On a cold winter night, a 24-year-old Kansas farmer named Clyde Tombaugh discovered a new world. Working at the Lowell Observatory, he had been comparing photographic plates of the night sky, hunting for a "Planet X" that astronomers believed was tugging on Neptune's orbit. After months of patient searching, he found a small dot that shifted between two photographs taken six days apart.
The new world was named Pluto, after the Roman god of the underworld. For 76 years, every textbook taught that our solar system had nine planets. Pluto was the smallest, the coldest, and the most distant — a tiny icy world taking 248 Earth years to orbit the Sun once.
But Pluto was always strange. Its orbit is tilted at 17 degrees from the path of the other planets. It is so eccentric that for 20 years of every orbit, Pluto is actually closer to the Sun than Neptune. And it is unbelievably tiny — smaller, in fact, than Earth's own Moon. By the 1990s, astronomers were beginning to whisper a question nobody wanted to ask: was Pluto really a planet at all?
In 2005, astronomer Mike Brown and his team at Caltech announced the discovery of a new world far beyond Pluto. They named it Eris, after the Greek goddess of discord — and discord is exactly what it caused. Early measurements suggested Eris was even larger than Pluto.
Suddenly astronomers had a problem. If Pluto was a planet, then Eris had to be the tenth. And what about all the other large icy objects being found in a region called the Kuiper Belt? Were there going to be twenty planets? Fifty?
In August 2006, the International Astronomical Union met in Prague and voted on a new definition. To be a planet, an object must do three things: orbit the Sun, be massive enough that its own gravity pulls it into a round shape, and have "cleared the neighborhood" around its orbit. Pluto passes the first two tests. But it shares its space with thousands of Kuiper Belt objects, so it fails the third. By a show of hands, Pluto was reclassified as a dwarf planet. Mike Brown himself wrote a book titled How I Killed Pluto and Why It Had It Coming.
Question 3 · 1 pt
Pluto failed the third rule, so…
Question 4 · 1 pt · Unscramble the sentence
Tap a word in the bank, then tap a numbered slot to place it. Correctly placed words turn green.
Question 5 · 1 pt · Unscramble the sentence
Question 6 · 1 pt · Sentence Expansion
Astronomers studied Pluto.
Rewrite the sentence so it answers all three prompts:
This simulation shows Pluto orbiting the Sun inside the Kuiper Belt — a region packed with thousands of icy objects. Use the toggle to compare it to Earth's "clean" orbital path.
Tracking: Pluto
SunPlutoNeptuneKuiper Belt Objects (60+)EarthMercury · Venus · Mars
Question 7 · 1 pt
After watching the simulation, I can see Pluto failed the third rule of planethood because…
Before you classify, look at the worlds you are about to compare. Each body is drawn to scale relative to the others. The Sun is not to scale — it would be hundreds of times wider than Jupiter.
Find the dashed rings — that's where Pluto, Eris, and the Moon are. They really are that small.
ESRT — page 15 · Solar System Data. The eight planets and their orbital characteristics are listed there. Pluto and Eris are not on the ESRT — that's the lesson.
Scale Drawing · Each Body Proportional to the Others
Body diameters are drawn to scale relative to each other (1 px ≈ 1,430 km). Sun and orbital distances are not to scale. Notice how tiny Pluto and Eris are compared to even Earth's Moon.
The graph below shows the mass of four solar system bodies. Because Earth is so much more massive than the others, the y-axis uses a logarithmic scale — each step is ten times bigger than the one below it.
Mass of four solar system bodies (kg)
Graph Question 1 · 1 pt
Based on the bar graph, which statement is best supported?
APluto is more massive than Earth's Moon.
BEarth's Moon is more massive than both Pluto and Eris.
CEris is more massive than Earth.
DPluto and Earth have approximately equal mass.
Graph Question 2 · 1 pt
Approximately how many times more massive is Earth than Pluto?
Tap an object in the word bank to select it (it will turn red), then tap the category zone where it belongs. Tap a placed object to send it back to the bank.
Note: Pluto and Eris are not included on the current ESRT. They were reclassified as dwarf planets by the IAU in 2006. · Press Esc or click outside to close.