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[TracyCoxx]

Originally Posted by sesame

The International Astronomical Union declared Pluto as the Nineth Planet of our solar system in 1930. Why has it suddenly been kicked out of the list?

Since this hasn't been answered, I'll go ahead and answer. Even though Pluto does have it's own moon, Pluto is smaller than the Earth's moon. Also, more and more objects have been discovered like large asteroids as well as large icy bodies beyond Pluto's orbit in the Kuiper belt. Not only that, but Pluto's orbit is inclined 17 degrees relative to the rest of the planets' orbits. So some astronomers have for years been wanting to downgrade Pluto's status as a planet.

Out of respect for Pluto's discoverer, Clyde Tombaugh, it has remained a planet. But after he died in 1997, there were increasing calls to downgrade Pluto. In 2006, the International Astronomical Union came up with some arbitrary categories of planetary bodies:
Planet
* is in orbit around the Sun
* has sufficient mass to be roundish
* has cleared its orbit of debris

A non satellite body fulfilling only the first two of these is a Dwarf Planet (which is what Pluto is now called). If it fulfills only the first criteria, it's a Small Solar System Body (SSSB).

So there are now 8 planets, and 5 dwarf planets in the solar system. They are Ceres (once considered a planet for many years), Pluto, Eris, Makemake, and Humea.