Within the last few years, astronomers announced the discovery of a terrestrial planet outside of our solar system with conditions that may be favorable for sustaining an atmosphere.
The characteristics of the planets in our solar system indicate that really only two fundamental types of planets exist: those that are similar to Earth (the terrestrial planets) and those that are similar to Jupiter (gas-giant planets). The terrestrial planets include Mercury, Venus, Earth, and Mars. The gas-giant planets include Jupiter, Saturn, Neptune, and Uranus. The size and other properties of Pluto, however, made it unlike either of the terrestrial or gas-giant planets. So, astronomers decided in 2006 that the faraway, frozen planet would no longer be considered a true planet. Pluto's status then changed to "dwarf planet."