Since 2006, the International Astronomical Union has classified Pluto as a:

In August 2006, the International Astronomical Union (IAU), meeting in Prague, defined the term "planet" for the first time. Under this definition, Pluto was reclassified as a dwarf planet.

The IAU definition of a planet:

A planet is a celestial body that:

• Orbits the Sun.
• Has enough mass for its own gravity to pull it into a nearly round shape.
• Has "cleared the neighbourhood" around its orbit, meaning it is the dominant body in its orbital zone.
• A dwarf planet meets the first two conditions but not the third.

Why Pluto failed the test:

• Pluto orbits in a crowded region of the outer solar system, among many other icy bodies.
• It has not cleared its orbit of these objects.
• The discovery of Eris in 2005, a body similar in size to Pluto, forced astronomers to decide whether to call many such objects planets or create a new category.
• After the decision, the solar system officially has eight planets.

About Pluto:

• Pluto was discovered on 18 February 1930 by Clyde Tombaugh at the Lowell Observatory in Arizona, USA.
• It is smaller than the Earth's Moon.
• It takes about 248 Earth years to complete one orbit of the Sun.
• Its orbit is highly elongated and tilted, and for part of its orbit it comes closer to the Sun than Neptune does.
• It has a thin atmosphere of nitrogen, methane and carbon monoxide.

New Horizons mission:

• NASA's New Horizons spacecraft, launched in 2006, flew past Pluto in July 2015.
• It sent the first close-up images, showing a large heart-shaped plain called Tombaugh Regio.

Pluto's status today:

• It is officially listed as minor planet number 134340.
• The decision remains debated by some scientists, but the IAU definition is the official one.

Why not the other options?

• ✗Pluto is no longer counted among the major planets.
• ✗Pluto is not a moon of Neptune; it orbits the Sun directly.
• ✗A comet is an icy body that develops a glowing tail near the Sun; Pluto is not a comet.