Short answers, pulled from the story.
The Solar System has eight planets under the IAU definition: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The definition was adopted in August 2006 and requires a planet to orbit the Sun, be rounded by its own gravity, and have cleared its neighborhood.
Pluto was reclassified as a dwarf planet in 2006 because it has not cleared its orbital neighborhood, one of the three IAU requirements for a planet. The discovery of Eris in 2005, an object 27 percent more massive than Pluto, forced an official definition, and Pluto, Ceres, and Eris were placed in the dwarf planet category.
Under the nebular hypothesis, an interstellar cloud collapses out of a nebula to create a young protostar surrounded by a rotating protoplanetary disk. Planets grow in this disk by accretion, the gradual accumulation of material, as dust forms planetesimals and then protoplanets through gravitational attraction.
The first confirmed exoplanet orbiting an ordinary main-sequence star was 51 Pegasi b, announced on the 6th of October 1995 by Michel Mayor and Didier Queloz of the University of Geneva. It orbits the star 51 Pegasi and is a hot Jupiter that lies very close to its parent star.
Terrestrial planets such as Mercury, Venus, Earth, and Mars are largely composed of rock and metal, while giant planets are significantly more massive and differ in composition. The gas giants Jupiter and Saturn are primarily hydrogen and helium, and the ice giants Uranus and Neptune are primarily water, methane, and ammonia under thick hydrogen and helium atmospheres.
The word planet comes from the Greek planḗtai, which in antiquity referred to the Sun, the Moon, and the five naked-eye lights that moved against the fixed stars: Mercury, Venus, Mars, Jupiter, and Saturn. The ancient Greeks called these moving lights planētes asteres, the wandering stars.