What if our Solar System could go back to hosting nine planets like in the good old days? I am not going to convince you that Pluto is the rightful owner of this title – it simply isn’t. Scientific evidence quite inevitably points towards the presence of a planet about five times as massive as the Earth outside the orbit of Neptune. With an orbit highly elliptical, this elusive planet has not been spotted yet, however its presence has been made reasonably evident in the structure of the outer Solar System.
The Kuiper belt, beyond the orbit of Neptune, is home to a plethora of small bodies called trans-Neptunian objects (TNOs). The positions and dynamics of most of these bodies follow the expected paths consistent with the properties of our Solar System. It is the absurd paths of some extreme bodies in this region which led to astronomers Konstantin Batygin and Mike Brown of Caltech to hypothesize the presence of Planet 9 (as it is called since Pluto lost its status as a major planet) in 2016. The object 2012 VP113 with a diameter of 600km, at a distance of about 80AU, was the first smoking gun of this hypothesis. Light, at a speed of 300,000 km/s takes a day to reach it from the sun, and then reflect off to reach the Earth. Moreover, closer studies of its orbital path revealed that its queer orbital path resembled that of another TNO called Sedna – their angle of approach to the same is similar. Given that the Solar System and its main objects formed from the same accretion disk, initially it was thought that Sedna’s peculiar orbit was due to sheer randomness. However since this characteristic is shared with 2012 VP113, it is fair to raise an eyebrow at how random this truly is.
Note that most of these TNOs have very similar perihelia – distances of closest approach to the sun. This supports the hypothesis for Planet Nine. The dotted red orbit is the predicted path of Planet Nine.
These outliers have highly elliptical orbital planes with orbital periods significantly larger than the norms – mostly measured in millennia. Some objects exhibit perihelion distances which are vastly beyond Neptune’s gravitational reach while harbouring retrograde orbits. These seemingly odd behaviours can be explained by the presence of an undetected planet in a trans-Neptunian orbit. However, some hypotheses suggest that perhaps these anomalies are due to something far more profound lurking in the backyard of the Solar System – a primordial black hole.
1. Planet Nine
Let’s assume that the body causing this chaos is in fact a planet. Studying the deviations of TNOs and other nearby bodies, computer-modelling suggests that this planet should have a mass between five and ten Earth masses with an orbital distance of between 400 and 500 AU. In comparison, the average distance of Pluto’s orbit is about 40 AU.
It is fundamental to note that even though this object is far more massive than the Earth, it is easily elusive due to its large distance from the sun. Hence, Planet Nine is most probably very faint and its massive orbit simply means that astronomers would have to map a large region of the night sky to observe it. This demands a large aperture telescope – Brown and his team are using the 8.2m Subaru telescope in Hawaii with a search area equivalent to 3,200 full moons, located in between Orion’s belt and Taurus. The difficulty of this search is increased a notch as it isn’t a 2D search – it’s three-dimensional. The exact distance to Planet Nine isn’t known which means that if nearer than expected, it will be brighter and if further away, fainter.
Batygin’s latest simulations and modelling suggests that “Planet Nine is smaller in all parameters by a factor of two compared to our original estimates”. The latest figures estimate that the orbital period is 10,000 years instead of the previous value of 20,000 while its mass seems to be closer to five Earth masses. If the orbit is indeed shorter, despite the smaller the size, it should be about 2.5 times brighter than the initial estimates made in 2016.
Their search has covered 50% of the sky in the expected range. Planet Nine seems to be a dull one – one with a relatively lower albedo. Its predicted position between Orion’s belt and Taurus further restricts their search to the months of winter. The Large Synoptic Survey Telescope, due to begin operating in 2022, will boost this search, if Planet Nine isn’t discovered by then. Even if this search does not detect Planet Nine immediately, it promises to discover countless TNOs – and if their orbits hint at the shared alignment, astronomers would have stronger clues as to the location of this elusive giant.
We are left with one key question – how did Planet 9 get to its current location? There are three possible scenarios. It may have been created at this far-flung point in space yet this seems unlikely as the Solar System is still young in its evolution for an accretion disk to have occurred at this distance. Perhaps Planet Nine was created much closer to the sun and later thrust into this far-flung region. This theory is not on firm footing as it requires a major event such as a nearby star passing the Solar System. Such an event would have left more clues in the Solar System we observe today and none can be seen whatsoever. Thirdly, Planet Nine may have been a free-floating planet that was captured by the sun. This field still requires thorough research and currently, there is little information regarding the statistics of such planets.
This leaves us with the final possibility and leads into the next segment. If the sun’s gravitational field can capture a planet this massive, it is equally possible that it can capture a primordial black hole of a similar mass.
2. Primordial Black Hole
Primordial Black Holes (PBHs) are small black holes which are remnants from the early universe. They formed shortly after the Big Bang as a result of the random density fluctuations in the early universe. Low mass PBHs are thought to have evaporated emitting Hawking radiation however larger mass PBHs may exist even today. Planet Nine was theorised to be a PBH in a recently published paper by astronomers Jakub Scholtz at Durham University and James Unwin of University of Illinois.
This theory seems to have fewer supporters in the scientific community. Proving this is a fundamental challenge as a black hole of the equivalent mass would have a radius of 5cm and this corresponds to a Hawking temperature of 0.004K. This is much colder than the Cosmic Microwave Background which has a temperature of roughly 2.7K. The power it radiates is infinitesimal in comparison making it much harder to detect.
If this is indeed a primordial black hole, it would leave a notably unique signature. Scholtz and Unwin suggest that the black hole with be shelled by a dark matter halo and their annihilation would generate gamma radiation which may be detected. In the future, they plan to use the Fermi Gamma Ray Space Telescope to look for these tell-tale signs.
With summer around the corner, Brown and Batygin’s search for Planet Nine has halted till winter approaches once again. It may be months or even years till we make a breakthrough. But even then, this is not an eternal mystery. “Nature has no obligation to you. Look at gravitational waves – they took 100 years to find,” says Batygin.
Planet Nine is more than a mere breakthrough in planetary sciences. Yes, it will be another testament to the power of mathematics and physics in successfully predicting yet another feat; but it will also be a unique looking glass that will give us another peek into how our Solar System evolved.
Image source: https://astronomy.com/news/2019/10/planet-nine-may-be-a-black-hole-the-size-of-a-baseball
Fauxtoez/Wikimedia Commons
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