What Is A Super-Earth?

Our solar system is home to three different types of planet: terrestrial, gas giant, and ice giant. 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 the Earth. More specifically, super-Earths are planets that have a mass between 1.5 to 10 Earths. 

Discovery

Transit method
Most exoplanets are found using the transit method, which is illustrated in this image. 

The first super-Earth ever discovered was also the first planet found beyond our solar system. In 1992, a team of astronomers detected two planets orbiting a distant pulsar. Analysis of their masses showed that both were approximately 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 since 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 the red dwarf star called Gliese 581. 

In 2009, NASA launched the Kepler Space Telescope. The primary objective of Kepler was to look for and study exoplanets. To find exoplanets, Kepler looks for any changes to 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 the nine years it was operational, it fundamentally changed the science of exoplanets. Over its lifetime, Kepler confirmed the existence of over 2,000 individual planets. Just two years into its mission, Kepler data revealed the existence of nearly 300 super-Earths, including Kepler’s first super-Earth found in the habitable zone. Called Kepler-22b, it has a size 2.5 times greater than Earth and orbits a star similar to our sun. 

Abundance of Super-Earths

Exoplanets discovered
Data on the number of exoplanets found from 2017. Image credit: NASA

Super-Earths have turned out to be one of the most common types of planet in our galaxy. Data from NASA shows that there are a total of 1,582 confirmed super-Earths around other stars. There are a total of 5,157 exoplanets confirmed as of September 1, 2022, and so super-Earths make up just over 30% of confirmed exoplanets. Furthermore, most rocky planets found in the habitable zones of other stars are also super-Earths. 

Habitability of Super-Earths

Habitable planets from Kepler
Artistic rendition of some potentially habitable planets found by Kepler. Image credit: NASA

Since super-Earths make up the bulk 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 be dependent upon how massive it is. For example, super-Earths that exceed 2-3 Earth masses may hold onto extremely dense, hydrogen rich atmospheres, or they may experience a runaway greenhouse effect similar to Venus. However, some research suggests that super-Earths around twice the mass of Earth could potentially develop conditions that are more suited to life than those on Earth are. A planet more massive than Earth will have a flatter topography due to the high surface gravity, and any oceans will be significantly larger and more shallow than those on Earth. This could potentially create massive chains of archipelago islands, which on Earth tend to house some of the highest amounts of biodiversity. A super-Earth could also retain more internal heat and maintain geologic processes for longer periods of time.

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