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Pluto at opposition
Posted: 27 June 2012

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Pluto finder Pluto's passage amongst the stars of Sagittarius. Astronomy Now graphic by Greg Smye-Rumsby

Dwarf planet Pluto comes to opposition on 29 June among the stars of Sagittarius and despite its +14 magnitude and current southerly declination it is a good target for imagers and owners of moderate to large telescopes. Of course it is far too small and distant to reveal a disc in amateur instruments, but keen observers can try to follow Pluto's movement against the background star field.

Pluto was discovered in 1930 by Clyde Tombaugh at the Lowell Observatory in Arizona, United States of America and for the next 75 years was considered the tenth planet. It was controversially reclassified as a Dwarf Planet in 2006, a class distinct from a 'Planet', by the International Astronomical Union and is now thought to be the largest Kuiper Belt Object, a mass of small bodies, remnants of the formation of the Solar System lying in a belt some 30 to 50 astronomical units (AU) from the Sun. Two other dwarf planets, Makemake and Haumea also reside in this icy belt.

Pluto's diameter is a mere 2274 kilometres but it has a very high albedo (measure of reflectivity) and this led to many erroneous past estimates of its size, quoted as high as 8000 kilometres. It has a highly eccentric orbit, ranging as close as 4.4 billion kilometres (30 AU) to the Sun and as distant as 7.4 billion km (49 AU). At perihelion it comes inside Neptune's orbit (most recently 1979-1999) and at this opposition it lies some 4,674 million kilometres from us. Pluto is the second largest trans-Neptunian object (TNO) after the Dwarf Planet Eris, which is 27 percent more massive. It was the discovery of Eris in 2005 that really sealed Pluto's fate.

If the skies are clear around the end of June and you have suitable instrumentation and a good southerly horizon then why not have a crack at tracking down Pluto. The bright open cluster M25 will be a big help as Pluto lies just over half a degree east to the east. Several images or sketches taken over the course of a few nights and carefully examined should betray its movement. For a local ephemeris visit the Minor Planets Center (MPC) website at Good luck!