How Many Planets Are There In The Solar System?
The Current Official Answer : 8
The simple official answer to this question is 8. The solar system planets in order from the Sun are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. This has been correct since 2006 when the International Astronomy Union actually defined what a "Planet" is and also what a "Dwarf Planet" is.
There are now 5 official dwarf planets: Ceres, Pluto, Haumea, Makemake, Eris.
The number of planets pre 2006 : 9
Prior to 2006, the official count of planets was 9 because Pluto was reclassified in this year from being a planet to being a dwarf planet.
The number of planets pre 1930 : 8
Before the discovery of Pluto (in 1930), there were only 8 planets - the same official 8 planets that we have today.
The number of planets pre 1854 : 8 - 23
From 1801 to the 1845 there were up to 23 planets. These were due to the discovery of Ceres, Pallas, Vesta and Juno early in the century - all of which were classified as planets. Then around 1845-49 more bodies were discovered (Astraea, Hebe, Iris, Flora, Metis and Hygiea) as well as Neptune (1846) and over a period of a few years (within which Parthenope, Victoria, Egeria, Irene and Eunomia were discovered) it was decided that the classification of Asteroid was needed to describe the bodies in this newly found "Asteroid Belt". Once the classification of asteroid became widely accepted, we were left with the 8 planets we have today. Currently we have over 300,000 catalogued asteroids, with probably a million or more waiting to be found.
The number of planets pre 1801 : 7
Of course before the discovery of Ceres (1801), the number of planets was 7: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus.
The number of planets pre 1781 : 6 or 8?
Before 1781 when Uranus was discovered, there were 6 planets - all of which are visible to the naked eye and were known to the ancients. However Mercury and Venus were often called by different names depending upon whether they appeared in the evening sky or the morning sky... so perhaps we can say there were 8 planets!
Of course before people actually thought of the Earth as a planet, the "Classical Planets" used for astrology included the Sun and the Moon... so you might say that we had 7 planets (i.e. The Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn).
The official planets and dwarf planets in order