This Day in Tech History

On This Day . . .

The Wonder Dog


February 18, 1930

Pluto is Discovered

Pluto, once believed to be the ninth planet, is discovered at the Lowell Observatory in Flagstaff, Arizona, by astronomer Clyde W. Tombaugh.


The existence of an unknown ninth planet was first proposed by Percival Lowell, who theorized that wobbles in the orbits of Uranus and Neptune were caused by the gravitational pull of an unknown planetary body. Lowell calculated the approximate location of the hypothesized ninth planet and searched for more than a decade without success. However, in 1929, using the calculations of Powell and W.H. Pickering as a guide, the search for Pluto was resumed at the Lowell Observatory in Arizona. On February 18, 1930, Tombaugh discovered the tiny, distant planet by use of a new astronomic technique of photographic plates combined with a blink microscope.

With a surface temperature estimated at approximately -360 Fahrenheit, Pluto was appropriately given the Roman name for the god of the underworld in Greek mythology. Pluto’s average distance from the sun is nearly four billion miles, and it takes approximately 248 years to complete one orbit. It also has the most elliptical and tilted orbit of any planet, and at its closest point to the sun it passes inside the orbit of Neptune, the eighth planet.


Image above: The artist’s concept above shows the Pluto system from the surface of one of the candidate moons. The other members of the Pluto system are just above the moon’s surface. Pluto is the large disk at center, right. Charon, the system’s only confirmed moon, is the smaller disk to the right of Pluto. The other candidate moon is the bright dot on Pluto’s far left. Image credit: NASA, ESA and G. Bacon (STScI)

Pluto is but a dot of light in even the largest Earth-based telescopes. Pluto is 2/3 the size of Earth’s moon but 1,200 times farther away, which makes viewing surface detail as difficult as trying to read the printing on a golf ball located thirty-three miles away.

Back Row: Jupiter, Saturn Middle Row: Uranus, Neptune Front Row: Earth, Venus, Mars, Mercury, Pluto

Back Row: Jupiter, Saturn Middle Row: Uranus, Neptune Front Row: Earth, Venus, Mars, Mercury, Pluto

After its discovery, some astronomers questioned whether Pluto had sufficient mass to affect the orbits of Uranus and Neptune. In 1978, James Christy and Robert Harrington discovered Pluto’s only known moon, Charon, which was determined to have a diameter of 737 miles to Pluto’s 1,428 miles. Together, it was thought that Pluto and Charon formed a double-planet system, which was of ample enough mass to cause wobbles in Uranus’ and Neptune’s orbits. In August 2006, however, the International Astronomical Union announced that Pluto would no longer be considered a planet, due to new rules that said planets must “clear the neighborhood around its orbit.” Since Pluto’s oblong orbit overlaps that of Neptune, it was disqualified.

Is Pluto a planet? Does it qualify? For an object to be a planet, it needs to meet these three requirements defined by the IAU:

  • It needs      to be in orbit around the Sun      – Yes, so maybe Pluto is a planet.
  • It needs      to have enough gravity to pull itself into a spherical shape – Pluto…check
  • It needs      to have “cleared the neighborhood” of its orbit – Uh oh. Here’s the rule breaker. According to      this, Pluto is not a planet.


Even though Pluto is a dwarf planet, and no longer officially a planet, it’ll still be a fascinating target for study. And that’s why NASA has sent their New Horizons spacecraft off to visit it. New Horizons will reach Pluto in July 2015, and capture the first close-up images of the (dwarf) planet’s surface.


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