Super-Earth

A super-Earth is a specific category of exoplanet. An exoplanet is a planet that orbits a star outside our own solar system.

This term describes the mass of the planet. It does not describe the surface conditions or whether the planet is habitable. A super-Earth has a mass higher than Earth's. However, its mass is substantially lower than the ice giants in our solar system. Uranus and Neptune are much heavier. Neptune has 17.1 times the mass of Earth. Uranus has 14.5 times the mass of Earth.

Astronomers define these planets primarily by their mass. Most scientists agree on an upper limit of 10 Earth masses. This limit is about 69% of the mass of Uranus. The lower limit for a super-Earth is not always the same. Some definitions start at 1 or 1.9 Earth masses. Other definitions use a starting point of 5 Earth masses. Some researchers use the term "gas dwarfs" for larger ones. Others call them "mini-Neptunes."

There are different ways to categorize these worlds. The Kepler space telescope team used a different definition. They looked at the radius, or the physical size, of the planet. They called a planet a super-Earth if it was between 1.25 and 2 Earth radii. This is larger than Earth-like planets but smaller than mini-Neptunes. Some authors suggest the term should only apply to rocky planets. These would be planets with solid surfaces or oceans. This would create a sharp boundary between liquid and atmosphere. This is different from the giant planets in our solar system.

Humanity has been discovering these worlds since the early 1990s. Aleksander Wolszczan and Dale Frail found the first super-Earths in 1992. These planets orbit a pulsar called PSR B1257+12. Two of these planets are named Poltergeist and Phobetor. They each have a mass about four times that of Earth. In 2005, Eugenio Rivera led a team that found a different kind. They found the first super-Earth orbiting a main-sequence star. This planet is Gliese 876 d. It has an estimated mass of 7.5 Earth masses. It orbits its star very quickly, in only about 2 days.

Many significant discoveries followed in the next few years. In 2007, Stéphane Udry found two super-Earths in the Gliese 581 system. One planet, Gliese 581c, has a mass of at least 5 Earth masses. It sits on the warm edge of the habitable zone. The habitable zone is where liquid water might exist on a surface. In 2009, scientists announced the discovery of COROT-7b. This planet has a mass of about 4.8 Earth masses. It is unique because its density suggests it is made of rocky silicate minerals. This is similar to the four inner planets in our solar system.

The Kepler space telescope mission provided massive amounts of data. In 2011, the mission team released a list of 1,235 planet candidates. This included 288 candidates that were super-Earth-size. The team also found 48 candidates in habitable zones. One important discovery was Kepler-22b. This planet has a radius 2.4 times larger than Earth. It orbits a star that is slightly dimmer than our Sun. Because the star is dimmer, the surface temperature might still allow liquid water.

These discoveries help us understand the scale of the universe. Astronomer Seth Shostak estimates there are at least 30,000 habitable worlds near Earth. The Kepler team estimated there are at least 50 billion planets in the Milky Way. Of those, at least 500 million might be in the habitable zone. These numbers show that super-Earths are part of a very large cosmic system. Studying them helps us learn how different types of planets form and exist.