Why exactly was Pluto kicked out of the group?

Someone submitted the discovery of the 10th planet, and the governing body that decides such things basically said “Pluto should have never really been classified as a planet because it’s way the hell smaller than we originally thought it was, so this 10th ‘planet’ you found, which is roughly the same size as Pluto, it doesn’t qualify either”.

It missed out on Part 2 of the new definition adopted of planethood:

“The committee’s original prime criterion was roundness, meaning that a planet had to be big enough so that gravity would overcome internal forces and squash it into a roughly spherical shape. But a large contingent of astronomers, led by Julio Fernandez of the University of the Republic in Montevideo, Uruguay, has argued that a planet must also be massive enough to clear other objects out of its orbital zone. ”

New York Times

As @FutureMemory said, it was prompted because an astonomer discovered an icy ball even bigger than Pluto much further out. He named it Xena.

That is a sad story for Pluto! It was always my favourite planet. So, hypothetically, if this ice ball was round enough and has enough mass, could it be considered our 9th planet?

No. It is only 6% larger than Pluto.

Also, they weren’t too happy about Pluto’s distance from our sun, and Xena is even further away than Pluto.

Man, these guys are harsh! So how much smaller was Pluto than the minimum mass for a planet?

“And by knowing its mass (0.0021 Earths), they could more accurately gauge its size. The most accurate measurement currently gives the size of Pluto at 2,400 km (1,500 miles) across. Although this is small, Mercury is only 4,880 km (3,032 miles) across. Pluto is tiny, but it was considered larger than anything else past the orbit of Neptune.”

” As planets form, they become the dominant gravitational body in their orbit in the Solar System. As they interact with other, smaller objects, they either consume them, or sling them away with their gravity. Pluto is only 0.07 times the mass of the other objects in its orbit. The Earth, in comparison, has 1.7 million times the mass of the other objects in its orbit.

Any object that doesn’t meet this 3rd criteria is considered a dwarf planet. And so, Pluto is a dwarf planet. There are still many objects with similar size and mass to Pluto jostling around in its orbit. And until Pluto crashes into many of them and gains mass, it will remain a dwarf planet. Eris suffers from the same problem.”

Universe Today

@marinelife Did we have the name wrong? It appears to be named Eris.

@FutureMemory I’m not sure. They seem to have changed the name.

Source

So Zena is now called Eris?

@Schroedes13 Apparently.

Xena was an unofficial name while its’ status was being established. Eris is the official name of the dwarf planet.

The basic reason is that Pluto fits very nicely into an entire class of objects in the outer solar system, the Kuiper Belt Objects — it is more like these objects than it’s like the 8 other planets. For example, if you took Pluto to Earth’s orbit, the heat of the sun would vaporize Pluto. Not so for other planets, which have sturdy, rocky cores.

The other planets have also all “cleared out their neighborhoods.” There isn’t a bunch of debris around the orbits of Earth, Mercury, etc—not so for Pluto, which is flying through an orbit crowded with Kuiper Belt Objects.

Similarly, the asteroid Ceres (the largest asteroid) was once thought to be a planet. But we see that Ceres fits much better into the class of objects called “asteroids,” distinct from planets—because it’s slightly smaller than planets and it hasn’t cleared out its neighborhood. Ceres is actually called a “dwarf planet,” just like Pluto.

A dwarf fortress is still a fortress.
A dwarf planet is still a planet.

Eris and Pluto and Ceres are not called large balls of icy rocks.

@mattbrowne I like your style. I’m still gonna call the poor, little guy a planet.