By: Elliott Deyell (’26)
We all remember, or at least have heard of, the days when Pluto was a planet in our solar system. Well, now there’s a new candidate for the ninth planet to orbit the Sun, accurately named Planet Nine. Unlike Pluto, which was demoted due to its small size and irregular orbit, Planet Nine is thought to be a massive, hidden world lurking in the far reaches of our solar system. Its existence is based not on direct observation, but on the strange gravitational effects it seems to have on distant objects beyond Neptune.
Astronomers have been speculating about the possibility of more planets in our Solar System for decades, with proposals like Planet X, proposed in 1985. Planet X was an attempt to explain the semi-regular mass extinctions on Earth, where scientists thought a massive planet was deflecting comets headed towards Earth. The theory was that, at certain times, the planet’s orbit would move away from the Earth, allowing comets to collide with our planet. Under scrutiny, however, the theory did not hold up and subsequently faded away.
Then, in 2016, Michael Brown and Konstantin Batygin of the California Institute of Technology first proposed Planet Nine. Its existence was hypothesized to explain the unusual clustering of objects in the Kuiper Belt, the region of our Solar System past Neptune that’s home to millions of comets and dwarf planets. This behaviour would be best described by a large planet whose gravitational field is acting on them. This planet would have to be much more massive than the Earth and hundreds of astronomical units (AU) away from the Sun. As such, it would be too dim to be detected by visible light, but bright enough for far-infrared light. So far, there has been no evidence to back this theory, at least not until recently.
This April, astronomer Terry Long Phan and his team from the National Tsing Hua University in Taiwan published a paper detailing their discovery of a promising candidate for Planet Nine. The team compared two infrared sky surveys from two astronomy satellites, the Infrared Astronomy Satellite (IRAS) and the AKARI satellite. They were searching through over 2 million objects for candidates that met the specifications of Planet Nine and found 13 potential candidates. Each candidate was about 500 to 700 times as far from the sun as the Earth is, and 10 to 17 times the mass of the Earth. After thorough analysis and visual examination, the majority of these objects were discarded, all but one.
Phan’s candidate for Planet Nine appears in both the 1983 survey by IRAS and the 2006 survey by AKARI, albeit it appears 47.4 arcminutes away. This difference is well within the range of what a planet the size of Planet Nine could travel over these 23 years. Despite this, the movement isn’t large enough to extrapolate the orbit of the object, so we cannot know for sure if this is Planet Nine. According to Phan, “Once we know the position of the candidate, a longer exposure with the current large optical telescopes can detect it,”. The team plans on using a current device like the Dark Energy Camera or the upcoming Vera C. Rubin Observatory.
A potential discovery like this is extremely exciting, especially when we consider why we have never detected Planet Nine before. Previous surveys by NASA’s Wide-field Infrared Survey Explorer (WISE) had ruled out the possibility of Jupiter-size planets out to 256,000 AU and Saturn-size planets out to 10,000 AU (One Astronomical Unit or AU is the distance from the Earth to the Sun). However, a smaller planet around the size of Neptune or Uranus could have been undetected. Phan had also searched WISE data to locate his candidate, but we need more info about the orbit to know where it’s moved.
Another mystery surrounding this potential discovery is how a planet would have ended up in an orbit around 280 to 1,120 AU from the Sun. This is far beyond the realm of the other planets, with Neptune being 30 AU from the Sun. To give you a sense of just how far 280 AU is, if a plane could fly from the Sun to planet Nine, it would take about 6,000 years. So, how did this planet form so far away? There are two main theories on how this could have happened. First, the planet could have formed closer to the Sun and been gravitationally scattered over time by the other planets. The other theory is that it was a rogue planet caught by the gravitational field of the Sun at a time when other solar systems were closer.
While this is very exciting, it is also not the first time a candidate has been proposed. In 2021, Michael Swan-Robinson of Imperial College London found an object of approximately 3 to 5 Earth masses in IRAS data. It was estimated to be about 225 AU from the Sun, and thus fit many of the requirements for Planet Nine. However, he only detected it on IRAS data and not AKARI, making Phan’s candidate more credible.
Furthermore, Mike Brown, who initially proposed Planet Nine, is skeptical of Phan’s candidate. He doesn’t deny that it could be a planet, but he does not believe it is the Planet Nine he theorized, as it lies on an orbit outside his initial projection. Remember, the reason a ninth planet was theorized was to explain clustering in the Kuiper Belt. At 280 AU, Phan’s candidate for Planet Nine would have a limited effect on our solar system.
Terry Long Phan’s discovery of a potential candidate for the ninth planet in our solar system is exciting, but further data is required to be certain. Hopefully, with the opening of the Vera C. Rubin Observatory in Chile and the use of the Dark Energy Camera, astronomers can determine whether a ninth planet is hiding in our solar system. This new revelation demonstrates just how little we know about our solar system, let alone the entirety of outer space. We have yet to uncover the many mysteries of outer space, but Planet Nine is a step towards expanding our knowledge of the universe.