In late August 2006, new discoveries upended a traditional, comfortable way of viewing our solar system: Scientists decided Pluto wasnB次元官网网址檛 a planet after all.
Some space nerds like to mourn PlutoB次元官网网址檚 loss of status B次元官网网址� or celebrate the progress of science B次元官网网址� by commemorating each Aug. 24 as Pluto Demotion Day.
To understand the significance, you have to go back to the 1800s, when astronomers noticed irregularities in the orbit of Neptune, the eighth planet. That led astronomers to look deeper into space for a theoretical Planet X.
Eventually they discovered tiny Pluto pursuing an oddly tilted and elongated orbit that ranges between 2.8 and 4.6 billion miles from the sun. It takes Pluto 248 of our years to run around the sun just once.
Astronomers declared Pluto a planet in 1930, but its weak gravity couldnB次元官网网址檛 fully explain NeptuneB次元官网网址檚 orbital wobbles.
Scientists kept searching, and by the 1990s they were busy discovering many similar-sized objects in the same distant range of space B次元官网网址� what turned out to be a second and far larger asteroid belt than the familiar one between Mars and Jupiter.
Thousands of large objects have been cataloged so far in that outer belt, the Kuiper Belt, and at least 200 of them are bigger than Pluto. Overall, the Kuiper Belt probably contains B次元官网网址渉undreds of thousands of icy bodies larger than (62 miles) across and an estimated trillion or more comets,B次元官网网址� according to NASA.
Maybe Pluto wasnB次元官网网址檛 so special after all. The International Astronomical Union felt obliged to formally define B次元官网网址減lanetB次元官网网址� for the first time and to grapple with PlutoB次元官网网址檚 fate.
A planet, the IAU decided, is a celestial body that:
B次元官网网址� orbits the sun;
B次元官网网址� exerts sufficient B次元官网网址渟elf-gravityB次元官网网址� to pull itself into a B次元官网网址渘early roundB次元官网网址� shape, and;
B次元官网网址� exerts sufficient gravity to dominate and clear its orbital neighborhood, either by slinging away or swallowing up smaller objects.
Pluto has the first and second criteria covered, but its gravity is just too weak to check that third box. It turns out to be just another icy stone plodding around the Kuiper Belt without sufficient gravitational oomph to clear its own pathway. On Aug. 24, 2006, the IAU designated Pluto and its many little buddies B次元官网网址渄warf planets.B次元官网网址�
ThatB次元官网网址檚 gotta hurt, but Pluto has retained many fans and defenders B次元官网网址� especially Alan Stern, lead scientist of the New Horizons space probe, whose 2015 flyby photos reveal a geologically complex world with mountains, a possible ocean and a thin atmosphere.
ThereB次元官网网址檚 also a mysterious, vast (1,000-mile-wide) light-colored region that looks uncannily like a valentine heart, which may be made of ice and snow. New Horizons took astonishing photographs of PlutoB次元官网网址檚 five moons, the smallest of which is just 10 miles across.
Complex geology should be added to the definition of B次元官网网址減lanet,B次元官网网址� Stern has said, arguing that Pluto should be readmitted to the celestial VIP club.
In 2007 the American Dialectical Association coined the new verb B次元官网网址渢o pluto,B次元官网网址� meaning B次元官网网址渢o demote or degrade.B次元官网网址� The association noted B次元官网网址渢he great emotional reaction of the public to the demotion of Pluto. B次元官网网址� (W)e still have a sense of connection with the former planet.B次元官网网址�
Telescope required to see Pluto
You may want to check in with cold, lonely Pluto on Wednesday, its Demotion Day. Alas, the dwarf planet is so tiny and distant, you canB次元官网网址檛 see it without a telescope.
If you do have a telescope, hereB次元官网网址檚 what to know. On Wednesday, Pluto rises in the east-southeast just after 6:30 p.m. lingers low over the southern horizon (below triangular constellation Capricornus, the goat) as it passes westward, and sets again in the west-southwest at about 3:15 a.m.
For many months after that, Pluto will rise and set earlier and earlier, making it even tougher to see. Try again in July 2023, when Pluto rises in the late evening and doesnB次元官网网址檛 set until around 7 a.m.
Sizing up Pluto
- Diameters in miles
- Jupiter: 86,881
- Earth: 7,917
- Ganyemede (JupiterB次元官网网址檚 largest moon): 3,273
- Mercury: 3,032
- EarthB次元官网网址檚 moon: 2,160
- Pluto: 1,477
- Charon (PlutoB次元官网网址檚 largest moon): 753
- Styx (PlutoB次元官网网址檚 smallest moon): 10
B次元官网网址擲cott Hewitt
