So, are there ten planets . . . or eight?

The Hubble space telescope has for the first time accurately measured the size of a new “planet” reported in our solar system last year.
It found that the object, 2003 UB313, is smaller than previously estimated with a diameter of 1,490 miles (2,384km), Nasa revealed this week.
But the planet – nicknamed Xena – is still larger than the ninth planet Pluto which is 1,422 miles across (2,275km). That is still smaller than the Moon.
The result will reopen the debate about how many planets there are in the solar system and whether Pluto is really a planet.
Pluto and Xena are similar icy worlds. So, many astronomers argue, if Xena cannot be considered a tenth planet, then why should Pluto be the ninth?
Xena lies around ten billion miles from the sun (16 billion km) which is around three times as far as Pluto.
Both are considered to be part of a class of icy world in the Kuiper Belt at the edge of the solar system and astronomers believe there are many thousands more waiting to be discovered.
Xena, named after TV’s Warrior Princess, pictured above in an artist’s impression by the Hubble team, takes 560 years to orbit the Sun, more than twice as long as Pluto.

Paul Sutherland

Paul Sutherland

I have been a professional journalist for nearly 40 years. I write regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy. I have also authored three books on astronomy and contributed to others.
Paul Sutherland

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Paul Sutherland

I have been a professional journalist for nearly 40 years. I write regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy. I have also authored three books on astronomy and contributed to others.

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