Is there or is there no planet 9? Is there a planet outside of our solar system with enough mass to explain the movements of distant objects? Or is a slice of icy material responsible? There is no direct evidence for a true planet 9 yet, but anything of sufficient mass affects the orbits of distant solar system objects.
A recent study suggests that a slice of icy material causes the strange motions of the outer objects of the solar system and that we do not have to invent another planet to explain these movements. The study is by
Professor Jihad Touma of the American University of Beirut and
Antranik Sefilian, a PhD student in the Cambridge Department of Applied Mathematics and Theoretical Physics. Their results will be published in the Astronomical Journal.
The idea of another planet in the remotest parts of our solar system is attractive. It enlivens the adventurer in all of us. And for the astronomer or the astronomers who may finally discover it, this would be a coronation. Who does not want to be known as the discoverer of a whole new planet in our solar system? It is much more exciting than being the person who has finally confirmed the mass of a slice of icy material.
As astronomers are better able to explore and understand the distant solar system, they have found more and more objects. Over the past 15 years, astronomers have discovered about 30 trans-neptunal objects (TNOs) that pass through highly elliptical orbits. The youngest was "The Goblin", a body with an orbit that is 2300 AU away from the Sun.
Since these bodies do not interact gravitationally with the other planets of the solar system, there must be another mass aggregate out there that shapes their orbits. And although the explanation of Planet 9 has gained in importance over the years, there is no direct evidence that a planet is responsible for shaping these strange orbits.
"The hypothesis of Planet Nine is intriguing, but if the ninth planet exists, then it has avoided detection so far.
Study co-author Antranik Sefilian, graduate student in Cambridge, Department of Applied Mathematics and Theoretical Physics.
The new study suggests that a disk of icy material is responsible for the highly elliptical orbits of distant objects. It is not the first theory to suggest this, but it is the first that can explain the observed orbits while taking into account the mass and gravity of the other eight planets of our solar system.
The 30 TNOs Moving These highly elliptical orbits belong to a larger group of TNOs and objects forming the Kuiper Belt. The Kuiper belt is made of material left over from the formation of the solar system. Most of these objects are almost circular in shape around the sun. But the 30, which do not move in almost circular orbits, have a different spatial orientation, and that requires an explanation.
The most discussed explanation is Planet Nine. Planet Nine would have to be about ten times more massive than Earth. This planet, hidden in the faint vastness of the solar system, would place those 30 bodies in their unusual orbits.
The problem is that no one has yet discovered Planet Nine, and it's only known by the observed effect.  "The Nine-Planet Hypothesis is intriguing, but if the ninth planet exists, it has so far avoided detection," said Antranik Sefilian, co-author of the Department of Applied Mathematics and Theoretical Physics at Cambridge. "We wanted to know if there is another, less dramatic and perhaps more natural reason for the unusual orbits that we see in some TNOs. We thought, rather than admitting a ninth planet, and then worrying about its formation and unusual orbit, why not just consider the gravity of small objects that represent a disk beyond Neptune's orbit, and see what they do to us does?  The new study is based on a detailed modeling of the solar system as well as on observations of other solar systems.
Touma and Sefilian modeled the overall spatial dynamics of TNOs with the combined effect of the giant outer planets and a massive, expanded disk of material beyond Neptune. They calculated a model that can explain the highly elliptical, orbitally focused orbits of the 30 TNOs. They also identified mass areas and shapes for the icy slices of material. They were also able to force incremental shifts in their orientations (or precession rate), faithfully reproducing TNO outliers.
In the vicinity, collective attractions between these objects could easily explain the eccentric orbits that we see in some TNOs, "said Sefilian, a Gates Cambridge scholar and a member of Darwin College.
So, case closed? Not quite.
"Although we have no direct observation data for the disk, we have none for Planet Nine. Therefore, we explore other possibilities. "
It is somehow easy to suggest another undiscovered planet with the right mass to explain these observed orbits. So far, however, such a planet has avoided detection. But in a sense, the disc of icy material theory suffers from the same. It's easy enough to suggest, and building a successful mathematical model that supports the ice disk theory at least proves it's possible, but it has not been discovered yet.
In fact, previous attempts to estimate the mass of icy objects beyond Neptune have summed up only about one-tenth of Earth's mass, which is not nearly enough to account for this weird collection of orbits to explain. The model developed by the two scientists behind this new study requires ten times more mass than that.
This is where the observation of other solar systems comes into play.
"The problem is, if you watch the disk from within the system, it's almost impossible to see it all at once."
"When we look at other systems, we often study a disk that surrounds the host star to account for the properties of planets in orbit," said Sefilian. "The problem is when you look at the disc from the system Although there is no direct observation data for the disk, nor for Planet Nine, we are exploring other possibilities, but it is interesting to note that observations of Kuiper belt analogs around other stars as well as from planetary formation models to show massive remnants of debris. "
Other solar systems have left a slice of icy material, the mass of which is sufficient to account for the highly elliptical orbits Objects on the edge of the systems. Could this also apply in our solar system? Could there be a slice of icy material as well as a Planet 9?
Sefilian believes it. "It's also possible that both things could be true – there could be a massive disk and a ninth planet – with the discovery of each new TNO, we gather more evidence that could help explain their behavior."
Das Back and forth of scholars looking for evidence that sometimes coincides and sometimes vehemently disagrees is shown in this issue
The study, particularly the Introduction and Conclusion, presents and cites other studies that support and reject it are still in the early days of a detailed understanding of the distant solar system, and as more powerful telescopes will be put online over the next few years, with more powerful computers and improved observation methods, it is only a matter of time before the distant orbits of these distant bodies are finally explained.