Ten things you might not know about Pluto

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A close-up view of the icy mountains and flat ice plains on Pluto in this photo released Sept. 17 (Reuters)

It’s the little planet that was – then wasn’t – and now is again (sort of).

Now officially listed as a “dwarf planet,” icy, tiny Pluto still has not been formally reinstated as the ninth planet in our solar system.

That’s because other, similar worlds (called Kuiper Belt objects) have been discovered lurking out on the far edges of the solar system. If Pluto’s a full planet, then they would have to be, too.

The eyes of humanity were upon it back in July, when NASA’s New Horizons spacecraft took stunning photos in a fly-by, following a nine-and-a-half year, 4.6-billion kilometer journey from Earth. Those photos are being released this month, including a new crop published by NASA just today.

A near-sunset view of the rugged, icy mountains and flat ice plains on Pluto taken by New Horizons. (Reuters)

It may have an identity crisis and be a little misunderstood, but there are still plenty of cool things to know about our tiny, icy fellow cosmic adventurer:

1) Pluto was discovered in 1930, by American astronomer Clyde Tombaugh. His photographic plates revealed a tiny, faint object moving against a background of fixed stars.

2) Pluto is smaller than Earth’s moon, and six other moons throughout the Solar System. The largest of those – Ganymede, which orbits Jupiter, and Saturn’s primary moon, Titan – are bigger than the planet Mercury, as well.

3) Average temperatures are unimaginably frigid, hovering around -230 degrees Celsius. Absolute zero – the coldest anything can ever be, where all atomic movement stops – is -273.

4) Beginning in 1979, Pluto was actually closer to the Sun than the eighth planet, Neptune. On February 11, 1999, Pluto’s elliptical orbit carried it back out past Neptune, where it will remain until the year 2247.

Pluto's largest moon Charon is shown in this NASA handout image, taken on July 14, 2015. (Reuters)

5) New Horizons discovered that Pluto and its largest moon, Charon, appear to be a “binary planet system,” with both bodies orbiting a common point somewhere in between. So far as we know, this is unique in the Solar System.

6) Pluto’s five moons orbit very close to their planet. Charon is less than 20,000 KM away. The distance from Earth to the Moon, by comparison, averages 384,400 KM.

Backlit by the sun, Pluto's atmosphere rings its silhouette like a luminous halo in this image. (NASA)

7) NASA scientists were amazed when New Horizons revealed signs Pluto has haze in its atmosphere. The working theory suggests the haze is caused by methane interacting with ultraviolet sunlight.

8) Pluto is 30 to 40 times farther from the Sun than the Earth is, depending on where it is in its wide elliptical orbit.

9) From Pluto, the Sun appears to be about 250 times as bright as a full Moon viewed from Earth. That sounds fairly bright – until you realize the sun is 400,000 times brighter than the full Moon here.

Eris: The Largest Known Dwarf Planet Credit & Copyright: W. M. Keck Observatory

10) If Pluto were to become a fully recognized planet again, status might also have to be granted to its moon Charon, as well as fellow large Kuiper Belt objects Eris, Makemake, Haumea, Sedna, Quaoar and who knows how many others that haven’t yet been discovered. Whether or not Pluto is a planet, our old nine-planet Solar System will never be again.