The enigmatic Planet 9 and the interstellar messenger hypothesis

In the last twenty years we have made important strides in exploring the outer solar system. We are talking about what is known as trans-Neptunian space, the eternal night beyond the realm of the giant planets. And in this exploration we have come across a surprising population of inhabitants, the so-called extreme trans-Neptunians, whose peculiar characteristics have caused an intense debate in the scientific community.

Some researchers see in this population the manifestation of an invisible presence, a new planet not yet discovered in the dark and cold confines of our solar system. Others, however, think that there is no such planet and that these extreme trans-Neptunian peculiarities are due to the incompleteness of our limited observations, the so-called observational biases.

A hypothetical huge and distant world

This hypothetical planet, and while its existence is confirmed or ruled out, is provisionally known as Planet 9. Remember that the solar system was left with only eight in 2006, when Pluto was removed from this category.

Planet 9 would not be a small object like Pluto or like many other trans-Neptunians that have been discovered in recent years. Detailed simulations have been made of what characteristics the body would have to produce the observed effects and the conclusion is that it should be a very large planet, with between 4 and 8 times the mass of the Earth. It would also be extremely far from the Sun: something like ten times the distance to Pluto. Probably even more.

Hypothetical orbit of Planet 9 with respect to the solar system and other extreme trans-Neptunian objects.
nagualdesign / WikipediaCC BY

If it existed, it would be a new type of planet, different from the others we know of in the solar system. Our planetary neighbors are basically classified into two types. They are either small rocky worlds with a solid surface (Mercury, Venus, Earth, and Mars), or they are gas giants (Jupiter, Saturn, Uranus, and Neptune).

Planet 9 would fall somewhere in between these categories. It could be what is known as a super-Earth, a rocky planet larger than our own, or a sub-Neptune, a gaseous world less massive and slightly smaller than Neptune.

We have located such planets in other stars but, being so far away, we know very little about them. Discovering one in our own solar system would open the doors to study in detail a category of planets that today is almost unknown.

The long journey of the asteroid CNEOS14

How could we detect this Planet 9? Is not easy. Being so far away, its brightness would be extremely dim and we would need powerful telescopes. The problem is that these telescopes usually have a very small field of view. It is like using a microscope to search a very large area for something small that we have dropped. In recent years, significant observational efforts have been made to try to discover this elusive world, so far without success.

A few months ago a scientific article by two Harvard researchers was published stating that a meteorite (CNEOS14) that fell in the Pacific in 2014 was not an object of our solar system. It would be the first interstellar object that we have detected, a small asteroid approximately one meter in diameter that impacted our planet while traveling through the solar system at 60 kilometers per second.

This high speed is precisely what led researchers to determine its provenance as a visitor from other stars. To do this, they first had to rule out that the object had been accelerated or deflected by the gravity of a planet in the solar system, which is easy to verify by reconstructing its trajectory and seeing that it had not passed close to any of the known planets.

Now, what if CNEOS14 had interacted with a yet unknown planet during its journey through the solar system? This was the question that we asked ourselves and that opened a new line of work.

an amazing coincidence

The first hint that there might be a connection between the CNEOS14 meteorite and Planet 9 appeared when we plotted on a sky map the orbit that the planet would have taken according to the most detailed simulations and superimposed the origin of CNEOS14. We found a striking match (see figure below) between the meteorite’s origin and the region where simulations predict Planet 9 is most likely to be found. The probability of such a match being the result of chance is on the order of 1% .

file 20230130 24 ze78ek.png?ixlib=rb 1.1
Possible trajectory of Planet 9 in the sky. The colored band indicates the region through which Planet 9 would move according to the simulations (Brown and Batygin, 2021). In red the regions where there would be more probability of finding it and in blue where less. The ellipses mark the direction of origin of CNEOS14 at different moments in time calculated by different authors. The blue ellipse is the direction calculated by these authors at the moment of crossing the supposed orbit of Planet 9. Figure reproduced from Astrophysical Journal (Socas-Navarro, 2023).

Following this thread, we did simulations reconstructing the trajectory of CNEOS14 and found three other statistical anomalies that would be highly improbable in an object that comes directly from the interstellar medium. Combining the probability of these irregularities we get that either there is something we don’t understand about objects in the interstellar medium, or there is a 99.9% chance that CNEOS14 ran into an unknown planet in the outer solar system. And that new world would be located right in the region predicted by the simulations.

These coincidences and statistical anomalies have led us to formulate the “messenger hypothesis”, referring to the use of the term delivery courier in astrophysics to denote particles that bring us information from celestial bodies, such as neutrinos, cosmic rays, or gravitational waves. According to this hypothesis, CNEOS14 would have been deflected in our direction by an unknown massive object in the outer solar system, possibly Planet 9, between 30 and 60 years ago.

If the conjecture is correct, tracing the trajectory of CNEOS14 backwards in time we would find the location of Planet 9 which, according to our calculations, would currently be very close to the point where the constellations Aries, Taurus and Cetus meet. We have an observation campaign underway at the Javalambre Observatory (Teruel) to carry out this search. The task is still difficult and will take time and work because the field to be scanned is still large and the object sought is very weak, but now it is encompassable.

Of course, today our hypothesis is no more than speculation, just like the very existence of Planet 9 is. However, it is a well-founded speculation that meets the three requirements to be taken seriously in science. : a) it is physically plausible; b) is well motivated; and c) it is empirically verifiable.

CNEOS14 could be pointing us to the position of Planet 9. Or perhaps it is a great cosmic coincidence. In any case, it is a beautiful story about which we could say that of If it’s not true, it’s ben trovato (if it is not true, it is well sought), an expression that, by the way, is attributed to an astronomer, the Renaissance friar Giordano Bruno.

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