A planet is an `astronomic object`_ that orbits a star, has enough mass to be rounded by its own gravity, but not massive enough to cause theromnuclear fusion, and has cleared is neighboring region of planetesimals. For example, Earth and Mars_.


1   Substance

The shape of all planet are ellipsoid, and roughly a sphere.

A planet may have moons.

Some planets have an atmosphere.

2   Properties

2.1   Poles

A pole is a point where the axis of rotation of a planet intersects the surface. All planets have exactly two poles.

2.3   Habitability

Every star has a habitable zone that is affected by the size of the star and its intensity. The habitable zone of the sun is about 0.95 AU to 1.37 AU. An AU is the Earth's average distance from the Sun (93 million miles). Earth's orbit could decrease by 4.5MM miles or increase by 34MM and still be in the habitable zone.

2.4   Lagrange point

A Lagrange point is a position in an orbital configuration of two large bodies, wherein a small object, affected only by the gravitational forces from the two larger objects, will maintain its position relative to them.

There are five Lagrange points for every pair of celestial objects.

2.5   Apparent diamater

We can talk about the "angular diameter" or "apparent diameter" of an object. This is how big it looks on the sky, rather than how big it really is. For instance, the Moon and the Sun have about the same angular diameter - half a degree - even though the Sun is much much bigger in actual size. This is of course because the Moon is much closer than the Sun.

3   History

3.1   Religious significance

The Greeks and Romans named the planets after their gods.

3.2   Orbit

Ptolemy thought the planets orbited Earth in deferent and epicycle motions. Although the idea that the planets orbited the Sun had been suggested a large number of times, it wasn't until the seventeenth century that this view was supported by evidence from the first telescopic astronomical observations, performed by Galileo Galilei. At about the same time, by careful analysis of pre-telescopic observation data collected by Tycho Brahe, Johannes Kepler found the planets' orbits weren't circular but elliptical. (It's perfectly possible to get a circular orbit, but the relationship between the bodies' velocities and separation needs to be exactly right. In practice it rarely is, unless we plan it that way (e.g, for satellites).)

An important factor to consider is the planet's escape velocity (how fast a particle of gas has to be moving to escape the planet's gravitational influence). Less massive objects (like the Moon and Mars) have lower escape velocities, so it is difficult for them to retain atmospheres. It's also important to remember that a strong magnetic field would only help prevent some forms of atmospheric escape (such as that involving solar winds). However, since there are multiple mechanisms by which a planet can lose it's atmosphere, the existence of a magnetic field is not always sufficient for atmospheric retention.

So we know that planets with low masses have trouble retaining atmospheres, but what about planets with higher masses? It turns out that exoplanets with masses a few times greater than Earth's tend to have gaseous envelopes (sort of like a mini-Neptune). This is because the planet's gravitational force is so large that it can accrete a lot of gas when it's forming, and its escape velocity is so large that it's hard for the gas to escape. In these situations, the surface pressure of the planet would be too great for us to live on it (but who knows, maybe there are some aliens that can).

The last thing to consider for habitability is atmospheric composition. Just because a planet has an atmosphere doesn't mean it's habitable (like Venus). To find out if a planet is truly Earth-like, we need to measure the composition of its atmosphere.

So, in general, there are four things we need to determine to find out if a planet is habitable: its orbital period (to determine if it's in the habitable zone), its mass (to find out if it's massive enough to retain at atmosphere), its size (to find out if it has a thick gaseous envelope), and its atmospheric composition (to determine if it has an Earth-like atmosphere). Luckily, we can make all of these measurements today on both Solar System bodies and exoplanets, and our precision is only getting better with time.

Knowing the properties of the host star is also important when determining each of these properties. Specifically, we need to know the luminosity of the star to determine how far away its habitable zone is, we need to know the size of the star to determine the size of the planet, and we need to know the mass of the star to estimate the mass of the planet. Also, since the compositions of exoplanet atmosphere are determined by studying the light from the star that passes through or reflects off of the planet's atmosphere, it's important to characterize the star's spectrum.

Also the planet can't be tidally locked to the star or rotate so slow that there are masive temperature swings. But at the same time almost none of that applies to subterranean life.

Venus and Uranus are the only planets of the Solar System that rotate clockwise, the Sun rises in the west and sets in the east.