An Earth - like planet is a planet that looks similar to Earth.
Theoretical use
The term is used mostly to see how a habitable planet would behave in given conditions, it is more easy to imagine an existing planet there. For theoretical models, an Earth - like planet has nearly all features of Earth (mass, gravity, geographic features, chemistry, luminosity, orbital period, orbit eccentricity, axial tilt, atmospheric properties, life and other features), but one or two things are different. To see examples of theoretical models, please go to Outer Planet, Elliptical orbit, Low - spinning planet, Tidal Locked Planet, Desert planet or Oceanic Planet.
Practical use
For terraformers, an Earth - like planet is a planet that has many similarities with Earth. Maybe it looked different first, but after a successfully terraforming process, the planet became similar to Earth. There is not a well fixed definition of an Earth - like planet, but the celestial body should respect majority of the rules listed below. Conditions are listed in order of their importance.
- The planet must host Earth - like life.
- The air must be breathable.
- There must exist enough water and that water must be suitable for life.
- Temperatures must be similar to Earth, without massive amounts of greenhouse gasses or shields.
- Luminosity must be suitable for Earth-like plants, radiation must be suitable for human body.
- In the ground, there must be traces of all microelements and not high amounts of toxic substances.
- Geographic features need to be similar to Earth (land, oceans, mountains, plains).
- Gravity, size and density must be close to Earth.
- Seasons need to exist, but not too long or too short, not too hard to survive (this implies for a moderate axial tilt or a slightly elliptic orbit).
- Day - night cycles are required to exist and have similar length with an Earth day.
- Tidal forces should be similar to what we know on Earth.
Please note that not all conditions are required.
Before terraforming
For scientists looking for a planet similar to Earth, the conditions to be an Earth - like planet are the following ones:
- Similar mass, gravity and density
- Similar average luminosity (also, similar temperatures with Earth, assuming the same greenhouse effect)
- Similar orbit and not a too tilted axis
- Presence of enough water
- Presence of all needed chemical elements, in good amounts.
Other additional conditions:
- Presence of liquid water on surface
- Presence of an atmosphere with similar pressure.
Earth - like planets are the best targets for future interstellar travelers. They are the most easy to terraform.