If Planet Nine is there, a large, mysterious planet lurking on the dark sides of the Solar System, this may not be where we thought it might be.
According to astronomers searching for the hypothetical object, new information taken into account could mean that its orbit is significantly more elliptical than recently predicted.
The hypothetical Planet Nine made a huge entrance in 2016 when astronomers Konstantin Batygin and Michael Brown of Caltech wrote an article in The Astronomical Journal. In it, they set out their case for an as yet undiscovered planet in the outer parts of the Solar System. According to them, the evidence lies in other objects from far beyond the orbit of Neptune.
These objects are called Extreme Trans-Neptunian Objects (ETNOs). They have large elliptical orbits, and never cross closer to the sun than Neptune’s orbit around 30 astronomical units, and swing out beyond 150 astronomical units.
Batygin and Brown found that these orbits have the same angle at the perihelion, the point in their orbit that is closest to the sun. The astronomers performed a series of simulations and found that the gravitational influence of a large planet could constrict the orbits in this way.
Since this article dropped, the theory has become very controversial, and many astronomers find the existence of Planet Nine unlikely, but so far we have no solid evidence whatsoever. The most crucial way the debate will be resolved is if we find the smooth thing – and a new update from Batygin and Brown can help us try to do that.
Their new paper has been accepted The astrophysical journal letters, and is available on the preprinted server arXiv.
The initial detection of a possible Planet Nine in 2016 is based on only six ETNOs – these objects are, after all, very small and very difficult to detect. Over time, more ETNOs have been discovered – today we know about 19 – which means we now have more data to calculate the properties of the planet.
In 2019, the astronomers reviewed the available information and came to the conclusion that they got some things a little wrong. According to the review, the mass of the planet was only five times the mass of the earth, rather than the ten they initially calculated, and its eccentricity – how elliptical it is – was lower.
And now they have updated the calculations again.
“However, they write in a post on the Find Planet Nine blog,” the question we asked ourselves during the pandemic is another question: are essential physics missing in our simulations? Through our constant and relentless examination of the model, we have discovered that the answer to this question is ‘yes’. “
According to them, their simulations assumed that any object moving beyond 10,000 astronomical units away from the sun would be lost in space. What they did not take into account was that the sun was not born in isolation, but probably in a large, strongly populated star-forming cloud along with other baby stars.
Under these conditions, the infant solar system would almost certainly have formed an inner portion of the Oort cloud, the shell of icy bodies surrounding the solar system, between about 2,000 and 100,000 astronomical units from the sun. The formation of giant planets such as Saturn and Jupiter would have thrown debris outward into interstellar space; but the gravitational perturbations of passing stars would have pushed them back into the sun’s gravitational influence, so that it eventually forms the inner Ear Cloud.
We tend to regard the Oort cloud as a kind of hanging thing, really doing nothing, but when Batygin and Brown performed a whole bunch of new simulations, taking into account this physics, they found objects in the inner region of the Oort cloud can indeed move a bit.
“However, Planet Nine is changing this image on a qualitative level,” the researchers said.
“Due to the long-term gravity of the planet Nine’s orbit, inner Oort Cloud objects are evolving on billion-year-old time scales and are slowly being injected back into the outer solar system. What happens to them then? We simulated and took this process into account with disturbances of the canonical giant planets, Planet Nine, passing stars, as well as the galactic tide, and found that these reinjected inner-Oort cloud objects can easily mix with the sensation of distant objects of the Kuiper belt, and even ‘ can display an orbital grouping. “
This means that some of the extreme trans-Neptunian objects we found could actually originate in the Oort cloud, which is really cool. However, the simulations of the team also showed that the grouping of the Oort Cloud objects would be weaker than those of the objects that came from the Kuiper belt.
This suggests that a more eccentric orbit for Planet Nine will explain the data better than the orbit found by the researchers’ 2019 article.
We will not know exactly how eccentric the orbit may be until more research can be done into the grouped objects to determine which of them originated in the inner Ear Cloud; but there is a limit to how eccentric the orbit can become before it no longer aligns with our observations of the outer solar system.
Because the hypothetical planet is so far away and so dull, our chances of spotting it are very low, so this information can be used to refine models and make us search in places where it may not be – hopefully this leads to the detection of this elusive animal.
Even though we never find it, the discoveries that led to it were amazing. A whole bunch of new Jovian moons and potential dwarf planets that are very far away are nothing to sneeze at.
Batygin and Brown’s new paper accepted The astrophysical journal letters, and is available on arXiv.