Star Trek 101: What is an M-class planet?

An M-class planet, in the Star Trek universe, is a planet that has conditions suitable for the development of carbon-based life: plenty of oxygen, nitrogen and liquid water, and an orbit and size that produce stable weather. M-class planets, also referred to by Vulcans as Minshara class, typically have an abundance of vegetation and animal life and often one or more sentient species.

In 2002, planet-hunters Geoff Marcy and Paul Butler announced the discovery of a planetary system around a star named 55 Cancri, 41 light-years away in the constellation Cancer. As of this year five planets have been confirmed to orbit 55 Cancri, including a terrestrial-type planet much larger than Earth but possibly with some similar characteristics.

It was hoped that the NASA Terrestrial Planet Finder, a space-based telescope designed to detect Earth-sized planets in other star systems, would find many extrasolar terrestrial planets; the project was put on hold indefinitely in 2007 because of Congressional spending limits. The European Space Agency plans a similar project called Darwin, with a launch date of no sooner than 2016.

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