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Planet Nine Mystery Or History?
Planetary observation suggest the presence of a ninth planet lurking in the outskirts of the Solar System. What evidence do we have to support this claim?
Date : 05/06/2020
Author Information
Uploaded by : Anwesha
Uploaded on : 05/06/2020
Subject : Physics
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.
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.
This resource was uploaded by: Anwesha