Planet X | A Hidden World In Our Solar System | Facts and Theories

  • Planet X is a hypothetical planet that is located about 400 astronomical units (AU) away. 
  • It may take up to 20,000 Earth years to complete one full orbit around our Sun. 
  • The planet has not yet been observed but mathematical calculations suggest it is there. 

Seeing is believing, but when it comes to Planet X, complex computations of the celestial body’s behavior and careful observations of orbital anomalies of the region beyond Neptune will have to do for now.

In 2015, researchers at California Institute of Technology found mathematical evidence indicating there may be a ‘Planet X’ in our solar system. They gave a nickname to this space object – Planet Nine.

The hypothetical planet revolves around the Sun in a large orbit far beyond Pluto. It orbits nearly 20 times farther from the Sun on average as compared to Neptune. The planet may take up to 20,000 Earth years to complete one orbit around the Sun. To put this into perspective, Neptune takes about 165 Earth years to complete a single orbit with an average orbital speed of 5.43 km/s.

Planet Nine is thought to be gaseous, similar to Neptune and Uranus. It is estimated to have a mass of 5 to 10 times than that of Earth and a diameter of 2 to 4 times Earth’s.

‘Planet X’ and ‘Planet Nine’ Are Two Different Things

Planet X is conspiracy theory about an invisible transdimensional planet that exists beyond the planet Neptune. It was first proposed in 1906 by an American Percival Lowell. The theory is now disproven.

The hypothetical Planet Nine, on the other hand, is based on detailed mathematical modeling and computer simulations which suggest that there is a trans-Neptunian planet far out there.

The evidence of the Planet Nine has been building over the past few years, as researchers have observed strange clustering in the orbits of minor objects beyond Neptune in the outermost region of the Solar system.

The planets, to scale | Credit: James Tuttle Keane/Caltech

If discovered, it would replace Pluto as the 9th planet of our solar system. That’s why Caltech researchers are calling it Planet Nine. However, some astronomers (and many laypeople) still regard Pluto as the 9th planet and thus use the term ‘Planet X’ or ‘Planet Next’ for the undiscovered body instead.

Reference: The Astronomical Journal | DOI:10.3847/0004-6256/151/2/22 | Caltech 

Although Caltech researchers presented the evidence of this hypothetical planet in 2015, the actual naming rights of the celestial body go the individual who actually discovers it.

If the ‘Planet X’ is found, its name must be approved by the IAU-WGPSN (short for International Astronomical Union Working Group for Planetary System Nomenclature).

Now that you have the basic understanding of Planet X, let’s dig deeper and try to understand why scientists think it is there.

What We Know About ‘Planet X’ So Far?

Researchers studying the Kuiper Belt — a circumstellar disc in the outer Solar System extending from Neptune orbit to 50 AU from the Sun — observed that some dwarf planets and minor, icy celestial bodies tend to follow orbits that cluster together.

Caltech team analyzed these orbits and predicted that a planet bigger than Earth (but smaller than Neptune and Uranus) might be hiding far beyond Pluto. The gravity of this undiscovered planet might explain the unique orbits of at least 5 minor celestial bodies (a group of extreme trans-Neptunian objects) discovered in the Kuiper Belt.

A group of extreme trans-Neptunian objects, Planet X and its orbit | Credit: JPL; Caltech

The orbits of these trans-Neptunian objects are similarly tilted and they make their closest approaches to the Sun in one sector. Their strange orbits could only be explained by a planet (Planet X).

Read: Researchers Discover A Planet Exactly Where ‘Vulcan’ Was Predicted To Exist In Star Trek

Some theories indicate that Planet X is the core of a giant planet that was ejected from its actual orbit by Jupiter during the formation and early evolution of our solar system. Unlike dwarf planets, Planet X gravitationally dominates its neighborhood. In fact, its dominating area is larger than any other known planet in our solar system.

4 Major Evidence of Planet X 

The gravitational pull of this hypothetical planet would explain four anomalies of the Solar system:

  1. The clustering of orbits of extreme trans-Neptunian objects.
  2. The high inclinations of these objects with orbits nearly perpendicular to the known-planets’ orbits.
  3. The high perihelia (closest point to the Sun) of Neptune-detached objects, such as Sedna.
  4. The high-inclinations of trans-Neptunian objects with semi-major axis less than 15,000 million kilometers.

Reference: ScienceDirect | DOI:10.1016/j.physrep.2019.01.009

Despite years of research, astronomers are still not sure the Planet X is out there. Unless it is observed, its existence is completely conjectural. However, we now have solid evidence that the planetary census of our solar system is incomplete.

Finding Planet X would help solve many mysteries of the solar system, including the origin of 2015 BP519, a ‘likely’ dwarf planet that measures approximately 550 kilometers and has been described as an extended scattered disc object.

Sensitive Sky Surveys

Although detailed sky surveys such as Pan-STARRS and Wide-field Infrared Survey Explorer were not able to detect Planet X, they haven’t ruled out the existence of such an object in the outer solar system either.

Researchers are currently using the world’s most advanced telescopes, including NEOWISE and Subaru Telescope, to search for the planet in its predicted orbit. Over the course of the next decade, we will have more sensitive sky surveys.

Read: Mercury (Not Venus) Is The Closest Planet To Earth | New Study

By 2030, astronomers will have seen it or will have better ideas about its location. And if it is not there, they will have alternate explanations for the observed peculiarities of the Solar System.

Written by
Varun Kumar

Varun Kumar is an experienced science and technology journalist interested in machines, AI, and space exploration. He received a Master's degree in computer science from Indraprastha University. To find out what his latest project is, feel free to directly email him at [email protected] 

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