The Mysterious Planet 9 – Is it Really Real?

The Mysterious Planet 9

There may be a hypothetical Neptune-sized planet lurking out in the far reaches of our solar system past Pluto. There is mathematical evidence found by Caltech researchers in 2015 suggesting that we may have an undiscovered 9th planet.

This planet, called “Planet X” or “Planet 9” could be 10 times as massive as the Earth, more than 20 times farther from the Sun than Neptune, and could have a year that is 10,000 to 20,000 Earth-years long.

This evidence is not proof of a ninth planet, it is still only theoretical as we do not have any direct observations of this so-called Planet 9, yet. Astronomers are searching for this possible planet. If a Planet 9 was to be discovered, it will be the first new planet in our solar system since Pluto was officially observed in 1930. 

How Did the Idea Originate?

How Did the Idea Originate

The idea for a ninth planet originated in the early 1900s. Astronomers suggested that the wobbles observed in the orbits of Uranus and Neptune could be explained by another planet further out in the solar system. When Pluto was discovered, astronomers thought this was the mysterious Planet 9 affecting the orbits of Neptune and Uranus, but it was later revealed that Pluto was too small to cause such an affect.

In 1989, NASA’s Voyager 2’s flyby collected data that showed there was not wobble in the outer planetsorbits. Leaving astronomers to seemingly be searching for something that did not exist. However, with the discovery of the Keiper Belt in 1992, and its continued study, came more signs that there very well might be a Planet 9. 

Why Do They Think There is Another Planet?

Why Do They Think There is Another Planet

Mike Brown and Konstantin Batygin from Caltech released an article in 2015 detailing their discovery of some interesting movements amongst objects in the Kuiper Belt. A dwarf planet names Sedna, and five other Keiper Belt objects are being tilted at the same angle and in the same direction, causing them to have orbits that are clustered together. They predict that this may be due to the gravitational pull from a massive planet in the Keiper Belt.

Since 2016, 11 objects have been identified as having strange orbits that are not due to the influence of Neptune. Could these orbits be the result of a Planet 9? Based on the apparent gravitational pull, scientists have estimated that Planet 9’s mass is likely 10 times that of the Earth.

Based on this and what we know about our solar system, it has been theorized what this planet may look like. It is theorized that Planet 9 is either a rocky super-Earth or a gaseous mini-Neptune. The planet would share many characteristics with Uranus and Neptune, indicating that it would be an icy planet with a solid core. 

Why Haven’t We Found It Yet?

Why Haven’t We Found It Yet

While the Sun is very bright here at Earth the further away you get the darker it is in space making the objects deep in our solar system faint and hard to detect. It is often said finding Pluto is like finding a piece of coal in the dark, and this object is predicted to be even further out from that.

Astronomers often do not focus on searching for one object at a time, but focus on a class of objects, such as a type of galaxy or a type of exoplanet. Batygin suggests that part of Planet 9’s elusiveness may be due to it being at the far edge of its own orbit, and likely reflecting little to if any sunlight, basically living in a shadow. However, while it will be hard to find, astronomers calculate that if it is there, today’s telescopes should be able to find it eventually. 

How Are We Trying to Find It?

How Are We Trying to Find It

The 8.2m Subaru telescope in Hawaii is aiding the search for Planet 9. While this is one of the best options currently available, the team only has access to Subaru approximately three nights a year to search for the dark planet. There is a chance that NASA’s Transiting Exoplanet Survey Satellite (TESS) may catch sight of Planet 9 while it is searching for exoplanets around distance stars.

The best hope to find Planet 9 lies with a future telescope currently under construction in Chile. In 2023, the 8.4m Vera Rubin Observatory will begin taking photographs of the entire visible sky (2/3s of the whole sky). It will uniformly capture our visible sky and take repeated observation every few nights.

This will allow astronomers to track the movements of objects in our solar system and beyond making the Vera Rubin Observatory our next best chance of finding Planet 9. Further research from Brown and Batygin have provided revised numbers on how long it may take Planet 9 to orbit the Sun.

Previously thought to have a 10,000 to 20,000 Earth-year orbit, it has now suggested that Planet 9 only takes approximately 7,400 Earth-years to orbit the Sun. A tighter orbit greatly improves the chances of discovery. 

What Else Could It Be?

What Else Could It Be

There are some sceptics who do not believe there is a ninth planet lurking in the Kuiper Belt. Some believe it is actually a primordial black hole causing the strange orbits of Kuiper Belt objects. A primordial black hole is one of the dense regions in space formed during the first seconds of the Universe’s existence.

Black holes are some of the densest objects in the Universe, making it a contender for it affecting orbits of objects in the Kuiper Belt. Others believe that evidence for Planet 9 is just a natural bias in sky surveys. They suggest that we do not know enough about Kuiper Belt objects to truly say if they are acting strangely or not, let alone what could be causing this so-called strange activity.

As searches for Planet 9 keep coming back empty handed, critics continue to say it is just a ghost in the data. Despite the critics, Planet 9 enthusiasts and researchers such as Brown and Batygin continue to hunt for this mysterious planet. 


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Cassie Hatcher

Cassie Hatcher

Cassie is a lifelong learner with a passion for communicating high level science in a conversational matter. She holds a B.S. and M.S. in physics and has written two astronomy theses, one of which is published. She earned an internship at NASA’s Goddard Space Flight Center in 2016 and got the chance to see the James Webb Space Telescope while it was being built.

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