Let’s Find Pluto!

At opposition this week and as bright as it will be for the next 190 years, it's time to find your way to Pluto, a frigid enigma at the edge of night.

Fringe Planet

When NASA's New Horizons probe flew past Pluto on July 14, 2015, it revealed a world more geologically active than anticipated. This image features Pluto's "Heart," a vast plain of nitrogen ice ringed by water-ice mountains covered in organic compounds that drifted down from the atmosphere.

Pluto may be a dwarf planet, but it can pose a gigantic challenge. Not only has the planet been fading since its 1989 perihelion, it's now playing hide-and-seek in star-rich Sagittarius at the pit of the ecliptic. Back when I first saw it in the early '80s, Pluto strode the star-barren fields of eastern Virgo at magnitude of +13.7.

These nights, it winks back weakly at magnitude +14.2. What used to be an easy catch for an 8-inch scope under dark skies will probably require a 10-inch now. Fortunately, telescope aperture sizes have increased and prices have dropped since the heyday of Madonna, keeping Pluto within the range of many observers.

I hope you'll be one of them. This is the perfect week to seek this icy dot, with the Moon departing the evening sky and darkness returning like a welcome breeze. Pluto reached opposition on July 10th just below the familiar Teaspoon asterism in eastern Sagittarius, so it's brighter now than at any other time this year. Make that brighter than it will be for the next 190 years.

Spoonful of Pluto

Pluto makes a tiny loop this apparition below the Teaspoon asterism in Sagittarius.

Because of its orbital eccentricity, the dwarf planet's distance from the Sun varies from 29.6 a.u. at perihelion, which occurred in 1989, to 49.3 a.u., when it reaches aphelion in Cetus in February 2114. The great grandchildren of today's parents will need a 16-inch scope to spot the magnitude +16 dwarf planet in that distant year. Not till 2207 will Pluto be this bright again.

Pluto culminates around 1 a.m. local time at mid-month, but you can begin looking around 11 p.m. when the dwarf planet stands about 20° high in the southeastern sky from mid-northern latitudes.

First, locate the Teaspoon, then focus your attention on Pi (π) Sagittarii (magnitude +2.8), the star just east and above the bottom of the spoon. Center this star in your finderscope and slide just under 1° southeast to +6.4 magnitude HD 179201. Pluto is currently 1° east and slightly north of the star. On the night of the 12th, it's conveniently just 3′ due west of 8th-magnitude HD 180332.

Zooming in for a closer look.

You're now ready to use the deeper map to wade through a sea of 13th- and 14th-magnitude stars to Pluto. Be methodical and make use of patterns like all seasoned star-hoppers do. I like joining field stars into triangles, "batons," and diamonds to work my way to a faint target. Because Pluto moves slowly night to night, the mini-asterisms you've created will make returning to the field to check on its motion much easier.

While some might be perfectly confident that they've seen Pluto on the first try, it's much more satisfying to return the next clear night and look it up again. Not only will you confirm your first observation but seeing it move from night to night firms our connection to this remote place, like going on a "ride-along."

A closer look at Pluto's track

Click this map for a large black-and-white .pdf that shows Pluto's position every 3-days with stars plotted to fainter than magnitude +14. It's formatted so you can print out and use at the telescope.

American astronomer Clyde Tombaugh used a related method to make his discovery of Pluto in 1930. He took pairs of photos on different nights and compared them in a blink comparator to determine if any objects had shifted their position. Little did he know that in 2006 his planet would be declared unfit for the job and reclassified as a dwarf planet.

This simulated flyover of two regions on Pluto, northwestern Sputnik Planum (Sputnik Plain) and Hillary Montes (Hillary Mountains), was created from New Horizons close-approach images.

Whatever you choose to call it, Pluto can speak for itself. Only 1,475 miles across or two-thirds the size of the Moon, its incredibly diverse landscapes include flowing nitrogen ice warmed by residual heat from its core, a now-frozen liquid nitrogen lake, glaciers, and a multi-layered atmosphere dotted with ethane clouds. I don't care how cold it is, I want to go there! And there's only one way to do it — through my telescope.

9 thoughts on “Let’s Find Pluto!

  1. Joe StieberJoe Stieber

    Bob — another fine article!
    I try to spot Pluto every year with my 12.5-inch dob, and I admit, I’m one of those satisfied with a single observation that matches the SkyTools chart that I use to place it. However, I have on a number of occasions followed-up on succeeding nights to establish the “ground truth” of those charts. I’ve also successfully compared my observations to contemporaneous photos when available.
    Like you, I construct various asterisms and geometric figures from the surrounding stars to guide my way. I too have heard some folks complain about the abundance of “confusing” Milky Way stars around Pluto now, but I’ve found they help me by providing plenty of fodder for my asterisms, and ultimately, reference points when Pluto is found.
    You mention that Pluto is currently at magnitude 14.2, but for those of us at the middle to upper latitudes of the USA, Pluto’s low altitude causes more-than-negligible atmospheric absorption. With Pluto at about 21.5°S declination now, it’s only at 28.5° altitude when it transits for an observer at 40°N latitude. SkyTools therefore shows a mean extincted magnitude of 15.0 at transit. Finding Pluto’s location has never been a problem for me. Rather, seeing it can be tricky — even with my 12.5inch scope. On the best nights, I can see it with direct vision, but less-than-ideal nights require averted vision. Of course, I need to travel from my suburban home to the darker skies of the NJ Pines to do so.

    1. Bob KingBob King Post author

      Thanks Joe. Extinction is an issue at my place as well at 47 N which is I why I reserve viewing it until I can get to country skies, which hopefully will be tonight!

  2. Lars Lindhard

    Pluto is too low in the sky for my latitude in Denmark these years, but back in 1988 I could see it in the suburban skies with my C-8. I have seen it several time in the 90’s with bigger telescopes. I will probably not be around when Pluto come over the local horizon again.

    1. Bob KingBob King Post author

      Hi Lars,
      I imagine that you have very long twilights, too. At least through July. At least if you don’t see it again, you got a great view at perihelion.

  3. goodricke1

    Bob, there’s an interesting video on youtube from 1980, where Clyde Tombaugh predicts that many more Pluto type objects will be discovered in the years ahead, and Pluto would then lose some importance. So I think he had an inkling!

  4. Alain FigerAlain Figer

    I made a succesful attempt to image Pluto on 14 September 19h28-40 UT as the night was especially transparent in the Southern Alps (altitude : 1850m ; latitude 44.6°). I observed from my balcony in a ski resort above some streetlights near the time of Pluto’s culmination at a height slightly more than 23°.
    Photos were taken using a Canon EOS 6D + 300mm SIGMA Teleobjective at f/2.8 on a tripod. Adding a Canon EF 2.0X Telephoto Extender I got in fact a 600mm at f/5.6.
    On 10 second exposures at 20000 ISO, start trails were visible – without any image tratment – up to mag 14.7 (according to Calsky) or 14.8 (according to Guide).
    Despite the low height above horizon, I was lucky enough to spot the trail of Pluto at mag 14.2 (Calsky) or 14.4 (Guide) ; in fact detection was made easier as part of Pluto’s trail was blended with that of a neighbour star of mag 14.2 (Calsky) or 14.3 (Guide).
    I will repeat these observations on another clear night to show Pluto’s trail unblended.


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