Can you observe and capture the distant dwarf planet Pluto throughout the month?

Dwarf planet Pluto reaches opposition on 20 July in Sagittarius, close to the border of Capricornus. The mag. +8.6 globular cluster M75 sits even closer to the border and is a nearby navigational guide (see chart, above). In July, Pluto moves westwards, starting 1.5° west-southwest of M75. Our challenge is to locate and capture this distant world, which appears like a 14th magnitude star in a telescope.

The best way to ‘see’ Pluto is to use the asteroid-tracking method of recording the suspected star field over the course of several nights, looking for the faint dot that appears to be moving. During July, Pluto’s magnitude remains a steady +14.3. We’d normally recommend at least a 300mm scope for this, but additional aperture can only help as Pluto is quite low in the sky. There are claims that Pluto has been seen in smaller instruments than this under pristine skies.

The sure-fire way to record it is to photograph the star field where Pluto is expected to be located. Here, M75 works wonders as a starting guide. From M75 head half-a-degree to the southwest to locate mag. +7.5 HIP 98851. A fraction under half a degree south-southwest of this star lies mag. +6.4 HIP 98785, and half a degree to the west-southwest of this star is mag. +6.0 HIP 98575. Keep the line going from HIP 98785 through 98575 for approximately twice that distance again to arrive at mag. +7.6 HIP 98135. Pluto tracks parallel to the line between HIP 98575 and 98135, running slightly north of it. A minimum of a 200mm–300mm lens attached to a DSLR or MILC is recommended for this challenge; use a medium to high ISO. You’ll need to aim to record stars slightly fainter than the faintest shown on our narrow-field chart (below) to succeed, so a tracking mount is recommended to allow you to extend the exposure time without star trails.

Record as many images as you can throughout the month, loading them into a layer-based editor one image per layer. Carefully align the stars between layers and flick between the frames to see if you can spot something faint moving in a straight line over the course of the month. If you can, that’s likely to be Pluto. If you can, check your results using the first few images and increase camera sensitivity (higher ISO and/or longer exposures) if you can’t see Pluto at all. Low altitude and haze will make the job more challenging, as will the presence of the Moon. The bright post-solstice skies at the start of July will also add an extra dimension of difficulty to this already tricky challenge.