This post was initially written in 2008, however we created a cool video to go together with it yesterday

Let’s find out why Pluto is no longer considered a planet.

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Pluto was an initial discovered in 1930 through Clyde W. Tombaugh at the Lowell Observatory in Flagstaff Arizona. Astronomers had long predicted that there would certainly be a 9th planet in the Solar System, i m sorry they called Planet X. Just 22 at the time, Tombaugh was provided the productive task of comparing photographic plates. These were two pictures of a region of the sky, taken two weeks apart. Any type of moving object, favor an asteroid, comet or planet, would appear to run from one photograph to the next.


After a year of observations, Tombaugh finally discovered an item in the ideal orbit, and also declared that he had found Planet X. Due to the fact that they had found it, the Lowell team were allowed to name it. They resolved on Pluto, a name argued by an 11-year old college girl in Oxford, England (no, that wasn’t called after the Disney character, but the roman inn god of the underworld).

The Solar system now had actually 9 planets.

Astronomers weren’t sure around Pluto’s mass till the discovery of its largest Moon, Charon, in 1978. And by discovering its mass (0.0021 Earths), they could much more accurately gauge that size. The most accurate measurement currently gives the dimension of Pluto at 2,400 km (1,500 miles) across. Although this is small, Mercury is just 4,880 kilometres (3,032 miles) across. Pluto is tiny, but it was considered larger 보다 anything else previous the orbit of Neptune.


Over the last couple of decades, powerful brand-new ground and also space-based observatories have fully changed previous knowledge of the external Solar System. Rather of gift the only planet in that region, prefer the remainder of the Solar System, Pluto and also its moons space now known to be simply a huge example that a arsenal of objects referred to as the Kuiper Belt. This an ar extends indigenous the orbit that Neptune out to 55 astronomical units (55 time the distance of the earth to the Sun).

Astronomers estimate that there room at the very least 70,000 icy objects, through the exact same composition together Pluto, that measure 100 km throughout or more in the Kuiper Belt. And also according to the brand-new rules, Pluto is not a planet. It’s just an additional Kuiper Belt object.

Here’s the problem. Astronomers had been turning up larger and larger objects in the Kuiper Belt. 2005 FY9, discovered by Caltech astronomer Mike Brown and his team is only a tiny smaller than Pluto. And there room several other Kuiper Belt objects in that very same classification.


Astronomers realized that it was only a matter of time before an object larger 보다 Pluto was discovered in the Kuiper Belt.

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And in 2005, Mike Brown and also his team reduce the bombshell. Lock had found an object, further out 보다 the orbit the Pluto the was probably the exact same size, or even larger. Officially called 2003 UB313, the object was later on designated as Eris. Due to the fact that its discovery, astronomers have identified that Eris’ size is roughly 2,600 km (1,600 miles) across. It additionally has around 25% more mass 보다 Pluto.

With Eris gift larger, made of the very same ice/rock mixture, and an ext massive than Pluto, the principle that we have nine planets in the Solar System began to loss apart. What is Eris, world or Kuiper Belt Object; what is Pluto, for that matter? Astronomers decided they would make a last decision around the an interpretation of a planet at the XXVIth general Assembly of the International huge Union, i beg your pardon was organized from august 14 to respectable 25, 2006 in Prague, Czech Republic.


Astronomers native the association were provided the chance to vote on the an interpretation of planets. One variation of the definition would have actually actually enhanced the number of planets to 12; Pluto to be still a planet, and so to be Eris and also even Ceres, which had actually been assumed of as the biggest asteroid. A various proposal maintained the complete at 9, specifying the planets as just the acquainted ones we understand without any type of scientific rationale, and a 3rd would fall the variety of planets down to 8, and also Pluto would certainly be the end of the earth club. But, then… what is Pluto?

In the end, astronomers voted for the controversial decision the demoting Pluto (and Eris) down to the recently created category of “dwarf planet”.

Is Pluto a planet? Does that qualify? For an item to it is in a planet, it requirements to fulfill these 3 requirements identified by the IAU:


It needs to be in orbit about the Sun – Yes, so possibly Pluto is a planet.It demands to have enough gravity to pull itself right into a spherical shape – Pluto…checkIt demands to have actually “cleared the neighborhood” that its orbit – Uh oh. Here’s the dominance breaker. Follow to this, Pluto is no a planet.

What walk “cleared that neighborhood” mean? as planets form, they come to be the leading gravitational body in your orbit in the Solar System. As they interact with other, smaller objects, they either consume them, or sling lock away with their gravity. Pluto is just 0.07 times the mass of the other objects in that is orbit. The Earth, in comparison, has actually 1.7 million time the mass of the various other objects in that orbit.

Any object the doesn’t satisfy this 3rd criteria is taken into consideration a dwarf planet. And so, Pluto is a dwarf planet. There space still many objects with similar size and also mass come Pluto jostling around in its orbit. And also until Pluto crashes into countless of them and gains mass, the will stay a dwarf planet. Eris suffers native the very same problem.

It’s not difficult to imagine a future, though, wherein astronomers discover a huge enough object in the remote Solar mechanism that could qualify because that planethood status. Climate our Solar device would have 9 planets again.


Even despite Pluto is a dwarf planet, and no longer officially a planet, it’ll still it is in a fascinating target for study. And also that’s why NASA has sent their new Horizons spacecraft off to visit it. Brand-new Horizons will reach Pluto in July 2015, and capture the first close-up pictures of the (dwarf) planet’s surface.Space enthusiasts will certainly marvel in ~ the beauty and remoteness of Pluto, and the ache deplaneting memories will certainly fade. We’ll just be able to appreciate it as Pluto, and also not worry how come categorize it. At the very least now you recognize why Pluto to be demoted.

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If you’d like more information around Pluto, we did two podcasts top top this topic in ~ Astronomy Cast. The first discusses the IAU’s decision, and also the second is around Pluto and the Icy external Solar System. Check them out.