Mass (1024kg or 1021tons) - This is the mass of the planet in septillion (1 followed by 24 zeros) kilograms or sextillion (1 followed by 21 zeros) tons. Strictly speaking tons are measures of weight, not mass, but are used here to represent the mass of one ton of material under Earth gravity.
Diameter (km or miles) - The diameter of the planet at the equator, the distance through the center of the planet from one point on the equator to the opposite side, in kilometers or miles.
Density (kg/m3 or lbs/ft3) - The average density (mass divided by volume) of the whole planet (not including the atmosphere for the terrestrial planets) in kilograms per cubic meter or pounds per cubic foot. Strictly speaking pounds are measures of weight, not mass, but are used here to represent the mass of one pound of material under Earth gravity.
Gravity (m/s2 or ft/s2) - The gravitational acceleration on the surface at the equator in meters per second squared or feet per second squared, including the effects of rotation. For the gas giant planets the gravity is given at the 1 bar pressure level in the atmosphere. The gravity on Earth is designated as 1 "G", so the Earth ratio fact sheets gives the gravity of the other planets in G's.
Escape Velocity (km/s) - Initial velocity, in kilometers per second or miles per second, needed at the surface (at the 1 bar pressure level for the gas giants) to escape the body's gravitational pull, ignoring atmospheric drag.
Rotation Period (hours) - This is the time it takes for the planet to complete one rotation relative to the fixed background stars (not relative to the Sun) in hours. Negative numbers indicate retrograde (backwards relative to the Earth) rotation.
Length of Day (hours) - The average time in hours for the Sun to move from the noon position in the sky at a point on the equator back to the same position.
Distance from Sun (106 km or 106 miles) - This is the average distance from
the planet to the Sun in millions of kilometers or millions of miles, also known as the semi-major axis.
All planets have orbits which are elliptical, not perfectly circular, so there is a point in the orbit at
which the planet is closest to the Sun, the perihelion, and a point furthest from the Sun, the aphelion.
The average distance from the Sun is midway between these two values. The average distance from the Earth
to the Sun is defined as 1 Astronomical Unit (AU), so the ratio table gives this distance in AU.
* For the Moon, the average distance from the Earth is given.
Perihelion, Aphelion (106 km or 106 miles) - The closest and furthest points
in a planet's orbit about the Sun, see "Distance from Sun" above.
* For the Moon, the closest and furthest points to Earth are given, known as the "Perigee" and "Apogee" respectively.
Orbital Period (days) - This is the time in Earth days for a planet to orbit the Sun from one
vernal equinox to the next. Also known as the tropical orbit period, this is equal to a year on Earth.
* For the Moon, the sidereal orbit period, the time to orbit once relative to the fixed background stars, is given. The time from full Moon to full Moon, or synodic period, is 29.53 days. For Pluto, the tropical orbit period is not well known, the sidereal orbit period is used.
Orbital Velocity (km/s or miles/s) - The average velocity or speed of the planet as it orbits the
Sun, in kilometers per second or miles per second.
* For the Moon, the average velocity around the Earth is given.
Orbital Inclination (degrees) - The angle in degrees at which a planets orbit around the Sun is tilted relative to the ecliptic plane. The ecliptic plane is defined as the plane containing the Earth's orbit, so the Earth's inclination is 0.
Orbital Eccentricity - This is a measure of how far a planet's orbit about the Sun (or the Moon's orbit about the Earth) is from being circular. The larger the eccentricity, the more elongated is the orbit, an eccentricity of 0 means the orbit is a perfect circle. There are no units for eccentricity.
Obliquity to Orbit (degrees) - The angle in degrees the axis of a planet (the imaginary line running
through the center of the planet from the north to south poles) is tilted relative to a line
perpendicular to the planet's orbit around the Sun, north pole defined by right hand rule.
*Venus rotates in a retrograde direction, opposite the other planets, so the tilt is almost 180 degrees, it is considered to be spinning with its "top", or north pole pointing "downward" (southward). Uranus rotates almost on its side relative to the orbit, Pluto is pointing slightly "down". The ratios with Earth refer to the axis without reference to north or south.
Mean Temperature (C or F) - This is the average temperature over the whole planet's surface (or for the gas giants at the one bar level) in degrees C (Celsius or Centigrade) or degrees F (Fahrenheit). For Mercury and the Moon, for example, this is an average over the sunlit (very hot) and dark (very cold) hemispheres and so is not representative of any given region on the planet, and most of the surface is quite different from this average value. As with the Earth, there will tend to be variations in temperature from the equator to the poles, from the day to night sides, and seasonal changes on most of the planets.
Surface Pressure (bars or atmospheres) - This is the atmospheric pressure (the weight of the
atmosphere per unit area) at the surface of the planet in bars or atmospheres.
*The surfaces of Jupiter, Saturn, Uranus, and Neptune are deep in the atmosphere and the location and pressures are not known.
Number of Moons - This gives the number of IAU officially confirmed moons orbiting the planet. New moons are still being discovered.
Ring System? - This tells whether a planet has a set of rings around it, Saturn being the most obvious example.
Global Magnetic Field? - This tells whether the planet has a measurable large-scale magnetic field. Mars and the Moon have localized regional magnetic fields but no global field.
The term "terrestrial planets" refers to Mercury, Venus, Earth, Moon, Mars, and Pluto.
The term "gas giants" refers to Jupiter, Saturn, Uranus, and Neptune.