Gliese 581c: The First 'Earth-like' Planet Found

The discovery of Gliese 581c was announced in the journal Astronomy and Astrophysics in 2007, thanks to the work of an international team led by Stephane Udry of the Geneva Observatory. The team detected Gliese 581c using the radial-velocity method, which identifies planets by the slight gravitational tugs they exert on their host stars.

Notably, Gliese 581c wasn't alone. It was one of two super-Earth planets (planets up to 10 times the size of Earth) found by Udry's team, both residing on the edge of the star's habitable zone. The researchers dubbed Gliese 581c "the known exoplanet which most resembles our own Earth" because of its mass, which was only about five times that of Earth. As the first Earth-like planet discovered, Gliese 581c generated a lot of excitement upon its announcement.

Gliese 581c orbits Gliese 581, an M-class or red dwarf star cooler than our sun. Due to its cooler temperature, the habitable zone of Gliese 581 is closer in than our own solar system's habitable zone. M dwarfs are favored for planetary searches due to their relative dimness, which makes it easier to spot planets passing across the star. Gliese 581c is the third planet from its host star Gliese 581. The others in this planetary system are Gliese 581b, Gliese 581d and Gliese 581e.

Despite its discovery, Gliese 581c has never been directly seen passing across its star. Hence, researchers rely on the planet's influence on other celestial bodies to infer its characteristics. Depending on the radius of the planet, Gliese 581c could either be similar to an Earth-like planet (with a smaller atmosphere) or resemble a Neptune-like planet (with a much thicker atmosphere).

Gliese 581c's close proximity to its star leads to a quick orbit — only about 13 days — likely resulting in tidal locking. This means one side of the planet always faces its star while the other side remains in perpetual darkness, which has significant implications for its habitability.

Its actual surface temperature could range between 32 degrees F (0 degrees C) and 104 F (40 C). This wide range is due to the fact that scientists haven't (so far) detected the planet's composition — whether it is rocky or watery. And composition determines temperature.

Models of the planet's orbit over time suggest that the heating from the tidal locking could mean a surface heat flux (amount of heat energy passing through a certain surface) three times greater than Jupiter's moon Io. This could mean volcanoes and plate tectonics are present.

When Gliese 581c was initially discovered, researchers were optimistic that it might reside in its star's habitable zone, the region where liquid water might exist on a planet's surface. However, later research suggested that the planet might be more similar to Venus in our own solar system, with extremely high surface temperatures and a runaway greenhouse effect under a thick atmosphere.

If Gliese 581c is indeed tidally locked, this could also complicate the picture of habitability. Earth's regular day-night cycle has played a significant role in the evolution and adaptation of life forms. On a planet with one side always bathed in daylight and the other always in darkness, it's unclear how life would adapt.

Research from 2013 also showed that the rocky centers of super-Earths are unlikely to evolve into terrestrial rocky planets with thin atmospheres (like the planets in our inner solar system). Rather, these planets tend to remain as small rocky cores engulfed by large hydrogen-rich atmospheres. A rocky surface is one of the requirements for habitability.

In recent years, Gliese 581c hasn't been the focus of scientific research. The explosion of exoplanets or extrasolar planets (planets outside the solar system) found by the Kepler and TESS space telescopes, has resulted in many other "Earth-like" planets being found, some with masses much closer to that of Earth.

Still, as future space missions dedicated to finding habitable planets continue, who knows what new insights we'll gain about Gliese 581c and other intriguing worlds like it?

This article was created in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.