The biggest victim of corporal restructuring ever: Pluto

Last week we lost a planet. Not that we misplaced it or anything like that, it was just 'downgraded' to a lower status than the other eight planets in the Solar System:

Astronomers have voted to strip Pluto of its status as a planet.

About 2,500 scientists meeting in Prague have adopted historic new guidelines that see the small, distant world demoted to a secondary category.

The researchers said Pluto failed to dominate its orbit around the Sun in the same way as the other planets.

The International Astronomical Union's (IAU) decision means textbooks will now have to describe a Solar System with just eight major planetary bodies.

Pluto, which was discovered in 1930 by the American Clyde Tombaugh, will be referred to as a "dwarf planet".

I have to say that it's partially sad, but definitely warranted. Pluto is so small and its orbit so different from all the other planets' that it couldn't really be considered a planet, especially if you consider that a larger object has recently been discovered farther away.

Here's a scientist's take:

"I have a slight tear in my eye today, yes; but at the end of the day we have to describe the Solar System as it really is, not as we would like it to be," said Professor Iwan Williams, chair of the IAU panel that has been working over recent months to define the term "planet".

The need for a strict definition was deemed necessary after new telescope technologies began to reveal far-off objects that rivalled Pluto in size.

Without a new nomenclature, these discoveries raised the prospect that textbooks could soon be talking about 50 or more planets in the Solar System.

And that would have been preposterous. So this is the new definition of a planet:

The scientists agreed that for a celestial body to qualify as a planet:
* it must be in orbit around the Sun
* it must be large enough that it takes on a nearly round shape
* it has cleared its orbit of other objects

And this is why Pluto is suddenly out:

Pluto was automatically disqualified because its highly elliptical orbit overlaps with that of Neptune. It will now join a new category of dwarf planets.
[...]
Pluto's status has been contested for many years. It is further away and considerably smaller than the eight other "traditional" planets in our Solar System. At just 2,360km (1,467 miles) across, Pluto is smaller even than some moons in the Solar System.

Its orbit around the Sun is also highly tilted compared with the plane of the big planets.
[...]
The critical blow for Pluto came with the discovery three years ago of an object currently designated 2003 UB313. After being measured with the Hubble Space Telescope, it was shown to be some 3,000km (1,864 miles) in diameter: it is bigger than Pluto.

So long Pluto:

Named after the god of the underworld in Roman mythology, Pluto orbits the Sun at an average distance of 5.9 billion kilometres (3.7 billion miles) taking 247.9 Earth years to complete a single circuit of the Sun.