Super-Earths: Exoplanets Close to Earth's Size
An artist's conception shows the size of super-Earth 55 Cancri e compared to Earth. A ground-based telescope in Spain was able to identify 55 Cancri e, which suggests that telescopes on the ground help in the search for habitable planets around other stars.
Credit: NASA/JPL

A super-Earth is a planet that is close to Earth's size and mass, but not so big that its surface is surrounded by a gas envelope — like a small version of Uranus. If the super-Earth is in the habitable zone of its star and has a life-friendly atmosphere, it could host life, but that is not a guarantee.

There are no known super-Earths in our solar system; they are all in extrasolar planetary systems. With telescope technology of today, most super-Earths have been found near M-class red dwarf stars (smaller than the sun) or G-class stars (similar to our sun). Stars that are bigger and shine brighter than the sun would simply wash super-Earths out, and make their tiny gravitational tugs hard to detect and the transit signals too small.

There is no agreed-upon definition as to how big a super-Earth can get, but many scientists think that planets that are no bigger than 1.6 times Earth's radius will most likely still be rocky, according to Sara Seager, a professor of planetary science and physics at the Massachusetts Institute of Technology who studies exoplanets. 

Planets that are much bigger, she added, could have an inhospitable atmosphere. The atmosphere may get thick and oppressive, unlike Earth's relatively thin and transparent atmosphere that lets sunlight in. But even if a planet is the right size, it's unclear how likely it is to host life.

“It's a habitable planet if, by definition, it has surface liquid water, but there is no guarantee a [given] planet has surface liquid water,” Seager told Space.com. “But some people might think it's inevitable.”

There are two primary arguments for how water arrived on Earth. Some scientists think that comets and other small bodies brought it from elsewhere in the solar system. Others think that oxygen (which is a common element in rocks, particularly in oxygen-hydrogen combinations) got released and reformed into water, which is also made up of hydrogen and oxygen.

“Some people might argue that every planet should have that release, but others would say no, you have to have the right conditions,” Seager said.

There are other things to consider as well, such as the planetary atmosphere. Current telescope technology can only measure the composition of planet atmospheres that are very large (Jupiter's size or bigger) and relatively close to Earth. And on a rocky planet, small amounts of elements matter. Earth scientists are concerned about global warming, which concerns parts per million of carbon dioxide, Seager pointed out.

Seager is part of a group working on the proposed Transiting Exoplanet Survey Satellite (TESS), which could be used to search for super-Earths. The project, led by MIT's George Ricker, will view planets that are only a few hundred light-years away. NASA's Kepler planet-hunting telescope, by contrast, seeks planets thousands of light-years away.

“The hope is that some of these planets have atmospheres that we can study,” Seager said. While super-Earths are still small and hard to analyze, she said it might be possible to learn about the atmospheric composition of those planets that are close to small stars.

But the challenge there is telescope time. While NASA's forthcoming James Webb Space Telescope (to launch in 2018) potentially can analyze atmospheres, it could take a dozen transits across a star to pick out the planet's atmosphere from its stellar host. This could take days of observing time, which is rare among space observatories; only a handful of Hubble Space Telescope projects get that much priority, for example.

In the meantime, Seager is waiting for her “favorite super-Earth” to show up. Some of the more massive types may have hydrogen in its atmosphere. Because hydrogen tends to puff up when the sun hits it, this makes the element (and atmosphere) easier to detect than most. But that world sure wouldn't be habitable. “It's a potent greenhouse gas, way more nasty than anything we have,” Seager said.

While many super-Earths have been discovered, there are a few that stick out:

  • Kepler-10b: This planet, announced in January 2011, is the first "unquestionably rocky" exoplanet, scientists said at the time. Precise measurements of the star allowed scientists to calculate the density at 8.8 grams per cubic centimeter, a rocky density similar to Earth taking into account Kepler-10b's more massive size. But Kepler-10b is so close to its parent star that its surface is likely liquid lava.
  • Kepler-452b: Announced in 2015, this is the planet that is both in the habitable zone of its G-type star — and the most similar to Earth's size. It's about 60 percent larger than our home planet. While its mass and density are not known, scientists predict it will be a rocky planet based on its size.
  • Gliese 667C c: This planet is about 1.8 times the radius of Earth and orbits its star (an M-class dwarf) once every 28 Earth days. It commonly tops lists of potentially habitable exoplanets. It is believed that six planets orbit this star (Gliese 667C c is one of three in the habitable zone), but just how many are there has been a subject of dispute.
  • Kepler-62f: A planet about 1.4 times the size of Earth and likely in the habitable region of its star. The discovery was announced in 2013. A modeling study of the planet suggests that it might be covered with water, leading to questions about what sort of life would be possible on such a world.

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