Pluto — Dwarf Planet Reading Comprehension Lesson

Reading Passage

Pluto — Dwarf Planet

Pluto is a small world in the outer reaches of our solar system, located in a region known as the Kuiper Belt — a band of icy bodies beyond the orbit of Neptune. For 76 years, from its discovery in 1930 until 2006, Pluto was classified as the ninth planet in our solar system. Then, in a decision that surprised and upset many people (especially children), the International Astronomical Union voted to reclassify Pluto as a ‘dwarf planet.’ This decision sparked debate among scientists and the public, and Pluto’s status remains a cultural touchstone for discussions about how science classifies the natural world.

Pluto was discovered on February 18, 1930, by 24-year-old American astronomer Clyde Tombaugh at the Lowell Observatory in Flagstaff, Arizona. He found it by comparing photographs of the night sky taken on different days, looking for a tiny point of light that had moved — a sign of a nearby solar system object. The discovery made international headlines; it was the first new solar system body found in 84 years. The name ‘Pluto’ was suggested by 11-year-old Venetia Burney of Oxford, England, who thought the name of the Roman god of the underworld was fitting for such a cold, dark, distant world. The name also happened to start with the initials P.L. — for Percival Lowell, the astronomer who had originally predicted a ninth planet’s existence.

Pluto orbits the Sun at an average distance of about 3.67 billion miles — about 39 times farther from the Sun than Earth is. Its orbit is much more elliptical (oval-shaped) than most planets, and it is also tilted compared to the plane of the solar system. A Pluto year — one complete orbit around the Sun — takes about 248 Earth years. Pluto is tiny: its diameter is only about 1,477 miles (2,377 km), smaller than the United States is wide. Despite its small size, Pluto has five known moons. The largest, Charon, is so big relative to Pluto (about half of Pluto’s diameter) that astronomers sometimes call the two a ‘double dwarf planet’ system, as they orbit their common center of gravity in space rather than Charon simply orbiting Pluto.

In 2006, the International Astronomical Union defined a ‘planet’ as an object that (1) orbits the Sun, (2) has enough mass for gravity to make it approximately spherical, and (3) has ‘cleared the neighborhood’ around its orbit of other bodies. Pluto fails the third criterion — it shares its orbital zone with many other Kuiper Belt objects. Therefore, Pluto was reclassified as a ‘dwarf planet,’ a new category that includes Pluto, the asteroid Ceres, and several other large Kuiper Belt objects. Many scientists, including the principal investigator of the New Horizons mission, Alan Stern, disputed this reclassification as scientifically flawed. The debate continues.

On July 14, 2015, NASA’s New Horizons spacecraft — which had been traveling for nine and a half years after its launch in 2006 — flew past Pluto at a distance of about 7,800 miles, becoming the first spacecraft ever to visit Pluto. The images and data it sent back astonished scientists. Instead of a featureless, icy ball, New Horizons revealed a world of stunning complexity: a giant heart-shaped plain (named Tombaugh Regio in honor of Pluto’s discoverer), towering mountains of water ice, possible cryovolcanoes (ice volcanoes), hazes in Pluto’s thin atmosphere, and a surface with varied colors and textures. Far from being just a boring ice ball, Pluto turned out to be a geologically active world with a rich and surprising story to tell.

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