Our solar system is home to three different types of planets: terrestrial, gaseous, and icy. However, these do not represent every type of planet. In fact, our solar system is unique in that it does not contain one of the most common types of planets in our galaxy: super-Earths. As their name suggests, super-Earths are rocky worlds whose mass exceeds that of Earth. More specifically, super-Earths are planets that have a mass between 1.5 and 10 Earths.
The first super-Earth discovered was also the first planet found outside our solar system. In 1992, a team of astronomers detected two planets orbiting a distant pulsar. Analysis of their masses showed that both were about four times the mass of Earth, making them too small to be gas giants. However, since they are in orbit around a pulsar, they are not defined as true exoplanets, as an exoplanet is defined as a planet that orbits a main sequence star. The first super-Earth discovered around a main-sequence star was Gliese 876d in 2005. Two years after the discovery of Gliese 876d, astronomers confirmed the existence of a super-Earth located in the habitable zone of a red dwarf called Gliese 581.
In 2009, NASA launched the Kepler Space Telescope. Kepler’s primary goal was to search for and study exoplanets. To find exoplanets, Kepler looks for any changes in a star’s brightness that may be caused by a planet passing in front of the star. Kepler remained in operation until 2018 and in its nine years of operation fundamentally changed the science of exoplanets. During his lifetime, Kepler confirmed the existence of more than 2,000 individual planets. Just two years into its mission, Kepler data revealed the existence of nearly 300 super-Earths, including the first Kepler super-Earth found in the habitable zone. Called Kepler-22b, it is 2.5 times the size of Earth and orbits a star similar to our sun.
It turns out that super-Earths are one of the most common types of planets in our galaxy. Data from NASA shows that there are a total of 1,582 confirmed super-Earths orbiting other stars. As of September 1, 2022, there are a total of 5,157 confirmed exoplanets, making super-Earths just over 30% of confirmed exoplanets. Additionally, most of the rocky planets that are in the habitable zones of other stars are also super-Earths.
Habitability of super-earths
Since super-Earths make up the majority of potentially habitable planets, it makes sense to wonder if planets larger than Earth could still be habitable. The potential habitability of a super-Earth will depend on how massive it is. For example, super-Earths that exceed 2-3 Earth masses may hold an extremely dense hydrogen-rich atmosphere or experience a Venus-like greenhouse effect. However, some research suggests that super-Earths with twice the mass of Earth could potentially develop conditions more suitable for life than those on Earth. A planet more massive than Earth will have a flatter topography due to high surface gravity, and any oceans will be significantly larger and shallower than those on Earth. This could potentially create massive island chains of archipelagos that tend to harbor some of the highest amounts of biodiversity on Earth. A Super-Earth could also retain more internal heat and sustain geological processes for longer periods of time.