Mass – This is the mass of the planet compared to the mass of the Earth.
Diameter – 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, compared to Earth.
Density – The average density (mass divided by volume) of the whole planet (not including the atmosphere for the terrestrial planets) compared to Earth.
Gravity – The gravitational acceleration at the surface, including rotation, compared to Earth.
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 – Initial velocity, compared to Earth, 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 – This is the time it takes for the planet to complete one rotation
relative to the fixed background stars (not relative to the Sun) compared to Earth. Negative numbers indicate retrograde (backwards relative to the Earth) rotation.
Length of Day – The average time compared to Earth 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 – This is the average distance from the planet to the Sun, also known as the semi-major axis, compared to Earth.
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 – 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 – This is the time compared to Earth 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.
Orbital Velocity – The average velocity or speed of the planet as it orbits the
Sun, compared to Earth.
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 compared to Earth. 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.
Axial Tilt – The angle 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, compared to Earth.
*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.
Surface Pressure – This is the atmospheric pressure (the weight of the
atmosphere per unit area) at the surface of the planet compared to Earth.
*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.
Index of Planetary Fact Sheets
– More detailed fact sheets for each planet