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Why Isn't Pluto a Planet

Interactive passage with audio narration, comprehension questions, and printable PDF.

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Format: Interactive (Online), Printable (PDF)
Grades: 5678
Subjects: scienceela
Standards: MS-ESS1-3
Languages: English, Spanish

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What's included

Reading passage

Reading comprehension

Audio narration

With word word highlighting

Comprehension quiz

Auto-graded

Writing activity

Open-ended response

Glossary & flashcards

Vocabulary practice

Differentiated version

Adapted for varied levels

Spanish translation

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About this reader

This engaging 400-500 word science reading passage for grades 6-8 explores why Pluto is no longer classified as a planet. Aligned with NGSS standards MS-ESS1-3 and MS-ESS1.B, the passage examines the 2006 decision by the International Astronomical Union to formalize the planet definition. Students learn about the three criteria objects must meet to be classified as planets: orbiting the Sun, having enough mass to be spherical, and clearing their orbital region. The passage explains how Pluto meets two criteria but shares its orbital space with many Kuiper Belt objects, leading to its reclassification as a dwarf planet. Through cause-and-effect reasoning, students discover how scientific definitions evolve as knowledge expands. The content includes audio integration, vocabulary development with 8-10 key science terms, Spanish translations, differentiated versions for struggling readers, comprehension questions, writing activities, and graphic organizers. This comprehensive resource helps students understand that science is a dynamic process where classifications change based on new evidence and improved understanding of our solar system.

Written by Workybooks TeamPublished by Workybooks

Sample passage and quiz content

CONTENT PREVIEW

Four images from NASA's New Horizons' Long Range Reconnaissance Imager (LORRI) were combined with color data from the Ralph instrument to create this global view of Pluto "Pluto in True Color" / NASA

Pluto is not classified as a planet because it does not meet all three criteria established by the International Astronomical Union in 2006. Scientists decided that a celestial body must fulfill specific requirements to be called a planet. This decision changed how we understand objects in our solar system.

The IAU established three criteria for planet classification. First, an object must orbit the Sun. Second, it must have enough mass for gravity to pull it into a spherical shape. Third, it must have cleared its orbital region of other debris. Pluto meets the first two criteria successfully. It orbits the Sun in an elliptical path. Its gravity has pulled it into a round shape. However, Pluto fails the third criterion because it shares its orbital space with thousands of other objects.

Evidence shows that Pluto resides in the Kuiper Belt, a region beyond Neptune filled with icy bodies. Scientists observe that Pluto's orbit overlaps with many similar-sized objects in this zone. The planet Neptune, in comparison, has cleared nearly all debris from its orbital path. Pluto's mass is too small to gravitationally dominate its neighborhood. This discovery led astronomers to create a new category called dwarf planets. Dwarf planets orbit the Sun and have spherical shapes but have not cleared their orbits.

The reclassification of Pluto demonstrates how scientific definitions evolve as knowledge increases. Before 2006, astronomers lacked a formal definition of what makes a planet. New telescope technology revealed hundreds of Kuiper Belt objects similar to Pluto. Scientists needed clearer categories to organize these discoveries. The IAU's decision was not about Pluto changing but about our understanding improving. This process shows that science adapts when new evidence emerges.

Understanding why Pluto is a dwarf planet matters because it teaches us how science works. Classifications help scientists communicate clearly about celestial objects. As technology improves, we can discover more about distant regions of our solar system. The debate about Pluto reminds us that scientific knowledge grows over time. New observations can lead to better ways of organizing information about our universe.

Interesting Fact: Pluto has five known moons, and its largest moon Charon is so big that Pluto and Charon orbit a point in space between them rather than Charon simply orbiting Pluto.

When did the International Astronomical Union formally define what a planet is?

2000200620101999

How many criteria must a celestial body meet to be classified as a planet?

TwoFourThreeFive

What does it mean for a planet to have 'cleared its orbital region'?

It has moved away from the SunIt has removed most debris from its orbital pathIt has a perfectly circular orbitIt has no moons

Where is Pluto located in our solar system?

Between Mars and JupiterIn the asteroid beltIn the Kuiper Belt beyond NeptuneCloser to the Sun than Earth

Which criterion for planet classification does Pluto NOT meet?

Orbiting the SunHaving a spherical shapeClearing its orbital regionHaving enough mass

Based on the passage, why did scientists create the category of dwarf planets?

To honor Pluto specificallyTo classify objects that orbit the Sun and are spherical but haven't cleared their orbitsTo remove Pluto from astronomy textbooksTo make the solar system smaller

What does the reclassification of Pluto demonstrate about science?

Scientific definitions never changeScientists make mistakes frequentlyScientific definitions evolve as knowledge increasesOld discoveries are always wrong

How does Neptune differ from Pluto in terms of its orbit?

Neptune is closer to the SunNeptune has cleared nearly all debris from its orbital pathNeptune does not orbit the SunNeptune is not spherical

Pluto physically changed in 2006, which is why it lost its planet status.

TrueFalse

The Kuiper Belt contains thousands of icy objects similar to Pluto.

TrueFalse