Why is Pluto no longer considered a planet?

Why is Pluto no longer considered a planet?

• A. It has too much junk in its orbit
• B. It is too far away
• C. It is not large enough
• D. It orbits a dwarf star

Answer: (A)

The idea being tested is what defines a true planet. A planet must do three things: orbit the Sun, be large enough to be rounded by its own gravity, and clear its orbital neighborhood of other debris. Pluto satisfies orbiting the Sun and being roughly spherical, but it has not cleared its path around the Sun. In the region where Pluto travels—the Kuiper Belt—there are many other objects of similar size, and Pluto hasn’t dominated or swept away those neighboring bodies. That lack of clearing is why it’s classified as a dwarf planet rather than a full planet. The phrase “too much junk in its orbit” captures that idea: there’s too much other material in Pluto’s orbital zone for it to be considered a planet. The other options don’t fit because distance from the Sun, size alone, or orbiting a star other than the Sun aren’t the defining criteria for planethood, and Pluto certainly orbits the Sun.

The idea being tested is what defines a true planet. A planet must do three things: orbit the Sun, be large enough to be rounded by its own gravity, and clear its orbital neighborhood of other debris. Pluto satisfies orbiting the Sun and being roughly spherical, but it has not cleared its path around the Sun. In the region where Pluto travels—the Kuiper Belt—there are many other objects of similar size, and Pluto hasn’t dominated or swept away those neighboring bodies. That lack of clearing is why it’s classified as a dwarf planet rather than a full planet. The phrase “too much junk in its orbit” captures that idea: there’s too much other material in Pluto’s orbital zone for it to be considered a planet. The other options don’t fit because distance from the Sun, size alone, or orbiting a star other than the Sun aren’t the defining criteria for planethood, and Pluto certainly orbits the Sun.