What is a Super-Earth?

A Super-Earth is a type of extrasolar planet (exoplanet) more massive than the Earth but not as large as a gas giant like Jupiter or Saturn. The concept is relatively new. Scientific papers variously define a Super-Earth as a planet between 1 and 10 Earth masses, or as a planet between 5 and 10 Earth masses. It could be a while before a standard definition emerges, if one does at all. Super-Earths are of interest to astronomers because they are a category of planets that does not exist in our own solar system and thus have an aura of mystery about them.

Most extrasolar planets discovered thus far are either Super-Earths or gas giants. This is because our detection technology is not yet sensitive enough to find exoplanets with a mass similar to Earth or less. The first Super-Earths discovered were PSR 1257+12 b and PSR 1257+12 c, planets with masses approximately 4 times greater than the Earth, orbiting the pulsar PSR 1257+12 located 980 light-years from the Sun. Though first discovered in 1992, it wasn't until 2003 that their existence was confirmed to the satisfaction of the astronomical community. These were the first extrasolar planets to be discovered, but not the first to be confirmed. They orbit a pulsar, so these planets are constantly bathed in x-rays, and are most certainly not habitable to life as we know it.

In 2005, the first Super-Earth orbiting a main sequence or Sun-like star was detected. The planet, Gliese 876 d, was discovered by a team led by Eugenio Rivera and is orbiting Gliese 876, a red dwarf star about 15 light-years away in the constellation of Aquarius. With an estimated mass about 7.5 times greater than Earth, Gliese 876 d likely has a surface gravity of about 3 g, which would be enough to make walking around like lifting twice your own weight. That's not the only feature of this Super-Earth that could scare off potential colonists — it's also located so close to its home star that its surface temperature is probably around 710°F (377°C), similar to that of Venus.

In April 2007, the discovery of the Super-Earth Gliese 581 c by a team headed by Stephane Udry based in Switzerland made major headlines, by being the first planet discovered in its home star's habitable zone. The habitable zone is that region around a star where temperatures have the potential to be similar to that of Earth and the existence of liquid water is theoretically possible. Gliese 581 c has a mass of about 5 Earth masses and orbits Gliese 581, a red dwarf star located 20.3 light years away from the Sun. Located about 0.073 astronomical units (11 million km) from its home star, Gliese 581 c is on the warm edge of its star's habitable zone.

Only a few Super-Earths have been detected so far, but many more are expected to be discovered as detection technology improves. As the field advances, Super-Earths are likely to be the exoplanets that grab the headlines the most, due to their relative similarity to Earth in comparison with gas giant exoplanets.