Super-Earth

Some planets are very big.

They are heavier than our Earth. But they are not as heavy as the giant gas worlds. We call these big worlds super-Earths.

The name only tells us how much they weigh. It does not tell us if they are hot or cold.

Some might have rocks. Others might have lots of gas.

Some of these worlds are in a special spot. They are near their star. This spot might let them have liquid water. Finding these worlds is very exciting!

A super-Earth is a type of exoplanet. An exoplanet is a planet outside our solar system. These worlds are heavier than Earth. But they are not as heavy as ice giants like Neptune.

The name only describes their mass. Mass is how much matter is in an object. The name does not tell us if they are hot or cold. It does not tell us if they can support life.

Some super-Earths might be rocky. Others might be made of gas. Some scientists call them gas dwarfs.

Some of these worlds sit in a habitable zone. This is a special area around a star. In this zone, a planet might have liquid water on its surface. For example, Kepler-22b is a planet in a habitable zone. It is 2.4 times the size of Earth. It orbits a star that is a bit dimmer than our Sun. This helps the surface stay at a good temperature for water.

A super-Earth is a special type of exoplanet. An exoplanet is a planet that orbits a star outside our solar system.

These worlds are heavier than Earth. However, they are much lighter than ice giants like Uranus or Neptune. Neptune is about 17.1 times the mass of Earth. The term "super-Earth" only describes how much mass a planet has. It does not tell us about the temperature or the weather. It also does not tell us if the planet can support life.

Scientists use mass to define these planets. Most agree the upper limit is 10 Earth masses. This is about 69% of the mass of Uranus. The lower limit can change depending on who you ask. Some say it starts at 1 or 1.9 Earth masses. Others say it starts at 5. Some people call them "gas dwarfs." Others use the name "mini-Neptunes."

Some scientists think the name should only apply to rocky planets. These would be planets with solid surfaces or oceans. This is different from the giant planets in our own solar system.

We have been finding these worlds for many years. The first super-Earths were found in 1992. Aleksander Wolszczan and Dale Frail found them near a pulsar. A pulsar is a type of star. Later, in 2005, a team led by Eugenio Rivera found the first super-Earth around a normal star. This planet is called Gliese 876 d. It orbits its star in only about 2 days. It is very heavy, with about 7.5 Earth masses. It might be too hot for liquid water.

Many discoveries happened in the following years. In 2007, Stéphane Udry found two super-Earths in the Gliese 581 system. One planet, Gliese 581c, has at least 5 Earth masses. In 2009, scientists found COROT-7b. This planet has about 4.8 Earth masses. It is very special because it is made of rocky minerals. In 2011, the Kepler space telescope found many more candidates. It found 288 candidates that were the size of super-Earths. Some of these might live in the "habitable zone." This is a place where liquid water could exist.

These planets help us understand the universe. They show us that there are many kinds of worlds. For example, Kepler-22b was found in 2011. It is 2.4 times the size of Earth. It orbits a star that is a bit dimmer than our Sun. This helps the planet stay at a good temperature. Astronomers like Seth Shostak think there are many more out there. He estimates there could be 30,000 habitable worlds near Earth. This shows how much there is left to discover.

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.