Transcript

TONY EASTLEY: It's official: the planet Pluto is no longer; it's been kicked out of the cosmic club. Its status has been officially downgraded and the solar system now consists of eight planets rather than nine.

The surprise decision by the International Astronomical Union came after a group of astronomers rebelled against the decision to expand the solar system to 12.

From London Stephanie Kennedy filed this report for AM.

NASA CONTROL: Three, two, one, we have admission and lift off of NASA's new...

STEPHANIE KENNEDY: The unmanned spacecraft missile blasted off earlier this year, and it's heading to the planet Pluto. But the ninth rock from the sun named after the god of the underworld in Roman mythology has suffered a bitter blow to its planethood.

VOTE CALLER: 137 in favour, 157 opposed, 17 abstaining, so the motion is carried.

STEPHANIE KENNEDY: Members of the International Astronomical Union have been meeting in Prague and they've decided this mysterious mass of ice and rock should be stripped of its planet status.

Professor Mike Cruise is from the University of Birmingham.

MIKE CRUISE: Over the years we've built better, more sensitive telescopes, much bigger ones, and they can see more objects in the solar system. And so we need to have a good definition of exactly what is a planet, and exactly what is a piece of rubble left over from when the solar system was formed.

And unfortunately for poor old Pluto, it's fallen into the rubble, rather than being retained as a planet.

STEPHANIE KENNEDY: Now the celestial object will be classified in the new category of "dwarf planets". Only last week, the IAU appeared ready to not only affirm Pluto's planet status but also enlarge the cosmic club to 12 planets. That would have included Zena, discovered by Professor Mike Brown.

MIKE BROWN: It has been a fun year having this thing that I found being called the tenth planet for so long, and so I'm a little sad to see that go, it now becomes the largest dwarf planet.

It's such the right scientific choice to make that my personal disappointment is tempered by the fact that scientifically I know this is the right decision.

STEPHANIE KENNEDY: Space scientists also agreed on a new definition of a planet.

Professor Cruise outlines the three requirements.

MIKE CRUISE: First of all they've got to all be around the sun, not around any other body. Secondly they've got to be big enough to sort of spherical in shape, that is to say the forces of gravity have got to be strong enough that they have gathered themselves into a ball.

And then the last issue is that they've got to have cleared the neighbourhood around them, they mustn't be existing with lots of rubble in their close neighbourhood.

STEPHANIE KENNEDY: There is one shining star in this cosmic story - Pluto's demise is a boon for the book publishing industry. Every book about the solar system will have to be re-written.

This is Stephanie Kennedy reporting for AM.