Ever since the discovery of Pluto in 1930, children grew up learning about the nine planets of our solar system. That all changed starting in the late 1990s, when astronomers began to argue about whether Pluto was a planet. In a highly controversial decision, the International Astronomical Union ultimately decided in 2006 to call Pluto a “dwarf planet,” reducing the list of “real planets” in our solar system to eight.
However, astronomers are now hunting for another planet in our solar system, a true ninth planet, after evidence of its existence was unveiled on Jan. 20, 2016. The so-called “Planet Nine,” as scientists are calling it, is about 10 times the mass of Earth and 5,000 times the mass of Pluto. [The Evidence for ‘Planet Nine’ in Our Solar System (Gallery)]
Did You Know? Three of the planets in the solar system are currently visible in the night sky in June 2017. They are: Venus, Jupiter and Saturn. You can find out how and when to see the brightest planets in June’s night sky here: The Brightest Planets in June’s Night Sky: How to See them (and When)
What is a Planet?
This seemingly simple question doesn’t have a simple answer. Everyone knows that Earth, Mars and Jupiter are planets. At least, they are for now. Both Pluto and Ceres were once considered planets until new discoveries triggered scientific debate about how to best describe them—a vigorous debate that continues to this day. The most recent definition of a planet was adopted by the International Astronomical Union in 2006. It says a planet must do three things:
- It must orbit a star (in our cosmic neighborhood, the Sun).
- It must be big enough to have enough gravity to force it into a spherical shape.
- It must be big enough that its gravity cleared away any other objects of a similar size near its orbit around the Sun.
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