Discovering a planetary satellite used to be a rare event. After the finding of Uranus's moon Miranda by Gerard Kuiper in 1948 and Jupiter's Ananke by Seth Nicholson in 1951, the total count of natural satellites stood at 31: Jupiter led with 12, followed by Saturn (9), Uranus (5), Neptune (2), Mars (2), and Earth (1). Telescopic observers added only a few more in the ensuing three decades — most notably Pluto's moon, Charon — as satellite discovery largely fell to NASA spacecraft. After Voyager 2's flyby of Neptune in 1989, the total had nearly doubled to 60.

However, the advent of sensitive electronic cameras has put telescopic observers back in the driver's seat, resulting in dozens of finds over the past few years. Indeed, some of the new objects are so small, only a few kilometers across, that they stretch the traditional notion of what constitutes a "moon." Sometimes faint objects appear fleetingly in images and are never seen again; others require patient follow-up observations over months or years to confirm their reality. While comets bear the surname(s) of their discoverer(s), and observers retain naming rights for asteroids, those who spot planetary moons must yield this privilege to the International Astronomical Union. By convention, the IAU does not name a satellite until its orbit is known precisely — a threshold sometimes not reached until decades after the initial discovery.

The following is a tabulation of known planetary satellites, last updated in July 2011. As of then, the count stood at 171, distributed as follows: Earth 1, Mars 2, Jupiter 65, Saturn 62, Uranus 27, Neptune 13. Among the dwarf planets, Pluto has 4, Eris 1, and Haumea 2.

In the table below and on the following pages, the Diameter of each satellite and the Orbital distance from its planet's center (semimajor axis) are in kilometers; Eccentricity (Ecc.) is the elongation of the satellite's orbit; and Inclination is the tilt of its orbit plane, in degrees, with respect to the planet's equatorial plane (for inner moons) or the ecliptic plane (for outer moons).

Confirmed List of Planetary Satellites
Satellite	Diameter (km)	Visual magnitude	Distance (km)	Orbital period (days)	Eccen- tricity	Inclina- tion (°)
EARTH
Moon	3,474	–12.7	384,400	27.3	0.055	5.1
MARS
Phobos	22	11.4	9,380	0.3	0.015	1.1
Deimos	12	12.5	23,460	1.3	0.000	1.8