by Ludwig Scheibe (TU Berlin), October 2024
Looking at all the planets of the Solar System arranged next to each other to scale, you can clearly see three categories: There’s the small rocky planets, with Earth being the biggest of them, the medium-sized Uranus and Neptune – about 4 times as large and 15 times as heavy as Earth – and the big gas giants Jupiter and Saturn. In our proverbial backyard, we do not see any planet with a size between Earth and Neptune. But looking beyond our own system, to the thousands of exoplanets discovered so far, we find exactly that. And what’s more: planets with a size between Earth and Neptune are even the class of which we have discovered the most by far.
Artist’s conception of CoRoT-7 b, the first super-Earth discovered – and the first rocky exoplanet at the same time. Credit: ESO/L. Calçada under CC by 4.0
A broad range
While we can assume that a small planet is mostly made of rock, and that Jupiter-sized planets are practically always gas giants, medium-sized planets – so super-Earths and Neptune-sized – vary in their composition over a large range. Super-Earths can be big rock spheres, similar to Earth only larger and perhaps with a denser atmosphere. An example would be CoRoT-7 b pictured above. Or they have a large amount of water and methane like Neptune, or are even small gas planets. Depending on your source, sometimes only the denser rocky planets are called “super-Earth”, while more gas- and ice-dominated worlds use the name “mini-Neptunes”. Unfortunately, there is no completely consistent definition of these terms.
Possible interior compositions of a super-Earth exoplanet. Left: rocky planet with an extended outer envelope of hydrogen and helium. Right: rock core, extended water mantle and thinner atmosphere. Credit: ©P. Baumeister
For many super-Earths, we do not even know for sure, what they are made of. The only physical quantities we usually have available for that, is their mean density: how much mass do they have at a certain volume, so at a certain size. Is it very low, then we can assume the planet is made of light materials like hydrogen and helium. Conversely, if the density is high, then the planet contains mostly heavy substances like rock or iron. The problem comes in if the density is not extremely high or low. Is that the result of a large rock core with an extended hydrogen atmosphere – a heavy and a light material together – or is the planet mostly made of water – a medium-heavy substance? This problem is still unsolved and many scientists are working on it, by incorporating extra information where available, like atmospheric measurements, studies of planet formation, or even measurements of the exact form of the planet.
The search for life
Artist’s impression of mini-Neptune K2-18b, which might be an ocean-world with dense hydrogen atmospjere and where first tentative indications of a biosignature where found. Credit: ©ESA/Hubble, M. Kornmesser
The smaller a planet is, the more difficult it is to find and to characterize. By planets’ standards, Earth is a rather small world. Super-Earths on the other hand, even those that mare made of rock and are thus most similar to Earth, can sometimes be twice as big as Earth. That’s why they are ideal targets for studies about whether life is possible on other worlds. Because for some Super-Earth’s, we already managed to measure the atmosphere, while this is still in its infancy for terrestrial planets. An example of these kinds of measurements is K2-18b, a planet that has about 2.5 times the diameter of Earth and orbits its star in the habitable zone. Already in 2019, water was detected in its atmosphere, and a study published in 2023 shows clear signs of methane and CO2, as well as at least tentative indications for a molecule called dimethyl sulfide, which is only produced by biological processes on Earth (Madhusudhan et al. 2023).
The fact that super-Earths are so common, even though we do not have one in our Solar System, coupled with their wide range of possible compositions and the fact that they could be similar enough to Earth to potentially host life as we know it, makes this planet class important targets for exoplanet research in the immediate future.