We have been stuck with 8 planets in our solar system ever since Pluto was downgraded to a dwarf planet in 2006. This ‘injustice’ may be corrected soon though, as astronomers believe that there might be a new 9th planet ready to be discovered after a 6-year search.
The idea for this planet comes from decades-long observations of small celestial bodies in the far reaches of our solar system. These bodies are observed for several reasons; to understand the formation history of the solar system, to test computer models, and of course to assess them as potential impact threats for the Earth.
What is Planet 9?
While studying these orbits, astronomers have found that they seem to be different from what was expected, suggesting that something is influencing them gravitationally. By looking at the differences between the predicted and observed orbit, it was possible to estimate an approximate mass, distance and location of this potential body, and the results suggest that it is potentially large enough to be a new planet. It is estimated to be between 5 to 10 Earth masses, and orbit between 400 and 800 Earth-Sun distances. The spread in parameters is because the models can be fitted by many different planets, a heavier one that is further away or a lighter one that is closer. Of course, it is also possible that there are two or more smaller bodies spread out over this range.
These findings were enough to start specialized searches with various telescopes peering deep into our own solar system. While it may sound paradoxical, it is much easier to observe far-away stars and even galaxies than it is to observe our own backyard. These distant places are bright and their stars light up the nearby regions, something that is not the case for our own distant solar system. In fact, this hypothetical planet is so far away that hardly any sunlight will reflect off of it. Its distance also makes it very cold, reducing any infra-red radiation that it might emit. For this reason, the search is done mainly in millimeter wavelengths with the 6-m Atacama Cosmology Telescope (ACT).
An ongoing, slow process
Over the last 6 years, 87% of the available sky has been observed with this telescope finding over 3500 candidates, but none of them confirmed yet. Once that is done, a similar search will have to be performed on the northern hemisphere. Still, even once we scan the whole sky, only about 10-20% of the possibilities will be excluded, and the search will have to be repeated with different telescopes in different wavelengths. Until then we can only hope for a stroke of luck from the ACT crew. Is Planet 9 really out there? Only time will tell. Until then, we can all argue about whether or not Pluto should become a planet again.