As it is known, Pluto, was discovered in 1930 but ceased to be considered as a planet in 2006 when the International Astronomical Association withdrew that title for not meeting the minimum planet requirements. What very few people know is that Pluto then became the first and most popular Kuiper Belt Object (KBO).
The Kuiper Belt is a large set of debris and remnants from the early formation stages of our Solar System made out of tens of thousands of objects (only 2,000 have already been catalogued) of different natures and sizes orbiting around our Sun in a disc shaped cloud with a huge extension with an inner perimeter beginning at a distance of 30 AU (1 Astronomical Unit or AU is the average distance from Earth to the Sun) and a still defined outer perimeter that ends at 50 AU (… but the belt goes on with gradual loss in density up to distances of more than 1,000 AU). A high percentage of those KBOs are bodies of water or methane ice with diameters of more than 100 Km in diameter that originate most of the short-period comets that most probably resulted in the formation of our atmosphere and our seas by impact with our planet.
Pluto was considered a planet since 1930 when it was discovered until 2006 when it ceased to be a planet but became the first and most popular Kuiper Belt object.
Since the prediction of the existence of the Kuiper Belt in 1951 and its subsequent confirmation in 1992, it has been found that the orbits of several KBOs in the most exterior areas do not respond to predictions made with the most advanced means of observation, measurement and calculation and that those orbits adopt a very unusual eccentricity format while also aligning preferably in a very particular direction against the odds of a more uniform distribution. The improbability of these orbits calls for the consideration of the existence of a relatively massive and undetected body with a gravitational influence that over the course of billions of years has been deforming, stretching and aligning the orbits of dozens of KBOs to leave them where they are now.
More precise simulations suggest that these orbits can be explained by taking into account a super-planet of 5 to 10 Earth masses, with an extremely eccentric orbit of 300 to 700 AU, a great inclination with respect to the plane of the other planets which takes 10,000 to 20,000 years to complete. The characteristics of this orbit make it currently impossible to detect that body directly.
The origin of this body (already dubbed as Planet 9) is not well known but the most probable hypotheses are:
Perhaps the most accepted one is that it formed at the same time as the rest of the planets in an area near the Sun but in a relatively unstable orbit that ended up succumbing to interaction with Jupiter or Saturn and from there was ejected to its current stable orbit that is so distant and lonely that since then it stopped its growth by accretion (capture of matter from its surroundings) preventing it from becoming another Jupiter and is no longer affected by the gravity of the rest of the planets in the Solar System.
It has also been considered that its peculiar orbit is explained by the possibility that it was a rogue planet born in another solar system but expelled from it by the same mechanism mentioned above and later captured by our Sun at some point during its solitary journey through our galaxy.
But the most recent hypothesis is that it is not even a planet. Some astronomers think it could be a primordial black hole the size of a basketball.
Primordial black holes are a hypothetical class of black holes formulated by Stephen Hawking in 1974. Formed during the first fractions of a second after the Big Bang, they are the prime suspects of concentrating the dark matter of the universe (which is known to exist but has not yet been identified). Due to Hawking’s radiation many of them are slowly dissolving until they disappear, but in that process they can reach any size, no matter how small, since they are not limited by the minimum possible size of 12,000 m in diameter (the Tolman-Oppenheimer-Volkoff limit) that applies to black holes formed from a star of 2.2 solar masses (the smallest size of star that can become a blackhole).
Unfortunately, black holes are, by definition, invisible in all radiation spectra and cannot be detected directly (as a star or a hot nebula would) but by the effects they produce around them. If, moreover, they are tiny in size then the odds for detectability are further reduced. Despite this, many black holes have been detected (none primordial though) by the gravitational waves they produce in their collisions (at least 10 binary systems of colliding black holes have already been detected by this recent method), by the altered orbits of nearby bodies, by the accretion disk that they generate when devouring a neighbouring star, etc.
If Planet 9 is a primordial black hole it will most likely be detected by observing how it diverts the orbit of a passing by comet, after all, it is in an area where comets abound.
On the other hand, there are also astronomers who believe that the data predicting the existence of Planet 9 are merely the product of an artefact due to the technical limits of the current observation capacities and a possible statistical bias in the processes of cataloguing celestial bodies when it is focused on reduced areas of space.
Anyhow, it is always worth knowing our solar neighbours in-depth (you never know what may be lurking outside) so there are already several lines of research to confirm the existence (or non-existence) of Planet 9 and to identify its nature if it does exist.
The most immediate initiative will occur in 2023 when the Transitioning Exoplanet Survey Satellite (TESS) will reach a working orbit. This satellite is designed to detect and survey exoplanets but is likely to provide details in this case as well (more about the TESS mission in the article “Exoplanets”).
In that same year, it is also expected that the Large Synoptic Survey Telescope (LSST) located in Chile starts operations. It has been designed specifically for the sweeping of large areas of space and their comparison over long periods of time allowing for the detection of very small objects within our Solar System, such as distant comets that can be influenced by a primordial black hole.
So, we will be watching …starting in 2023, you may be told that our Solar System has a new planet …or maybe, a new primordial black hole.