7 mins read 15 Oct 2021

The Most Mysterious Body in the Solar System - Planet Nine

One of the most exciting debates in astronomy in recent years is related to the existence of a planet that is yet to be confirmed, lurking at the very edge of our own Solar System. Eloise Birchall writes about the great mystery of Planet Nine. 

Illustration of Planet Nine. Credit: Nagualdesign/T. Ruen/ESO.

The last time a new planet was discovered in our Solar System was in 1930 when Pluto was discovered (not counting the more recent discoveries that led to the reclassification of Pluto as a dwarf planet), and before that, it was Neptune in 1846. Prior to that, it was Uranus in 1781, with the other planets of the Solar system have been known since ancient times, due to the ability to see them with the naked eye. All this is to say that when the existence of a new planet for our Solar system was proposed a few years ago, people were keen to see if it was real or not.  

Now, currently, the answer to this seems to be “probably not”, but there are still those who believe that we may find Planet Nine one day. And although it may not be quite as hot of a topic as it was a few years ago, there are still plenty of people looking at it and discussing the likelihood of its existence and ways we can go looking for it.

Firstly, I’d like to talk about the methods being used to try to find Planet Nine. The idea is based on a tried and tested method that led to the discovery of Neptune in 1846 and is also a method used to find exoplanets today. 

Neptune was found because astronomers noticed that there were disturbances to the motion of Uranus (which resides closer to the Sun) that were best described by the gravity of an object beyond the orbit of the first Ice Giant. By studying the movements of Uranus, the position of the “culprit” in this orbital mystery was able to be narrowed down, and then, the mathematics came good - Neptune was found!

In other Solar systems, we can clearly see the effects of these gravitational disturbances on orbits, and they can be used to detect exoplanets with a method called transit-timing variation (TTV). When a planet orbits a star, typically the orbit takes the same length of time, but if an orbit is perturbed by the presence of another planet, the period of time can change. 

The transit method involves looking at a star and noting the dimming of the light coming from that star - charts of this are called a light curve - and once a few consecutive transits have been observed, the period of the planet can be calculated fairly well. This method is used frequently to detect hot Jupiters (giant planets close to their host stars). To detect smaller planets, and planets further from their host stars, methods like TTV can be used.

Transit timing variation involves looking at many transits of a star and then noting differences in the duration of the period of the transit. Since orbits can be disturbed by the presence of another body in the system, additional planets can then be inferred.

But what does all of this have to do with Planet Nine? This method, the one used to find Neptune and how we find some exoplanets, is how the idea of an additional planet got started in the first place, and how astronomers are seeking to prove or disprove the existence of Planet Nine. 

Beyond the orbit of Neptune, there is the scattered region of icy bodies, known as the Edgeworth-Kuiper belt, which you can think of as being similar to the asteroid belt, featuring more objects and spreading out to great distances from the Sun. 

These Edgeworth-Kuiper belt objects ( or KBOs for short) are in the outer Solar system, with some presenting peculiar orbits that could be explained by interactions with a larger object, further out than the known planets. 

If this object is out there, it needs to be in a highly eccentric orbit and with a mass greater than the Earth (potentially approaching the size of Neptune). In theory, such a planet could cause the orbits of a certain population of KBOs to be extremely odd, and perpendicular to the rest of the Kuiper belt.

Better understanding the behaviour of KBOs and their orbits is needed to determine whether there is a Planet Nine out there that we should be looking for. Attempts have been made by many groups, and even citizen science projects have been undertaken in the hope of trying to find it. In April 2017, the ABC ran a program called Stargazing Live, running alongside the program was a massive citizen science effort to search for Planet Nine. 

Australia’s also involved in the hunt for the enigmatic body, with the SkyMapper telescope at Siding Spring Observatory used to map the Southern Sky in detail and looking for any differences - SkyMapper’s purpose is to look for transient events; those that appear one night and change the next. This is done by taking a number of images of the sky each night and then comparing them - typically it is used for the detection of supernovae, but planets, asteroids and KBOs are also transient objects. Although this effort did not find Planet Nine, four smaller objects were found, which is still an awesome achievement.

In the last few years, the search for Planet Nine has continued, as surveys of KBOs have expanded there have been many trials and tribulations for the existence of the elusive planet. Is there clustering in the Kuiper belt that matches the presence of Planet Nine, or is it coincidental?

It also turns out that there are other surveys that have data about KBOs as a waste product of their observations. One such survey is the Dark Energy Survey (DES), which has added a number of KBOs to the set that can be studied for clustering. Other surveys are used as well to help gather enough samples to make the argument for or against the existence of Planet Nine more statistically significant.

Early this year, a paper announced that with additional KBOs they could not detect the clustering that was indicative of Planet Nine but using those in the original paper they could detect the clustering. 

However, proponents of the existence of Planet Nine have rebutted this, due to survey bias. They’ve claimed that as the data from the DES was used, and the DES was not designed for detection of KBOs or Planet Nine, and as such only looks at a specific location, then that maybe there still is the same amount of clustering.

At the moment, we still don’t know whether Planet Nine is out there or not, but it certainly is exciting to see the process of proving or disproving its existence in action. This is science in action - happening in real-time before our eyes. 



Eloise’s interest in space started when she was seven years old and an astronomer gave a talk at her primary school. Her interest in astronomy competed with her interest in fashion design when deciding what to study at university, but astronomy won in the end!

Eloise is a data scientist, with a Masters in Astronomy and Astrophysics, who is passionate about science communication. Her favourite astronomy topics are all things related to planets, especially planet and star formation, and being taught planet formation from the perspectives of geologists and astronomers in her undergraduate inspired her to pursue a Masters in this field.

Twitter: @ellykaybee