8, and I think two dwarfs.
Correct, Sprocky, IT
Pluto is no longer considered a planet. Pluto was first discovered in 1930 by Clyde W. Tombaugh. Astronomers weren't sure about Pluto's mass until the discovery of its largest Moon, Charon, in 1978. And by knowing its mass (0.0021 Earths), they could more accurately gauge its size. The most accurate measurement currently gives the size of Pluto at 2,400 km (1,500 miles) across. Over the last few decades, new observatories have completely changed previous understanding of the outer Solar System. Instead of being the only planet in its region, Pluto and its moons are now known to be just a large example of a collection of objects called the Kuiper Belt. Astronomers estimate that there are at least 70,000 icy objects, with the same composition as Pluto, that measure 100 km across or more in the Kuiper Belt. Astronomers had been turning up larger and larger objects in the Kuiper Belt. And in 2005, Mike Brown and his team discovered an object, further out than the orbit of Pluto that was probably the same size, or even larger. Officially named 2003 UB313, the object was later designated as Eris. Since its discovery, astronomers have determined that Eris' size is approximately 2,600 km (1,600 miles) across. It also has approximately 25% more mass than Pluto.
With Eris being larger, made of the same ice/rock mixture, and more massive than Pluto, the concept that we have nine planets in the Solar System began to fall apart. What is Eris, planet or Kuiper Belt Object; what is Pluto, for that matter? Astronomers decided they would make a final decision about the definition of a planet at the XXVIth General Assembly of the International Astronomical Union, which was held from August 14 to August 25, 2006 in Prague, Czech Republic.
Astronomers from the association were given the opportunity to vote on the definition of planets. In the end, astronomers voted for the controversial decision of demoting Pluto (and Eris) down to the newly created classification of "dwarf planet".
For an object to be a planet, it needs to meet these three requirements defined by the IAU:
• It needs to be in orbit around the Sun
• It needs to have enough gravity to pull itself into a spherical shape
• It needs to have "cleared the neighborhood" of its orbit – According to this, Pluto is not a planet.
What does "cleared its neighborhood" mean? As planets form, they become the dominant gravitational body in their orbit in the Solar System. As they interact with other, smaller objects, they either consume them, or sling them away with their gravity. Pluto is only 0.07 times the mass of the other objects in its orbit. The Earth, in comparison, has 1.7 million times the mass of the other objects in its orbit.
Any object that doesn't meet this 3rd criteria is considered a dwarf planet. And so, Pluto is a dwarf planet. There are still many objects with similar size and mass to Pluto jostling around in its orbit. And until Pluto crashes into many of them and gains mass, it will remain a dwarf planet. Eris suffers from the same problem.