Science Lesson Plan · Grades 4–6

Pluto, the Dwarf Planet Lesson

Teach students about Pluto and its reclassification as a dwarf planet. The lesson explains what the International Astronomical Union decided in 2006, why Pluto no longer meets the definition of a planet, and what makes it a dwarf planet. Covers Pluto’s size, rock-and-ice composition, unusual elliptical orbit, and its location in the Kuiper Belt beyond Neptune. Includes comprehension questions in multiple-choice format and an answer key.

Lesson Overview

Pluto and the Definition of a Planet

The lesson explains the three criteria the IAU uses to define a planet — orbiting the sun, having enough mass to be roughly spherical, and clearing the neighborhood around its orbit. Pluto meets the first two but not the third, which is why it was reclassified as a dwarf planet.

The four-page lesson pairs a reading passage with multiple-choice comprehension questions. Students learn about the Kuiper Belt, Pluto’s 248-year orbit, and how its unusual tilted orbit sometimes brings it closer to the sun than Neptune. A complete answer key is included.

How to Use This Lesson

1. Read the lesson passage together or have students read silently (pages 1–2). Ask students to explain in their own words why Pluto is no longer a planet.

2. Discuss the three-part definition of a planet from the IAU. Have students apply each criterion to Pluto and explain which one it fails.

3. Assign the comprehension questions (page 3) independently. Review answers using the key on page 4.

Printable Resource

A classroom-ready PDF with everything in one printable resource.

Keep Exploring Science