Recognize these planet names: Vulcan, Neptune, Pluto, Nemesis, Tyche, and Planet X? They all have one thing in common: their existence was predicted to account for unexplained phenomena in our solar system.
While the predictions of Neptune and Pluto proved correct, Nemesis and Tyche probably don’t exist. Now we have another contender, Planet Nine—the existence of which astronomers predicted last month—but we may need to wait ten or more years for it to be confirmed.
Compare this to Vulcan. While many claimed to have observed the predicted planet, it took 75 years and Einstein’s general theory of relativity to consign it to the dustbin of history.
Somewhere Out There
Astronomers are finding new exoplanets in other parts of the galaxy all the time. So why is it so hard to pin down exactly what is orbiting our own sun?
One reason is that very different methods are used to identify planets in other solar systems. Most involve observing periodic changes in the star’s light as the planet swings around it, as intercepted by telescopes such as Kepler.
Inside our own solar system, we can’t see these effects when we’re looking out into the darkness rather than towards the sun. Instead, planet-hunters use indirect means. Slight wobbles and perturbations in the orbits of planets, comets and other objects may reveal the gravitational presence of ghostly neighbors we didn’t know we had.
This method has been used often over the past two centuries to predict new planets.
The Planet That Arrived Late
In 1843, French mathematician Urbain Le Verrier published his provisional theory on the planet Mercury’s orbital motion.
Three years in the writing, it would be tested during a transit of Mercury across the face of the sun in 1845. But predictions from Le Verrier’s theory failed to match the observations. Mercury was late by 16 seconds!
