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The International Astronomical Union (IAU) caused a cosmic stir when it kicked Pluto out of the planet gang, demoting it to ‘dwarf planet’ status. The decision by the IAU – the body responsible for naming and categorising astronomical objects – was, and still is, hotly contested.

The trouble started in 2005 when Mike Brown, professor of planetary astronomy at the California Institute of Technology (Caltech), discovered Eris; a celestial body larger than Pluto, and orbiting far beyond the then ninth planet. The self-confessed “killer of Pluto” is also responsible for discovering a host of other dwarf planets and ‘trans-Neptunian objects’. Brown’s discovery of Eris caused such an upset, that on 24th August 2006, the IAU reclassified the Solar System’s objects. The classification now incorporates ‘planets’ (the eight that we’re familiar with), ‘dwarf planets’ (Pluto, Eris, etc.), and ‘small solar system objects’ (including asteroids, comets, and Kuiper Belt objects).

Pluto was discovered back in 1930 by Clyde Tombaugh, a young American astronomer given the task of finding ‘Planet X’. Tombaugh found Pluto using a blink comparator; a device which could flick between two images of the same patch of sky taken a few nights apart to detect objects that appeared to jump from one position to another. The planet had been theorised to exist by the astronomer Percival Lowell in a region now known as the Kuiper Belt (big brother of the Asteroid Belt); a disc of distant bodies in the cold outer reaches of the Solar System. But Pluto has turned out to be one of possibly hundreds of yet-to-be-confirmed dwarf planets. The IAU currently recognises five: Ceres (in the Asteroid Belt), Pluto, Makemake, Haumea (Kuiper Belt objects), and Eris (Scattered Disc object, in an area beyond the Kuiper Belt). Brown has suggested a further six as being “virtually certain”: 2007OR10, Quaoar, Sedna, Orcus, 2002MS4 and Salacia – bringing the current total to eleven dwarfs.

A handful of modern day astronomers find themselves in a similar position to the one faced by Tombaugh in the 1930s; seeking a new Planet X, proposed to exist in the far reaches of the Solar System. The existence of this unseen planet has been suggested based on the clustering of dwarf planets and other trans-Neptunian objects which may be caused by the gravitational tug of a hypothetical giant. A visual search made by the WISE space telescope (NASA’s Wide-field Infrared Survey Explorer) earlier this year was unsuccessful in finding a new gas giant. It was, however, unable to discount the existence of a rocky ‘super Earth’, which would be far more difficult to detect due to potential size and albedo (the astronomical term for reflectivity).

In July 2015, NASA’s New Horizons space mission is due to rendezvous with Pluto, its companion Charon, and their four tiny moons, before heading off into the uncharted territory of the Kuiper Belt. It’s hoped that the mission will unveil the mysteries of the king of the dwarfs, revealing surface composition and structure down to a resolution of 230 feet – close enough on Earth to reveal individual streets and buildings. Mission scientists also hope to discover a faint ring system, thought to be the result of a cosmic impact. “Everything we know about Pluto comes from studying it from billions of miles away,” says the mission’s principal investigator and planetary scientist, Alan Stern, at the Southwest Research Institute.

“But the lesson of planetary science is that when we see things up close, our ideas from afar are often overturned.” Right now, the Hubble Space Telescope is hunting for the next destination for New Horizons. In a statement, Matt Mountain, director of the Space Telescope Science Institute (STScI) in Baltimore, which operates Hubble, said “The planned search for a suitable target for New Horizons further demonstrates how Hubble is effectively being used to support humankind’s initial reconnaissance of the Solar System.”

What surprises are lurking within the distant realms of our planetary neighbourhood? Will we discover the planet previously known as ‘X’? How many new dwarfs will be unveiled? Hopefully, New Horizons, assisted by Hubble, will help answer these questions and many more.

© 2014 Melanie Davies