Pluto imaged at last

When I first took an interest in astronomy Pluto was the last of the planets and marked the boundary of the solar system. No trans-Neptunian objects had been discovered and the Oort Cloud was only a theory. I could only dream of one day owning a telescope capable of seeing this 14th magnitude world which lies over 3 billion miles from the Sun.

I eventually managed to see all the other challenging planets with my telescopes: Mercury, Uranus and Neptune. Pluto however remained beyond reach and after it was declassified as a planet the interest in actually seeing it through the eyepiece diminished.

It was only in 2019 that my interest returned with the realization that my present scope was easily capable of imaging Pluto, which has now entered the starfields of the Milky Way and is very difficult to identify visually from a star chart. I decided to emulate Clyde Tombaugh’s discovery technique and take images of Pluto’s position a few days apart, then look for its shifting position against the background stars.

23 June 2019

In this first image Pluto is arrowed but looks just like any of the hundreds of background stars in the constellation of Sagittarius. Its identity is only revealed by its motion, as seen in the next image.

28 June 2019

Five days later Pluto has moved across the field and is seen almost on top of a background star.

01 July 2019

Another four days have passed and Pluto is now over on the right of the field.

Technical Notes

Planewave CDK-14 scope and FLI Proline P9000 CCD camera. Each image is a 15 minute exposure through a clear filter. The field in each case is 24 arcminutes wide, or 4/5 the size of the full moon.

The bright star near the bottom of the field with faint diffraction spikes is magnitude 8, so all of the stars in the image are well below naked-eye visibility.

North is up in each image, which may prompt some readers to ask why Pluto – which orbits the Sun anti-clockwise as seen from above the north pole, the same direction as Earth and the other major planets – seems to be moving ‘backwards’ through the stars. The explanation is that Pluto was near opposition to the Sun at the time, meaning that Earth was overtaking it on the inside as it crawls along in its 248-year orbit. Hence Pluto appears to move backwards with respect to the background stars for about 4 months of the year. All the outer planets exhibit this retrograde motion as Earth overtakes them.