The Discovery of Pluto: A not quite 8th planet

The planet Neptune wobbled in its orbit around the Sun. That could only mean one thing, astronomers said: There was a ninth planet out there, somewhere, lurking in the fringes of the solar system.

Astronomers would spend decades searching in vain for the missing planet. But then, success: by a 24-year-old farmboy from Kansas, on Feb. 18, 1930 — 95 years ago today.

Evidence Of A Ninth Planet ... But No One Could Find It

In the 1840s, French astronomer Urbain Le Verrier noticed the outermost planet of our solar system at the time, Uranus, wobbled a bit in its orbit. He reasoned that there must be another planet out there affecting Uranus’ orbit. He did the math to predict where a new planet might be found.

That planet would be found on Sept. 23, 1846 and named Neptune.

In 1906, astronomer and philanthropist Percival Lowell, noticed similar oddities to Neptune‘s orbit. That could only mean there was a ninth planet out there, he reasoned. Lowell referred to it as Planet X and worked with other astronomers and mathematicians to calculate where it might be found.

Lowell and his team began the search for Planet X in the observatory he built near Flagstaff, Arizona. He bought powerful new telescopes and an astrograph — a telescope equipped with a camera — for his facility and began using a blink comparator that would allow astronomers to compare photos taken days apart to hunt for objects that move.

Lowell searched for Planet X until his death in 1916. Meanwhile, a boy in Kansas named Clyde Tombaugh had become obsessed with astronomy and the writings of astronomers such as Lowell and his studies and theory about the possibility of canals on the surface of Mars.

Tombaugh at his family's farn in Kansas with his homeade telescope in 1928. Source: WikiMedia Commons

Tombaugh built the first of several homemade telescopes to observe the stars and planets. In 1928 — at age 22 — Tombaugh read about the Lowell Observatory’s research on Mars and sent some of his own sketches of the surface of Mars to the astronomers there. Impressed with his work, the Lowell Observatory hired Tombaugh to do manual labor around the facility — shoveling coal and snow and giving tours to guests — to raise money for his college fund.

Eventually, Tombaugh joined the search for Planet X. He would use the Observatory’s astrograph to photograph parts of the sky where Planet X was calculated to be. And then he’d go back later and use the blink comparator to compare the photos.

On Feb. 18, 1930, Tombaugh spotted an object in the blink comparator that met the expected motion and magnitude of what they had expected Planet X to be. He had taken the photos the previous month.

Tombaugh — at this point, 24 years old — alerted his supervisors. They alerted other astronomers, asking them to confirm Tombaugh’s discovery. They did, and the discovery of Planet X was announced on March 13, 1930.

An 11-year-old English schoolgirl, Venetia Burney, suggested the name Pluto, the god of the underworld.

Tombaugh’s newfound fame earned him a scholarship to the University of Kansas, where he earned bachelor’s and master’s degrees. He’d teach at Northern Arizona University and New Mexico State University.

Tombaugh would go on to discover nearly 800 asteroids and a comet. He died in 1997 at age 90.

Tombaugh's photos that led to the discovery of Planet X.

Why Is Pluto No Longer A Planet?

Pluto never really fit in with the other planets in the solar system. Its orbit was out of plane with the other planets. From 1979 to 1999, it was closer to the sun than Neptune.

When other bodies were found in the solar system that had similar issues, the International Astronomical Union realized it had to redefine what was a planet.

The new criteria to be considered a planet:

1. It must be in an orbit around the sun

2. It must have sufficient mass to assume hydrostatic equilibrium. In other words, it must be round - not lumpy, like many asteroids.

3. It must have "cleared out its neighborhood" by being the dominant gravitational force in its vicinity.

Pluto failed to meet that third criterion, so it was reclassified as a “dwarf planet” on Aug. 24, 2007.

Critics point out that by that criterion, the Earth is also not a planet: We constantly observe close calls with asteroids in and near Earth orbit.

Who Demoted Pluto?

Mike Brown, professor of planetary astronomy at Caltech in Pasadena, California, gets the credit for demoting the poor little planet.

Brown’s team discovered a dwarf planet, Eris, in January 2005. At the time, it appeared to be larger than Pluto. If Pluto hadn’t been demoted, Brown says, hundreds of celestial objects would have to be considered planets.

So Brown led the charge to reclassify Pluto — despite the fact that it cost him the chance to go down in history as a planet discoverer himself.

Brown’s Twitter feed was @plutokiller. He titled his autobiography “How I Killed Pluto and Why It Had It Coming.”

“I thought it was funny to talk about ‘killing Pluto’ because Pluto was god of the underworld,” Brown says. “But no one really picked up on that.”

What We've Learned About Pluto

We’ve learned a lot about poor, poor little Pluto — thanks to NASA’s New Horizons probe, which visited the dwarf planet in 2015.

1 - Instead of an ancient, inert rock, we’ve found an icy world with active processes creating fresh surfaces. The rugged surface of Pluto is younger than the Appalachian Mountains. One mountain chain near Pluto’s equator is more than 2 miles tall.

2 - We knew Pluto was covered by ice. What we didn’t know: It’s covered with four types of ice — methane, nitrogen, carbon monoxide and water. Each go through freezing and precipitation cycles. Much of the water ice is beneath the surface.

3 - Pluto is covered by a thin, blue soot-like haze. Scientists think it’s created by sunlight interacting with nitrogen and methane in the atmosphere.

5 - Pluto has an ionized tail like a comet — probably from nitrogen from its atmosphere leaking into the solar wind. Up to 500 tons of nitrogen escape the planet every hour. The tail extends for tens of thousands of miles.

6 - The mass of Charon is so large — and the mass of Pluto is so small — that Charon doesn’t actually orbit the dwarf planet. Instead, both orbit a fixed place in space between them.