If Planet Nine is out there, a large, mysterious planet lurking on the dark edges of the solar system, it may not be where we thought it would be.
According to astronomers looking for the hypothetical object, new information taken into account could mean that its orbit is significantly more elliptical than the most recent prediction.
The hypothetical Planet Nine made a grand entrance in 2016, when astronomers Konstantin Batygin and Michael Brown of Caltech published a paper in The Astronomical JournalIn it, they set out their plea for an undiscovered planet in the outer regions of the solar system. The evidence, they said, lay in other objects far beyond Neptune’s orbit.
These objects are called Extreme Trans-Neptunian Objects (ETNOs). They have enormous elliptical orbits, never intersecting closer to the sun than Neptune’s orbit by 30 astronomical units, and swing out farther than 150 astronomical units.
Batygin and Brown found that these orbits have the same angle at perihelion, the point in their orbit closest to the sun. The astronomers ran a series of simulations and found that the gravity of a large planet could cluster the orbits in this way.
Since that article was dropped, the theory has become highly controversial, with many astronomers finding Planet Nine’s existence unlikely, but so far somehow we don’t have hard evidence. The most decisive way the debate will be resolved is if we find the slick thing – and a new update from Batygin and Brown could help us do that.
Their new paper has been accepted The Astrophysical Journal Letters, and is available on the preprint server arXiv.
The first detection of a possible Planet Nine in 2016 was based on just six ETNOs – after all, these objects are very small and very difficult to detect. Over time, more ETNOs have been discovered – today we know about 19 – meaning we now have more data to analyze to calculate the planet’s characteristics.
In 2019, the astronomers revised the available information and concluded that they had gotten a few things slightly incorrect. The planet’s mass was only five times the mass of the Earth, according to the revision, instead of the 10 they had initially calculated, and its eccentricity – elliptical as it may be – was lower.
And now they have updated those calculations again.
But, they wrote in a post on the Find Planet Nine blog, the question we asked ourselves during the height of the pandemic is another question: Are essential physics missing from our simulations? we have found that the answer to this question is ‘yes’. “
Their simulations, they said, assumed that any object farther than 10,000 astronomical units from the sun would be lost in space. What they didn’t take into account was that the sun was not born in isolation, but probably in a large, densely populated star-forming cloud with other baby stars.
Under these conditions, the infant solar system would almost certainly have formed an innermost part 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 gigantic planets such as Saturn and Jupiter would have thrown debris out into interstellar space; but the gravitational perturbations of passing stars would have pushed them back into the sun’s gravitational influence, eventually forming the inner Oort cloud.
We tend to think of the Oort cloud as just hanging around, actually not doing much, but when Batygin and Brown ran a whole bunch of new simulations, taking these physics into account, they found that objects in the inner region of the Oort cloud can indeed be a move a little.
“Planet Nine, however, changes this picture on a qualitative level,” said the researchers.
Because of the long-term gravity of Planet Nine’s orbit, objects in the inner Oort cloud evolve on a time scale of a billion years and are slowly re-injected into the outer solar system. So what happens to them? Perturbations of the canonical giant planets , planet Nine, passing stars, as well as the galactic tide, and have found that these re-injected objects in the inner Oort Cloud can easily mix with the count of distant objects in the Kuiper Belt and even exhibit orbital clustering. ”
This means that some of the extreme trans-Neptunian objects we found could actually be from the Oort cloud, which is really cool. However, the team’s simulations also showed that the clustering of the Oort Cloud objects would be weaker than that of the objects coming closer from the Kuiper Belt.
This suggests that a more eccentric orbit for Planet Nine would explain the data better than the orbit the researchers found in the 2019 paper.
We will not know exactly how eccentric that orbit might be until more research can be done on the clustered objects to determine which of them originate from the inner Oort cloud; but there is a limit to how eccentric the orbit can become before it is no longer consistent with our observations of the outer solar system.
Because the hypothetical planet is so distant and so dim, our chances of seeing it are very low, so this information can be used to refine models and keep us from looking for it where it might not be – hopefully leading this to a detection of this elusive beast.
Even if we never find it, the discoveries it has led to have been amazing. A whole bunch of new Jupiter moons and super distant potential dwarf planets is nothing to sneeze at.
Batygin and Brown’s new document has been accepted The Astrophysical Journal Letters, and is available on arXiv.