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The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy , it usually applies to planets or asteroids orbiting the Sun , moons orbiting planets, exoplanets orbiting other stars , or binary stars .
Prior to this, "small" exoplanets such as Gliese 876 d, which has an orbital period of less than 2 Earth-days, were detected very close to their stars. OGLE-2005-BLG-390Lb shows a combination of size and orbit that would not make it out of place in the Solar System .
Kepler's 3rd law of planetary motion states, the square of the periodic time is proportional to the cube of the mean distance, [4] or a 3 ∝ P 2 , {\displaystyle {a^{3}}\propto {P^{2}},} where a is the semi-major axis or mean distance, and P is the orbital period as above.
which gives an angular distance from the pericenter at arbitrary time t [3] with dimensions of radians or degrees. Because the rate of increase, n , is a constant average, the mean anomaly increases uniformly (linearly) from 0 to 2 π radians or 0° to 360° during each orbit.
In tightly packed planetary systems, the gravitational pull of the planets among themselves causes one planet to accelerate and another planet to decelerate along its orbit. The acceleration causes the orbital period of each planet to change. Detecting this effect by measuring the change is known as transit-timing variations.
Note that the semi-major axis is proportional to the 2/3 power of the orbital period. For example, planets in a 2:3 orbital resonance (such as plutinos relative to Neptune) will vary in distance by (2/3) 2/3 = −23.69% and +31.04% relative to one another. 2 Ceres and Pluto are dwarf planets rather than major planets.
The Moon is in a supersynchronous orbit of Earth, orbiting more slowly than the 24-hour rotational period of Earth. The inner of the two Martian moons, Phobos, is in a subsynchronous orbit of Mars with an orbital period of only 0.32 days. [7] The outer moon Deimos is in supersynchronous orbit around Mars. [7]
Radial velocity curve with peak radial velocity K=1 m/s and orbital period 2 years. The peak radial velocity is the semi-amplitude of the radial velocity curve, as shown in the figure. The orbital period is found from the periodicity in the radial velocity curve. These are the two observable quantities needed to calculate the binary mass function.