With the rebranding of Pluto to a dwarf planet a space has opened up in our solar system for a ninth planet. And people have been searching for it for the last few years; the mysterious Planet Nine.
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Planet Nine is thought to be larger than Earth and orbit well beyond the distance of Pluto, and astronomers are looking for it, as well as evidence it may or may not exist.
In 1846, Neptune was found by studying the orbit of Uranus. The orbits of the planets are all affected by the gravity of the other planets of the solar system. By looking at the orbit of Uranus and seeing it was wobbling and not behaving as would be expected if it was the most distant object in the solar system, astronomers were able to locate the cause: Neptune.
Beyond the orbit of Neptune lies the Kuiper Belt, where there are many small icy bodies, some like Pluto and some smaller or slightly larger.
These Kuiper Belt Objects (KBOs) are remnants from when the solar system was forming, and would be expected to have somewhat regular orbits.
However, it has been found the KBOs do not always behave as expected. There are some on very eccentric orbits at and in completely different directions compared to the rest of the KBOs. It is these objects and their peculiar orbits that were the first indication there might be something bigger lurking out in the distant solar system, hoping to take the title of Planet Nine.
Given the way these KBOs orbit, it is expected Planet Nine might be quite large - bigger than the Earth and possibly as big as Neptune; as well as quite far away, and on a very eccentric (oval) orbit.
A few years ago, a large citizen science project was undertaken in hopes of finding Planet Nine, as part of the Stargazing Live television program.
This project used data from the SkyMapper telescope at Siding Spring Observatory in Coonabarabran. The SkyMapper telescope takes images of the whole southern sky every night, and differences in the images between the nights help identify certain astronomical events. Most of the time SkyMapper is looking for transient events like supernovae, but in this case, it was used to look for changes that could be our mystery planet, Planet Nine.
Although the Stargazing Live effort to find Planet 9 was unsuccessful, the epic tale of Planet Nine does not end there.
As our understanding and tracking of KBOs and their orbits improves, the evidence for and against the existence of Planet Nine continues to pile up. Astronomers are trying to determine if the strange cluster of orbits they see in the Kuiper Belt are just coincidence or if they have been perturbed by a Neptune-sized planet hiding in the shadows.
As astronomers continue searching for the elusive Planet Nine, there have been studies that indicate it exists and others that suggest it doesn't. Watching the quest for scientific knowledge play out makes for interesting reading.
If you're wondering whether Planet Nine is out there, the answer is "we aren't sure yet".
But who knows, maybe soon we'll have a ninth planet in the solar system again.
- Eloise Birchall has a Masters of Astronomy and Astrophysics from the Australian National University and is passionate about science communication and outreach.
