Once, cartographers drew dragons on the unknown bits of maps. Now, some people like to fill the outer solar system with an undiscovered planet or two, often known as Planet X. The reasons vary – claimed sightings, distortions in the orbits of other planets or patterns to cometary observations. But does this idea have more substance than the ancient versions? The answer depends on how large, and how distant a planet you're discussing.
What We Know Is Out There
Biden however, is hardly a giant object. At 450km wide it is smaller than several asteroid belt residents. Ice is easier for gravity to shape than rock, so if Biden is, as suspected, mostly ice, it will be classified as a dwarf planet on the basis of its shape, but it sets few imaginations aflame.
Sedna, the first object detected at these distances is approximately 1000km across, similar in size to Ceres, the inner solar system's largest asteroid. Something larger than this may be lurking in the icy outskirts of the Sun's influence. However, people talk of Planet X not Dwarf Planet X. Could there be something out there that is at least the size of Mercury?
While bigger objects should be easier to find, that doesn't work if they are too dark. Sedna reflects a third of the light it receives and organic compounds may darken the surfaces of some outer solar system objects.
Moreover, we only found Senda and Biden because they are both unusually close to us at the moment. Sedna's distance ranges from 76 Astronomical Units (the distance of the Earth from the Sun) to 937AU. It was found at 90AUs. Only a tiny portion of its 11,400 year orbit is spent this close or closer. Similarly Biden's orbit extends to 452AU from the sun, but it was detected at just 83AU. So planetoids of at least similar size could well be undetected, much further out in their orbits.
Scott S Sheppard/Carnegie Institution for Science. The orbits of Sedna (orange) and Biden (red) with Neptune and the Kuiper Belt to scale.
Closer in, the Kuiper Belt is a flat disk usually defined as extending from the orbit of Neptune (30AU) to 50 AU. Where once Pluto was the sole known inhabitant of this region we have now added Haumea, Makemake and a thousand objects too small to be classified even as dwarf planets.
That's Not a Planet, This Might Be A Planet
The idea of Planet X is rather more grand than these frozen worldlets, however. Neptune was discovered in 1846 thanks to anomalies in the orbit of Uranus that suggested the presence of a large object further out exerting gravitational influence. In turn Percival Lowell noted oddities in Neptune's travels he blamed on a yet further planet. Pluto turned up very close to Lowell's had prediction, and was at first hailed as vindicating him. Further observations revealed Pluto to be far too small to have the influence suggested.
The idea of a mysterious object large enough to be messing with Neptune's orbit refuses to die but has become a sort of Bigfoot or Loch Ness Monster, existing more in the more imaginative parts of the web than peer reviewed journals.
However, while it is possible for amateur cryptozoologists to seek strange footprints in remote locations, telescopes capable of detecting something this faint don't come cheap. While serious attempts have been made to detect the influence of such planets, for most astronomers the need for something to be tugging the outer planets around vanished when Voyager 2 spacecraft showed Neptune is slightly lighter than previously thought. The new mass explains Neptune's and Uranus' orbits perfectly. Moreover, none of the spacecraft currently exiting the solar system are detecting gravitational tugs from large unknown objects.
Other people read the tea leaves of cometary patterns to conclude that something is perturbing their orbits.
While some comets are thought to come from the Kuiper Belt, most originate in the Oort Cloud, a region beyond 2000AU from the sun that may extend out a hundred times that, out to the point where the Sun's gravitational influence no longer outweighs that of other stars. While the number of comets existing in this space is thought to be vast, their orbits normally keep them where we cannot see.
Sometimes however, something interacts with an Oort Cloud comet, changing the orbit so it comes plunging into the inner solar system. Occasionally, further interactions with a planet keep it trapped here, becoming a regular visitor to our skies.
In 1984, in the wake of evidence that a comet was responsible for the extinction of the dinosaurs, people started to notice patterns in the timing of mass extinctions. Perhaps a planet, or borderline star, was interfering with the Oort cloud every 26million years, sending a burst of comets into the inner solar system, enough of which hit Earth to wipe millions of species from the fossil record. The extinction periodicity remains controversial, but efforts to find a cause have failed.
An updated version proposes Tyche (Nemesis’s sister in mythology), a planet of 1-4x the mass of Jupiter orbiting within the Oort Cloud and accounting for many of the comets entering the inner solar system. However, observations of Tyche's supposed location reveal nothing and most astronomers are highly skeptical.
In science, as in life, it is always hard to prove non-existence, but so far the case for Planet X doesn't look good.