Pluto has certainly had its share of media attention in the past, and the spotlight is set to shine in its direction again as NASA’s New Horizon probe nears the dwarf planet.

Discovered in 1930, Pluto was declared the ninth planet in our Solar System, and it remained undisputed for many years. But as larger astronomical objects with similar orbits to Pluto were discovered, its planetary status was put into question. In 2006, the International Astronomical Union (IAU) reclassified Pluto as a dwarf planet. This was a controversial decision but based on the IAU requirements for a planet, Pluto just didn’t measure up.

In order to be determined a planet the object needs to:

1.     be in orbit around the Sun

2.     have sufficient mass to assume a nearly round shape (hydrostatic equilibrium), and

3.     have cleared the neighbourhood around its orbit

It is the third condition that Pluto fails. It has not “cleared its neighbourhood” as its mass is not large enough to do so.

Being reclassified as a dwarf planet does not make Pluto any less interesting… or worthy of a visit. The NASA New Horizons mission was approved in 2001, and in 2006, the same year of reclassification, the probe was launched on a path to Pluto. On July 14th, it will make its closest approach, but already we are learning much more about the former planet. The blurry images beamed back to Earth are revealing new information about Pluto. The latest photos, taken by LORRI (Long-Range Reconnaissance Imager) at 77 million kilometres away show the difference in Pluto’s faces.

“These new images show us that Pluto’s differing faces are each distinct; likely hinting at what may be very complex surface geology or variations in surface composition from place to place,” said New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute in Boulder, Colorado. “These images also continue to support the hypothesis that Pluto has a polar cap whose extent varies with longitude; we’ll be able to make a definitive determination of the polar bright region’s iciness when we get compositional spectroscopy of that region in July.”

The images and data returned from the closest approach will be much more detailed and give scientists some interesting information about the geology, atmosphere and temperatures of Pluto.

But Pluto’s planetary status and all the other dwarf planets and large Kuiper Belt objects will always depend on that definition set by the IAU. The status failure point of “clearing the neighbourhood” is one that isn’t well defined – what qualifies as a neighbourhood in Space? The neighbourhood of the dwarf planets is the Kuiper Belt and it has been argued that if Earth was in the same location, that the mass of our home planet wouldn’t clear the belt either. On the other hand, Pluto is actually a binary system because Charon and Pluto always face each other locked in mutual gravitational embrace. Pluto’s orbit around the Sun is also much more tilted and much more elliptical than the planets. Interestingly if this planetary definition requirement is defined further it could not only mean the return of Pluto to the Solar System family, but the addition of other astronomical objects which meet the criteria.

To learn more about Pluto keep an eye on the NASA/JHL website www.pluto.jhuapl.edu/

Source: New Horizons www.pluto.jhuapl.edu