PUBLISHED : Wednesday, 10 February, 2010, 12:00am
UPDATED : Wednesday, 10 February, 2010, 12:00am

Farther than Pluto

When the faraway icy planet Pluto was discovered in 1930, scientists said our solar system had nine planets. But in January 2005, Mike Brown and his team of scientists at the Palomar Observatory in the United States made a discovery. They saw a large object moving very slowly across the sky.

At first they named it Xena, after a warrior princess on a popular US television show. They calculated that Xena was three times farther away than Pluto. It took nearly 560 years to go around the sun. Even more interesting, it was 25 per cent bigger than Pluto. Had Brown and his team found the solar system's 10th planet? It even had its own moon.

The planet controversy

In 2000, the famous Hayden Planetarium in New York made headlines. When it reopened, its model of the solar system only had eight planets. The problem was that astronomers were beginning to realise there were probably a lot of large objects circling the sun. They are in a part of the solar system called the Kuiper Belt.

The Kuiper Belt is beyond Neptune. Scientists have now found more than 1,000 objects in it. Four of them are half or more the size of Pluto. For this - and other scientific reasons - in 2006, the international union of astronomers decided Pluto was no longer a planet. Pluto and Eris became two of five 'dwarf planets' - the others are Ceres, Makemake and Haumea.

Solar system in the news

Early last month, reported that the dwarf planets are probably not alone. Scientists even think there may be a planet far out in space as big as earth or Mars.

The problem is that our telescopes are not powerful enough to see it yet.

'When the solar system's story is finally written, it's much more likely that it will have closer to 900 planets rather than the nine that we grew up with,' a US scientist told Scientists think there are probably more big planets beyond the Kuiper Belt. They think there is probably another belt. They call it the Oort Cloud. It is 50,000 times farther from the sun than the earth is.

now do this

1 When scientists discovered Eris, they thought it was the solar system's ... planet.

a. eighth

b. ninth

c. 10th

2 Eris takes ... years to travel around the sun.

a. 25

b. 560

c. 480

3 The Oort Cloud is ... times farther from the sun than the earth is.

a. 50,000

b. 25,000