The Search for Planet X

With more than 1,600 planets detected around other stars, surely we know how many planets live closer to home in our own solar system, right? Not exactly! Let's sort through the tumultuous history, including a recent intriguing discovery, of the search for the mysterious, distant Planet X.

Sabrina Stierwalt, PhD,
February 1, 2016

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How many planets are in our solar system? That is, how many planetary neighbors are there orbiting around our star, the Sun? Now that, thanks to missions like Kepler, we have more than 1,600 confirmed exoplanets (planets outside of our solar system) and over twice as many candidate exoplanets, you would think we would know, without a doubt, how many planets are much closer to home in our own solar system. The debate rages on, however, and the flame was rekindled last month when astronomers once again claimed to have found evidence of a mysterious Planet X.

To understand this latest discovery, let’s first dive into a bit of the history of searches for a mysterious, distant planet lurking beyond Neptune in the outskirts of our solar system.

What Is a Dwarf Planet?

In the mid-1800s, our solar system was home to seven known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, and Uranus. Around that time, however, astronomers had noted unexplained wobbles in the orbit of Uranus that kept it from following a perfectly straightforward orbit. To explain these wobbles, the French mathematician Urbain Le Verrier and the British mathematician John Adams predicted that a large planet must exist beyond Uranus and that the gravitational tug from this planet was causing Uranus’s orbit to stray. In 1846, the astronomer Johann Galle discovered Neptune.

The mass of the newly discovered Neptune, however, could not explain all of the discrepancies in the orbit of Uranus, and so in 1906, the American tycoon and astronomer Percival Lowell began a search for a planet he thought must exist beyond Neptune. Then, sure enough, in 1930, Clyde Tombaugh discovered Pluto. For almost fifty years, Pluto was thought to have solved the mystery of the Uranian wobbles.

However, as astronomers learned more about the icy world, it was finally determined in 1978 that Pluto was too small to inspire the necessary gravitational tug. The search for the mystery planet was back on until 1992 when more detailed observations from the Voyager 2 probe revealed that astronomers had been overestimating Neptune’s mass. Adjusting the mass eliminated the orbital wobbles.

Although it was no longer needed to explain any orbital discrepancies, Pluto still reigned as the ninth and most distant planet in our solar system for a total of more than 75 years until 2006. In August of that year, astronomers from the International Astronomical Union voted to demote Pluto to dwarf planet status.

As more and more of our solar system was explored, astronomers and planetary scientists discovered a wealth of objects beyond the orbit of Neptune in a band called the Kuiper Belt. These so-called Kuiper Belt Objects (KBOs) vary in size from small chunks of ice to several kilometers across. The recent New Horizons probe is the first mission to get an up close look at the Kuiper Belt.

With more studies of the Kuiper Belt, the reality became clear: if Pluto were discovered today, it would not be considered a planet. Pluto is likely a KBO! It’s probably the most massive KBO, but a KBO nonetheless. Thus, Pluto is now dubbed a dwarf planet instead. A dwarf planet shares many of the properties of a bonafide planet—it directly orbits the Sun and it is massive enough for its shape to be in equilibrium under its own gravity. But it lacks the size necessary to clear any debris from the neighborhood within its orbital path.


Before Pluto was demoted, the debate over an additional planet in our solar system was reignited in 1986, this time not by astronomers but by paleontologists. David Raup and Jack Sepkoski of the University of Chicago claimed that their investigations of the fossil record determined that mass extinctions happened cyclically. They blamed some kind of extraterrestrial source.

Soon after, two teams of astronomers including Whitmire, Jackson, Davis, Hut, and Richard Muller all independently came up with a possible solution. They hypothesized that the Sun was not a solo star, but was actually in a binary system with a brown dwarf with highly elliptical orbit. Brown dwarfs fall somewhere between a planet and a star. With masses on the order of 10 to 80 times the size of Jupiter, our most massive planet, they cannot sustain the nuclear reactions required for stardom. They are stars that fail to shine.

The prediction was that this brown dwarf would get close to Earth approximately every 26 million years due to a very large, elliptical orbit. On its way in, the dwarf would perturb comets in the Oort cloud, a cloud of comets and other icy bodies predicted to surround the Sun (and our entire solar system) at a distance of more than one thousand times farther than the Kuiper Belt. This disturbance in the Oort Cloud would then cause those comets to rain down on Earth wreaking havoc, and this failed star was thus dubbed Nemesis. Only recently (circa 2010) newer analyses of the fossil record have suggested that mass extinctions are not as periodic as we once thought, and the Nemesis hypothesis has mostly been put to rest.


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