Dwarf planet Pluto continues to make certain of the solar system’s “proper” planets appear lacking following the discovery of a fourth moon orbiting the former planet.

Pluto’s rising moon count is also concerning mission controllers of the New Horizons space probe, who fear the vessel could collide with hitherto unknown satellites as it approaches the far flung body in 2015.

On the subject of Pluto, National Geographic has put together a great primer on what was once considered the solar system’s outer most planet.

Mike Brown, who in January 2005 discovered what was at first hailed as the solar system’s tenth planet – initially dubbed Xena but later named Eris – writes how the discovery of the now dwarf planet led to Pluto, long regarded as the ninth planet, likewise being downgraded in stature, in his new book, How I Killed Pluto and Why it Had it Coming.

I hobbled back from the rocky beach up to the house. I woke Diane and told her that when the press called tomorrow I was going to have to tell them why the new proposed definition of planet was no good and why, in the end, it made sense all along for there to be just eight planets. I told her that I was going to have to kill Pluto and that Xena would go down as necessary and important collateral damage. All along, Diane had been more practical than I was. “Just let it be a planet,” she would say. “Try not to worry about it so much,” she had told me all year. “Relax” was her usual advice.

Following recent observations, doubt has been cast over the size of Eris, a dwarf planet orbiting the Sun at a (massive) distance of up to 14,510,993,390 kilometres, and previously thought to be bigger than Pluto, a finding that eventually resulted in Pluto being classed as a dwarf planet in 2006, after being regarded as a planet since its discovery in 1930.

Occultation measurements are by their nature very accurate, and although more precision is needed before we get a definitive answer, Eris is certainly a lot smaller than it was thought to be. “Almost certainly Eris has a radius smaller than 1,170 km [727 miles],” Bruno Sicardy, of the Paris Observatory, said in an email to Sky and Telescope Magazine. Pluto has a radius of approximately 1,172 kilometers (728 miles). Pluto therefore has an average density of 2.03 grams/cm³ and Eris has an average density of 2.5 grams/cm³.

While the new figures don’t show a great deal of variance in the size of either object, they could re-open the discussion over whether Pluto should continue to be regarded as a dwarf planet.

The appearance of outer most (dwarf) planet Pluto has changed significantly in the last two years, after remaining relatively unaltered for the previous 50.

The new images, taken in 2002 and 2003, confirm that Pluto’s surface is actively changing. For reasons that are still mysterious, Pluto’s appearance remained constant for some 50 years of observations before its surface colour became 20 to 30 per cent redder over two years at the beginning of the decade. Over the same period, Pluto’s northern hemisphere also brightened, while its southern hemisphere darkened. This appears to be due to ice vaporising in the sunlit north and refreezing in the wintry south, NASA says.

An explanation may lie in Pluto’s long yet erratic seasons, which can last up to 120 years, with the transformation from winter to summer (such that it is, given temperatures hover at around -200° celsius) causing nitrogen ice to melt and freeze in different hemispheres, thus causing changes in surface colours.

Today is Pluto Day, at least if you live in the US state of Illinois, where lawmakers have also decreed that Pluto is a planet, in contravention of the International Astronomical Union 2006 ruling declaring it be classified a dwarf planet, as it is “too small” to be a full-sized planet.

Clyde Tombaugh who discovered Pluto in 1930, happens to hail from Illinois, which goes someway in explaining the recent actions of the Illinois senators.

A February 26 resolution adopted by the state senate honors the Streator,Ill.-born astronomer who discovered Pluto in 1930 by declaring March 13, 2009, “Pluto Day” in the State of Illinois. The resolution states Dr. Clyde Tombaugh first noticed Pluto when he worked at the Lowell Observatory in Flagstaff, Ariz. It also says Pluto was “unfairly downgraded” when the International Astronomical Union voted to demote it.

Update: Pluto, together with Eris, are in fact classed as Plutoids not dwarf planets.

Astronomers Neil deGrasse Tyson and Mark Sykes recently debated what it takes for a solar object to be classified as a planet, but failed to reach a consensus.

Maybe it could be be put to a popular vote, based on an observation made by Tyson:

He added that Americans are much more attached to Pluto than people elsewhere. “If you go to Europe and talk about Pluto they look at you like, ‘Yeah, it’s that little thing out there’. You come to America, it’s ‘Pluto – that’s my favourite planet!’”

I do along with the idea to use roundness as a way of sorting planets from plutoids though.

Don’t how I missed this, but apparently the Solar System’s outermost “member”, Pluto, together with co-dwarf planet Eris, have now been deemed “plutoids” rather than planets.

I know Pluto lost its status as a fully-fledged planet sometime ago, but it seems I’m not the only one miffed by the new plutoid designation…

Last week, the International Astronomical Union (IAU) designated Pluto and its cousin Eris as “plutoids”. “It sounds like ‘haemorrhoids’,” says Alan Stern, who is chief scientist for NASA’s New Horizons mission to Pluto. He says that planetary scientists have been left out of the IAU’s decision-making. Now these scientists will get the chance to present arguments for and against at “The Great Planet Debate” in August at Laurel, Maryland.