As more and more extra-solar planets are discovered, scientists are considering what planets may or may not support life. Though it will be a very long time before we might conceivably reach even the closest of our neighbor stars, this study is a great potential source for science fiction writers.
Over the past several years, astronomers increasingly have been using three, somewhat overlapping, terms to describe a range of extra-Solar planets that have been, or may soon be, found. The three types of planets (or moons) are defined to be smaller than the gas giants found in the Solar System (i.e., Uranus, Neptune, Saturn, and Jupiter).
At least one recently discovered super-Earth and water world even may be capable of supporting microbial life. As the new decade began on January 1, 2010, more than a few astronomers appear hopeful that, within a few years or months, even Earth-like planets may be discovered that can potentially support Earth-type plants and animals.
Super-Earths: Bigger than two Earths but not as massive (and hopefully not as gaseous) as Uranus.
Water Worlds or Ocean Planets: Planets ranging from super-Earths to those smaller than Earth that may have deep oceans but little, if any, habitable land.
Earth-like Planets: Eventually, some terrestrial planets will be found in orbit around their host star's "habitable zone" and so may potentially have liquid water on their surface. They could be constrained in size to between one-half to twice Earth's mass or to between 0.8 to 1.3 times Earth's diameter.
Other odd-sounding planets include:
Iron Planets: A theoretical type of planet that consists almost entirely of iron and therefore has a greater density and a smaller radius than other terrestrial planets of comparable mass. Mercury in the Solar System has a metallic core equal to 60–70% of its planetary mass. Iron planets are believed to form in the high-temperature regions close to a star, like Mercury, and if the protoplanetary disk is rich in iron.
Coreless Planets: A theoretical type of terrestrial planet that consists of silicate rock but has no metallic core, i.e. the opposite of an iron planet. The Solar System contains no coreless planets, but chondrite asteroids and meteorites are common in it. Coreless planets are believed to form farther from the star where volatile oxidizing material is more common.
Carbon Planets: A theoretical class of planets, composed of a metal core surrounded by primarily carbon-based minerals. They may be considered a type of terrestrial planet if the metal content dominates. The Solar System contains no carbon planets, but does have carbon asteroids.
USE IN X-PLORERS (AND OTHER SCIENCE FICTION)
The uses in such a list should be apparent to any game-master, or sci-fi writer. Truth is quite often stranger than fiction after all. Imagine the strange cave formations in a Coreless Planet. X-Plorer teams have endless opportunities for explorations. As for life, consider the environment of a planet, and let your imagination drift as to what sorts of beings might live here, or how life from Earth might have to adapt itself.