Clyde Tombaugh still discovered the dwarf planet, but this is the latest “precovery” image to be unearthed.
Clyde Tombaugh may have introduced the world to Pluto in 1930, but five years before, the Carnegie Institution for Science snapped a photo of the icy dwarf planet without quite knowing it.
But don’t worry, we don’t need to rewrite the history books. This isn’t the first time this has happened. In fact, it’s called precovery – the process of discovering astronomical bodies in older photos and data from before their official discovery. As it turns out, it’s quite common.
The Carnegie Institution for Science plate is no less remarkable, though precovery images of Pluto stretch back to 1909. The photo was recently unearthed in the archives, with the two men who snapped the photo printing a paper in 1931 as a sort of “we saw Pluto in 1925” brief article. Published in Astronomische Nachrichten, Gustav Stromberg and Nicholas Mayall showed that plates from December 1925 correspond to Pluto.
It wasn’t identified at the time in 1925, but rather found on a second search after Pluto’s discovery. It also speaks to how hard it is to figure out quite what you’re looking at in the sky. Objects at the outer boundaries of the solar system move across the sky very slowly. To discover a planet or planet-like object, you need to look in the same area of sky over time and note objects moving in position among the stars, which in human terms stay relatively fixed in the sky (though they change positions over tens of thousands of years.)
That’s why Uranus was identified as a planet by William Herschel in 1781, but was identified as a star as early as 1690. Uranus and Neptune aren’t visible to the naked eye under most conditions, so there was no ancient foreknowledge of their positions, unlike the other planets. Galileo spotted Neptune in 1612, just a few years after he discovered the Jovian moons. But he, too, mistook it for a star.
Of the dwarf planets, the next earliest precovered is Eris, the Pluto sized body that added on to the case for its demotion. (Which is to say, astronomers anticipate finding thousands of bodies at or near Pluto’s size in the outer solar system.) Though announced by Mike “Pluto killer” Brown in 2005, precovery images go back to 1954.
Many telescopes are set to do all sky-surveys in the coming years. While some have their aims on exoplanets, such as the MINERVA telescope in Arizona, there are others like the Large Synoptic Survey Telescope which will be surveying the entire sky for any sort of phenomena – transiting exoplanets, distant galaxies, supernovae, and even small solar system objects like asteroids and comets.
So there may be an asteroid or two that are discovered in one of these surveys that were precovered long ago. But in most cases, there’s a difference between spotting something in the sky and correctly identifying what it is and publishing the results. That’s why Clyde Tombaugh will likely still be considered the discoverer of Pluto, but others spotted and photographed it far beforehand.
Source: Carnegie Science