On 22 September 1846, the German astronomer Johann Gottfried Galle received a mysterious letter from his friend Urbain Le Verrier. Le Verrier had been studying the orbit of Uranus and puzzling over certain changes to the orbit that were hard to explain.

Le Verrier’s conclusion was that Uranus must be under the gravitational influence of an unknown planet orbiting further away from the Sun. He set to work calculating the position of this new body and sent the details to Galle telling him where to look.

The night after he received this letter, Galle discovered Neptune in exactly the place his colleague predicted. It is still one of the great discoveries in the history of science.

But even after the discovery of Neptune, astronomers were puzzled by mysterious perturbations in the orbits of these distant planets. Their conclusion was that there must be an even more distant planet that was exerting a gravitational pull on the others.

This planet, they calculated, must be about the size of the Earth and more distant even the Neptune. They called this body ‘Planet X’ and the search for it captured the media’s imagination.

It wasn’t until 1930, however, that the matter was laid to rest. In January that year, the astronomer Clyde Tombaugh, spotted a faint object moving against a background of stars. This turned out to be Pluto, a discovery that made headlines all over the world.

Aineias, NASA, ESA, and M. Buie (Southwest Research Institute)

Although a scientific triumph, this new planet was much smaller than Planet X was supposed to be. And further measurements and calculations lead to new estimates of the mass of Neptune, which in turn suggested that Planet X might not exist after all. The media, and eventually astronomers, forgot all about it.

Since then, astronomers have discovered hundreds of so-called trans-Neptunian bodies, one of which, Eris, is even bigger than Pluto. All of these lie in the distant reaches of the Solar System, most in a region known as the Kuiper Belt. So it’s not surprising that thoughts of Planet X have been long forgotten.

Except that last month, Planet X raised its head again after astronomers announced the discovery of the most distant known body in our Solar System, currently known as 2012 VP113.

Astronomers find new dwarf planets on a fairly regular basis but there was something about this new one that was odd. It turns out that some of the orbital characteristics of 2012 VP113 are remarkably similar to several other dwarf planets orbiting at distances greater than 30 astronomical units.

In particular, they all seem to share a similar argument of periapsis, which is related to the body’s point of closest approach to the Sun.

One way this can happen is if these dwarf planets are being “herded” by a larger one. In other words, the similarity between these orbits implies the existence of a Planet X somewhere in the distant reaches of the Solar System.

The initial calculations suggested that this planet ought to have a mass of somewhere between twice and 15 times that of Earth and that it should orbit the Sun at a distance of between 200 and 300 astronomical units. That raised hopes, as well as global media coverage, that Planet X might once again be within astronomer’s sights.

But that thinking may now need to be revised. Given a little more thinking time, one astrophysicist says that these initial calculations look wildly optimistic.

Today, Lorenzo Iorio at the Ministero dell’Istruzione, dell’Universit`a e della Ricerca in Italy, says that it is possible to constrain the position of Planet X more tightly using the orbital measurements of other planets. And his calculations show that Planet X is very unlikely to orbit at distances of only 200 or 300 AU. “Such a scenario is strongly disfavored by the latest constraints,” he says.

Instead, Plant X can only orbit at much greater distances, if it exists at all. Iorio calculates that a planet twice the mass of Earth cannot orbit any closer than about 500 AU. And a planet 15 times the mass of Earth must be at least 1000 AU distant.

What’s more, these constraints are likely to get tighter in the not too distant future. Iorio points out that the New Horizon is space probe currently on its way to Pluto and that this will produce much more precise measurements of Pluto’s orbit.

“The New Horizons spacecraft should constrain the distance of a putative body [a little smaller than Earth] to [a distance of] 4700 AU,” he says.

NASA

That’s bad news for any modern-day Galles hoping to get out their telescopes and lens clothes in the hope of discovering a mysterious new body.

Ref: arxiv.org/abs/1404.0258 : Planet X revamped after the discovery of the Sedna-like object 2012 VP113?