At the end of August 2006, new discoveries upset a traditional and comfortable way of seeing our solar system: scientists decided that Pluto was not a planet after all.
Some space nerds like to mourn Pluto’s loss of status — or celebrate advancements in science — by commemorating Pluto Demotion Day every August 24.
To understand its significance, you have to go back to the 1800s, when astronomers noticed irregularities in the orbit of Neptune, the eighth planet. This led astronomers to search deeper in space for a theoretical Planet X.
Eventually, they discovered tiny Pluto pursuing a strangely inclined and elongated orbit that is between 2.8 and 4.6 billion kilometers from the sun. It takes Pluto 248 of our years to circle the sun just once.
Astronomers said Pluto was a planet in 1930, but its low gravity couldn’t fully explain Neptune’s orbital wobbles.
Scientists kept searching, and by the 1990s they were busy discovering many similarly sized objects in the same deep space – what turned out to be a second asteroid belt much larger than the one that was. familiar between Mars and Jupiter.
Thousands of large objects have been cataloged so far in this outer belt, the Kuiper Belt, and at least 200 of them are larger than Pluto. Overall, the Kuiper Belt likely contains “hundreds of thousands of icy bodies over (62 miles) in diameter and about a trillion or more comets,” according to NASA.
Maybe Pluto wasn’t so special after all. The International Astronomical Union felt compelled to formally define “planet” for the first time and address the fate of Pluto.
A planet, decided the IAU, is a celestial body which:
— revolves around the sun;
— exerts sufficient “self-gravity” to put itself in an “almost round” shape, and;
— exerts sufficient gravity to dominate and clear its orbital neighborhood, either by throwing or engulfing smaller objects.
Pluto covers both the first and second criteria, but its gravity is simply too low to tick that third box. It turns out to be just another icy rock trudging around the Kuiper Belt without enough gravitational force to push through. On August 24, 2006, the IAU designated Pluto and its many little buddies “dwarf planets”.
It must hurt, but Pluto has held back many fans and advocates, especially Alan Stern, lead scientist on the New Horizons space probe, whose 2015 flyby photos reveal a geologically complex world with mountains, a possible ocean and a thin atmosphere.
There’s also a mysterious, vast, light-colored region (1,000 miles wide) that looks suspiciously like a Valentine’s Heart, which can be made of ice and snow. New Horizons took stunning photographs of Pluto’s five moons, the smallest of which is just 10 miles across.
Complex geology should be added to the definition of “planet,” Stern said, arguing that Pluto should be readmitted to the celestial VIP club.
In 2007, the American Dialectical Association coined the new verb “pluto”, which means “to demote or degrade”. The association noted “the public’s great emotional reaction to Pluto’s retrograde. … (W)e still have a sense of connection to the old planet.
Telescope needed to see Pluto
You may want to check in with cold and lonely Pluto on Wednesday, its retrograde day. Alas, the dwarf planet is so small and so distant that it cannot be seen without a telescope.
If you have a telescope, here’s what you need to know. On Wednesday, Pluto rises in the east-southeast just after 6:30 p.m., lingers low on the southern horizon (beneath the triangular constellation of Capricorn, the Goat) as it crosses west, and sets again in the west-southwest around 3:15.
For many months after that, Pluto will rise and set earlier and earlier, making it even harder to see. Try again in July 2023, when Pluto rises in the late evening and does not set until around 7 a.m.
- Diameters in miles
- Jupiter: 86,881
- Land: 7,917
- Ganyemede (Jupiter’s largest moon): 3,273
- Mercury: 3,032
- Earth’s Moon: 2,160
- Pluto: 1,477
- Charon (largest moon of Pluto): 753
- Styx (the smallest moon of Pluto): 10
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