds, but it doesn’t seem to have one. Whether that’s because it’s too close to its star or not is another question.
However, the “gas dwarf” planets would have the same rocky structure but keep their thick atmospheres. (It’s still an open question whether Jupiter and the like have rocky centers, but those cores aren’t large if they are present.) Based on this study, the authors of the suggest that rocky planets could grow much bigger than previously thought, as long as they form far out from their host stars. That’s a claim we can’t test yet, because our methods are best suited for detecting exoplanets with small orbits.
I’m using weasel words deliberately: unlike Kepler-10c, we don’t have a lot of direct data about mini-Neptunes yet. That’s because the two main methods we have to hunt exoplanets measure different properties. The technique used by the Kepler observatory can find the size of the planet by the amount of light it blocks from the host star, but not the planet’s mass. Measuring masses requires determining how much the planet’s gravity swings the star around on each orbit, which is biased toward very heavy planets.
For that reason, we know more about the sizes of exoplanets than their masses. Kepler-10c was found to be roughly 2.4 times the diameter of Earth by the amount of light it blocked, which is smaller than Neptune. Astronomers used careful follow-up observations nearly ten years later to measure its mass using the way it affects its host star, but that won’t be possible for most exoplanets: they are either too low-mass or orbit too far to have much of an effect. (In the Solar System, only Jupiter is massive enough to produce a noticeable effect on the Sun, so it’s likely the only planet any bug-eyed alien astronomers would detect.)
For that reason, it will be difficult to confirm the presence of mini-Neptunes in most cases. But that’s the way exoplanet research has gone so far. Twenty-five years ago, we only knew about the Solar System (though I doubt many seriously thought those were the only planets in the Universe). Based on that limited data, it made sense to think that rocky planets were small and orbited close to their host stars, while giant planets were gaseous and orbited farther out. Now after thousands of exoplanet discoveries, we know there are more possible planet types than we expected, from super-Earths to giant planets orbiting their stars at roastingly close distances.
In science, imagination isn’t the limiting factor to discovery. Instead, we’re limited by the evidence: what kinds of things we can learn from experiment and observation. In the case of the mega-Earth and the mini-Neptunes, the evidence shows that galaxy of exoplanets is more wondrous than we expected. Every discovery brings us closer to understanding how planets form in all their marvelous variety.