Pluto: from Planet X to ex-planet

For anyone who went to school before 2006, the solar system was a simpler place. There were nine planets circling the sun, from boiling Mercury to distant, mysterious Pluto.

Pluto was discovered by American astronomer Clyde Tombaugh in 1930, after a long search for an unseen “planet X” beyond Neptune that was thought to be disrupting the orbits of the giant planets.

Generations of children remembered the order of nine planets orbiting the sun with mnemonics like My Very Excellent Mother Just Served Up Nine Pizzas. Now she only Serves Up Noodles.

Pluto’s status as a planet was called into question as early as 1992.

More powerful telescopes had begun to pick up other objects forming the Kuiper Belt – a region of space beyond the orbit of Neptune. It became increasingly clear that Pluto was just one of thousands of bodies in the belt.

Then, in 2005, a team led by American astronomer Mike Brown discovered Eris, another distant object orbiting the sun that appeared to be more massive than Pluto.

And then there were eight

In 2006 the International Astronomical Union (IAU) held a meeting to decide what to do about Eris and Pluto. Should Eris become the 10th planet in the solar system, or should Pluto be stripped of its status? What was a planet anyway?

After lengthy debate, the IAU proposed a new definition of “planet”. In order to be worthy of the name, an object must:

• orbit the sun
• be massive enough for its own gravity to pull it into a roughly round shape
• have “cleared the neighbourhood” around its orbit

Astronomers decided that Pluto did not meet the last of the three criteria. It had not swept away other smaller objects in its orbital region.

The decision remains controversial, as only a few hundred of the union’s 10,000 or so members voted for the momentous change.

Some astronomers think the “clearing the neighbourhood” definition is too vague, or impossible to achieve for celestial bodies in the crowded Kuiper Belt.

It is not the first time the list of planets has shrunk: between 1801 and 1850 Ceres was counted as a planet in many textbooks, before being reclassified as an asteroid.

Geology of Pluto, captured by New Horizons (Picture: Nasa)

The five dwarfs

No longer a full planet, Pluto is classed as a “dwarf planet”. There are five others: Ceres, the possibly rugby ball-shaped Haumea, Makemake and Eris.

Some astronomers thought Eris could be bigger than Pluto, but Nasa appear to have settled that argument today as the space agency’s New Horizons spacecraft approaches Pluto after a decade-long journey.

Alan Stern, the mission’s principle investigator, said: “We have discovered that Pluto is a little bit larger then we anticipated. We now have good measurements of its diameter and its radius: its radius is 1,185km, plus or minus 10.”

That means Pluto is bigger than Eris, is the largest object in the Kuiper Belt and the biggest dwarf planet – though there could be hundreds or thousands more out there waiting to be discovered.

Long-range New Horizons picture showing Pluto and Charon (Picture: Nasa)

Elvis is orbiting the sun

Pluto is the Greek god of the underworld and Charon, the largest of its five moons, is the ferryman who takes the dead across the river Styx into Hades.

The tradition of naming planets after gods and goddesses from various mythological traditions continued with the dwarf planets. Eris is the Greek deity of strife or discord and its single moon is Dysnomia or lawlessness, the daughter of Eris.

Haumea is named after the Hawaiian goddess of childbirth, Makemake is a god in the Rapa Nui mythology of Easter Island and the Italian discoverer of Ceres named it after the Roman goddess of agriculture.

But more and more “minor planets” – objects that orbit the sun – are being discovered: the list already runs at half a million.

Astronomers are running out of deities and turning to pets, lovers and popular culture for inspiration. Elvis, the four Beatles, James Bond and Buddy Holly all have minor planets named in their honour.

Despite being downgraded, Pluto has the distinction of giving its name to whole classes of objects: dwarf planets beyond Neptune are sometimes called Plutoids, and Plutinos are objects that have the same 2:3 orbital resonance as Pluto – travelling around the sun twice for every three orbits that Neptune makes.

What do we know about Pluto?

The New Horizons mission has already told us more about Pluto than we ever knew. As well as sending back magnificent images, the spacecraft has measured Pluto accurately and changed scientists’ views about its composition.

As far as we know, Pluto’s surface is covered in mostly nitrogen ice, and this sits on a dense rocky core. It has a thin atmosphere and there could be an ocean of liquid water hidden under the ice.

Mr Stern said the new estimate of Pluto’s diameter “has important scientific implications, because it is larger and we have known its mass very precisely for a long time”.

“That means it’s a little less dense, which will raise the fraction of ice in the interior that modelers will need to compensate the rock in the interior. It also means, because it’s larger the troposphere, the lowest layer in the atmosphere, is shallower than some models had predicted.”

At 2,370km or 1,472 miles across, Pluto is smaller than the eight planets. Its diameter is less than 20 per cent that of the Earth’s.

New Horizons has given us the closest ever look at Pluto’s rust-coloured surface, revealing bright spots that could be ice caps, markings that look like a whale’s tail and most dramatically, a lighter region that forms a perfect heart shape.

The craft is carrying a portion of Clyde Tombaugh’s ashes as it flies over the “heart” of Pluto. He died in 1997, while his discovery was still rated as the ninth planet in the solar system.

His widow Patricia said he would have been disappointed to learn of Pluto’s demotion, but would have understood the decision as he “was a scientist”.