The hunt for Pluto began with a search in 1906 by Percival Lowell, an American businessman and astronomer who claimed to see canals on Mars that were actually just reflections of blood vessels in his eyeball. Lowell believed that slight perturbations in the orbit of Uranus could not be explained by the gravity of Neptune alone and that another massive body was responsible, which he nicknamed “Planet X”.
Today, thanks to data from the Voyager 2 spacecraft, we know that Lowell’s estimate of Neptune’s mass was just a little off. However, Lowell was sure that his calculations were correct. Lowell founded the observatory that bears his namesake in Arizona, and a decades-long search began. Lowell believed the planet to be about 7 times the mass of the Earth, or smaller than Neptune, and about 43 AU from the Sun, or 1.5 times the distance Neptune orbits from the Sun.
Lowell died in 1916, never seeing the fruits of his labor, despite the fact that Pluto was imaged in other astronomical sky surveys as early as 1909 but no one noticed its movement. A Kansas farm boy named Clyde Tombaugh built his own telescopes in the mid-1920s and sent drawings through them to Lowell Observatory. Impressed, they offered him a job, and Tombaugh began spending hours using a “blink comparator” to look for the ninth planet in photographic plates, carefully lined up with each other and rapidly flashed back and forth to reveal evidence of any objects that moved between the different nights on which the photographs were taken.
After less than a year on the job, Tombaugh discovered a moving object, which was thought to be the new Planet X, though a dissatisfied Tombaugh spent years continuing his search, discovering many new asteroids, variable stars, and two comets, but no new planets – though he probably could’ve detected dwarf planet Makemake if it hadn’t been positioned in the middle of a star cluster at the time of his search.
Pluto was named for the Roman god of the underworld, as suggested by 11-year-old schoolgirl Venetia Burney, who picked it as Pluto was known to be able to render himself invisible. Pluto was automatically a disappointment to its discoverers; it was six times dimmer than Lowell’s calculations for Planet X and initially thought to be the mass of the Earth. Pluto orbits in a resonance, taking two trips around the Sun every time Neptune makes two, keeping them far away from each other despite crossing orbital paths.
Continually better observations and the discovery of Pluto’s moon Charon allowed its mass and radius to be pinned down accurately, and we know today that Pluto is slightly smaller than our own Moon at 2377 kilometers in diameter and 17% of the Moon’s mass (or 0.2% of Earth’s). Pluto’s crossing of Neptune’s orbit, combined with the discovery of other objects similar to it in size and orbit, led to its downfall and loss of planetary status. The most prominent of the new dwarf planets, Eris, which is roughly the same size as Pluto and actually more massive, was briefly considered the tenth planet before the idea was scrapped.
In 2015, the New Horizons spacecraft flew by Pluto. It was one of only two dwarf planets to be studied by a space probe. Clyde Tombaugh’s ashes are in the spaceship, so he was technically the first person to go to the outer planets and leave the Solar System. Plans for a follow-up Pluto orbiter or lander have been proposed but are unlikely to happen for at least a few decades.
When can I see Pluto?
The best time to look at Pluto is when it is in opposition, which will happen in late July for the foreseeable future. It is low in the sky in the constellation Sagittarius, soon to move into Capricornus.
How Good of a Telescope Do I need to see Pluto?
Pluto is gradually getting further from the Sun and dimmer as a result. At its perihelion (29.6 AU, or 29.6 times the Earth-Sun distance) in 1989, Pluto was magnitude 13.7 in brightness, but has since retreated from the Sun and is now about half as bright. At magnitude 13.7, Pluto was comparable in brightness to the brightest moons of Uranus and Neptune, but without being near a glare-producing planet, it required less effort to see. It was within the range of telescopes as small as 4 to 5 inches in aperture back then.
Right now, Pluto is hovering at magnitude 14.3–14.4 and losing about 1/10 of a magnitude of brightness every 3 years. It will dip below 15th magnitude in the 2040s and continue to dim until it reaches its aphelion and thus its lowest brightness in the early 2100s. So don’t wait; it’s probably brighter and easier to see with a telescope right now than it will ever be for all but the youngest observers.
Theoretically, a 6″ or 8″ telescope is powerful enough to see Pluto under dark skies when it is as bright as it is now, but few observers are likely to have dark enough skies and enough experience to do so, and it’s not worth trying. Light pollution affects Pluto greatly on account of its dimness. A 10” telescope can show you Pluto fairly conspicuously under dark (Bortle 3 or better) skies, while a larger telescope is needed in light-polluted environments. A 12″ telescope will show you Pluto under Bortle 4-5 skies, while a 14-16” telescope is needed in suburban Bortle 5-6 skies, and places with more light pollution simply won’t allow you to glimpse the faint, icy world no matter how powerful the instrument. These thresholds will all increase as time goes on and Pluto dims.
What does Pluto look like in a telescope?
Pluto will appear as a star-like point; no telescope can resolve its disk, which is under 0.1 arc seconds wide; some red giant stars appear larger in the sky, albeit similarly unresolved. The easiest way to confirm you’ve found Pluto is to sketch its position against the background stars and come back a few days or weeks later. You will notice that one “star” has “moved”. Alternatively, familiarizing yourself with the immediate star field around Pluto and using very high magnification to isolate it (as a basically point source, it doesn’t dim at higher magnifications) can also allow you to nail down its position.
Pluto’s large moon Charon is half its size and quite a bit duller, but still technically bright enough to see with 20” or larger instruments; unfortunately, it lies too close to Pluto to possibly resolve as a separate object. Pluto’s other moons—Kerberos, Styx, Hydra, and Nix—are tiny and far too small to observe with amateur telescopes.