After more than 75 years, the time has come to say good-bye to our solar system's smallest and most distant planet. Last summer, members of the International Astronomical Union (IAU) ruled that as Pluto didn't meet their latest definition, it should no longer be considered a planet. The tiny former-planet, discovered by Clyde Tombaugh in 1930, has been demoted from a 'planet' to a 'dwarf planet'. The official number of planets in our solar system now stands at eight instead of nine.

According to the IAU's new definition, a celestial body must meet three criteria to be classified as a planet in our solar system: 1) it must orbit the sun; 2) it must be spherical; and 3) its orbit must not cross that of another planet. Unfortunately for fans of the 'ninth rock from the sun', Pluto fails to fulfil the IAU's third criterion — its orbit intersects Neptune's.

The union's decision to reclassify the tiny ball of ice was far from unanimous. Many IAU members are angry that the vote to determine Pluto's future was taken at the end of last summer's conference. More than 2,500 members attended the conference, but only 430 members were present at the vote. 'As voting took place on the last day, many had already left and attendance was poor,' explains Siraj Hasan, director of the Indian Institute of Astrophysics.

Despite the controversy surrounding the ruling, for most astronomers, Pluto's demotion came as no surprise. In fact, Pluto's days as a planet had been numbered since the early 1990s, when scientists began to discover objects of the same size or even greater than Pluto in the Kuiper Belt, an outer region of the solar system. For example, 'UB313', more commonly known as 'Xena', is larger than Pluto and orbits the sun, yet astronomers were reluctant to call it a planet because its orbit crosses Neptune's. The logical question arose: why should Pluto be labelled a planet if the same label wasn't available to Xena?

What effect will Pluto's reclassification have on the world? For scientists, it seems that the consequences will be negligible. 'Other than rewriting astronomy textbooks, this is not going to change the way we are looking at Pluto from the point of research,' says Hasan. Moreover, according to California Institute Technology's Michael Brown, a member of the team that discovered Xena, scientists tend to avoid using general, non-scientific terms, such as 'planet' and 'star', although he admits that they hold great importance for everyone else. For starters, museum exhibitions and textbooks will need to be updated. In addition, from now on, students will learn about the solar system's eight planets, trading the old memory trick 'My Very Excellent Mother Just Sent Us Nine Pizzas' for 'My Very Excellent Mother Just Sent Us, well, something that starts with 'N''.

Although it may be a sad farewell for some, is there something positive to take from Pluto's demotion? Michael Brown insists that people can 'learn that science is capable of correcting itself when it makes errors, which is a useful lesson to see in action.'

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