Rocky planet
From RationalWiki
A rocky planet, also called "terrestrial planet" is a planet consisting of metals, in the form of rocks. Rocky planets are one of two types of planets, the other being the gas giants. Rocky planets can also be classed Dwarf Planets like Pluto when they're too small to be classed as actual planets.
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[edit] Life on rocky planets
Some rocky planets, such as Earth, are capable of supporting carbon-based "life as we know it". Conditions necessary for life are generally thought to be:
- Orbit around a yellow star (like the Sun), a red giant, or a stable combination of those stars.
- Presence of water, and carbon based organic molecules. The former is believed to be fairly common while higher resolution spectroscopy equipment has started to detect fairly large organic molecules.
- Orbit within the habitable zone.
- Presence of an active core that causes volcanic and tectonic activity as well as a magnetic field to protect the surface from cosmic and solar radiation (the possibility of life evolving despite this is possible but probably far more difficult).
- The size and density of the planet have to be such that the planet's gravity can hold an atmosphere, but is not too strong to crush lifeforms.
Out of all the known planets and exoplanets, Gliese 581c and Mars are the only planets (besides Earth) to even remotely meet these criteria. However, it should be noted that current methods for detecting exoplanets are biased towards finding ones that certainly aren't Earth-like.
We must however not limit ourselves to searching for live on Earth-like planets inside the habitable zone, as alternative scenarios, such as volcanic activity providing heat on planets or moons outside of the habitable zone (similar to "smokers" on Earth's ocean floors), silicon-based life that can survive where carbon-based life cannot, and even floating, balloon-like lifeforms living in the atmospheres of gas giants, have been theorized.
[edit] Rocky planets throughout the universe
In our solar system, the inner planets (Mercury, Venus, Earth and Mars) are rocky planets. The famous exoplanet Gliese 581c is also a rocky planet.
Recently Pluto has been removed from the list of rocky planets, because it is similar in size to several other objects that are generally not classified as planets. Indeed there is a fine line between small rocky planets, moons and large asteroids.
Few rocky exoplanets have been discovered so far, this is most likely because their small size and mass makes them them difficult to detect.
Stars with planetary systems are believed to be quite common, however, and as new detection technologies are developed, more rocky planets will almost certainly be discovered.
