For many of us, living in an eight planet solar system is a fundamental pillar in our
scientific knowledge. With Pluto having been demoted
to a dwarf planet in 2006, Neptune is the last
planet to have been discovered in our solar system. Neptune was first observed with a
telescope in 1846 and since then there s been widespread discussion about whether or not
there s another planet beyond it. This hypothetical planet is what astronomers refer to as Planet
X.
But could Planet X really exist? Discovering a ninth planet in our solar system almost
two centuries after the last discovery may seem
far fetched, but it s actually way more likely than you
might think. In fact, the existence of Planet X would account for a lot of anomalies that
scientists have noticed, such as the seemingly erroneous
orbits of some trans-neptunian objects, or TNO s. Let s dig a little deeper.
The story of how Neptune was discovered can provide some important insight into the possible
existence of Planet X. Neptune is not visible to the naked eye, having only been observed
via telescope for the first time in 1846. Uranus had been discovered in 1781 and, in
the interim, calculations had been made about its orbit. Astronomical tables calculating
the orbit of Uranus were published in 1821 using Newton s laws, which is the scientific
standard. Using these, one could presumably reliably pinpoint the position of Uranus at
any given time in the future. However, there was a problem. Subsequent observations of
Uranus showed deviations in its orbit, which seemed to defy Newton s laws. There were few
reasons for this that we could explain. It was possible that the effect of the Sun s
gravity differed from Newton s description when the colossal distance between it and
Uranus was taken into account. It was also possible that the error had been made in observation.
The third possibility however was that Uranus s orbit was being altered, or perturbed, by
another gravitational body.
As we now know, subsequent searching led to the discovery of Neptune, and the understanding
that it had, in fact, been affecting Uranus orbit. Neptune has 17 times the mass of Earth,
and is also denser than Uranus, despite Uranus larger
diameter. But what does this mean for Planet X?
Well, even before the discovery of Neptune, there was debate within the scientific community
about whether or not one planet alone could cause the discrepancy in the Uranian orbit.
A German astronomer and director of the Gotha Observatory at the time, is reported to have
hypothesised as early as 1934 that one planet would not have sufficient gravitational pull
to explain the motions of Uranus, and conjectured that there must be two planets beyond Uranus.
After the discovery of Neptune, there was still widespread speculation about the existence
of a further planet. In 1848, French physicist Jacques Babinet hypothesised that there was
a planet with 12 times the mass of Earth beyond Neptune, which he referred to as Hyperion.
Scottish astronomer George Forbes calculated that there must be two trans-Neptunian planets,
based on the study of the paths of several comets conducted by Camille Flammarion in
1879. By the turn of the 20th century, William Henry Pickering, former director of the Harvard
College Observatory, was leading two searches for trans-Neptunian planets- however both
searches came up empty handed. Does this mean that Planet X simply doesn t exist?
The fact that we haven t yet found a ninth planet doesn t necessarily mean that it s
not out there though. It s possible that we just haven t
made the technological advancements necessary to
see it yet. Remember, Neptune isn t visible to the human eye. Although it has brightened
since 1980 with a now average apparent magnitude
of 7.78, it s still easily outshone by multiple celestial bodies, including Jupiter's Galilean
moons, the dwarf planet Ceres and even several asteroids: specifically 4 Vesta, 2 Pallas,
7 Iris, 3 Juno, and 6 Hebe. Until the invention of the famous Hubble Space Telescope, it was
a challenge to observe Neptune even with a telescope.
Now, with advances in telescopic recording and other technology, it s easier to visualise
Neptune- but even now, some of the best footage we have of Neptune, such as this false colour
image taken from Voyager 2 in 1996, leaves a lot to be desired with regards to exact
details.
With an average distance of 4.5 billion km from the sun, Neptune is simply too far away
for us to garner much information about it at all. It
s for that reason that it s easy to see how another
planet even further away could still be evading us.
Now, you might be asking- what about Pluto? True, Pluto is even further away than Neptune
and yet has been observed since 1930. In fact, at this time it was even believed to be Planet
X! But the discovery of Pluto was a stroke of luck. Clyde Tombaugh, a Kansas farmboy,
had been serendipitously handed the job of locating Planet X at the Lowell Observatory,
replacing Percival Lowell who was widely known for his dedication
to finding Planet X until his death. Taking photos of patches of the night sky two weeks
apart, he tracked the entire zodiac. Then, by chance, in January 1930, two of his photos
of Gemini detected movement. This breakthrough led to the discovery of Pluto, which he named
in honour of Lowell. Ultimately, it transpired that Pluto did not have the gravitational
pull necessary to be the planet they were actually looking for, but it renewed hope
that another planet could be found with the right technology- and the right timing. We
re still discovering TNOs in this century, such as Eris, and with each improvement of
technology we come closer and closer to settling this case.
However, there are other explanations that scientists have been investigating for these
orbital discrepancies. Scientists from the University
of Cambridge and the American University of Beruit published their theory in 2019 that
the unusual orbits of TNOs can be explained by their combined gravitational force as they
orbit the sun, as opposed to the existence of a ninth planet beyond Neptune. They propose
there could be a disc made up of smaller, icy TNOs and that these together could constitute
a mass up to ten times the size of Earth s. If true, this is a game changer for how scientists
have been pursuing the questions about the unusual orbits of these exoplanets- and could
be a nail in the coffin for Planet X. There s more that could disprove the theory of the
ninth planet though. Voyager 2 s space mission flyby of Neptune in 1989 reassessed the mass
of the giant planet and subsequent calculations put it at 0.5% less than previously thought.
Though seemingly a miniscule percentage, it made a massive difference. In actuality, Neptune
s mass was more comparable to that of Mars and when that fact is taken into account the
anomalies in Uranus orbit practically disappear. The readjusted mass of Neptune explains why
Uranus orbit seemed erroneous- and explains away the scientific need for a ninth planet
to exist. This is why many scientists today have reached a consensus that if a ninth planet
does exist, it s not the one that scientists of the 20th century expected.
But does this mean the end for any hopes of discovering another planet out there? Not
quite. Just because there is no longer any evidence pointing to a giant planet close
enough to alter the orbits of Neptune and Uranus, doesn t mean that there isn t another
planet further out. Like we said before, Neptune is so far away that we don t have the technology
yet to get a clear footage of it, so it s not beyond the imagination that a planet could
exist beyond it with an orbit so vast that we haven t managed to track it down yet. And
there is still evidence to support this idea.
Take the orbit of the planetoid Sedna, a TNO that was discovered in 2003. It s orbit is
incredibly erratic and cannot be explained by any gravitational
body we ve observed thus far. While some theories proposed are that its orbit has been
affected by the gravitational pull of an undiscovered star, other theories propose that the gravitational
pull of a distant giant planet could explain it too.
However, while Mike Brown, the astronomer who discovered Sedna, has stated that the
balance of probabilities lies with there being another planet beyond Neptune, that it s unlikely
the planet would achieve anything beyond dwarf planet status due to the amount of uncleared
planetoids and gravitational objects in its vicinity.
Is there a Planet X out there? While the jury is still out on what exactly is out there,
we can say for certain that with the advancements we ve been making in technology and with the
determination of astronomers worldwide, someday we ll find out.
The New Horizons Probe that flew by Pluto in 2015 didn t detect any large unknown planets
on it s journey, but it did capture the surface and secrets of Pluto like we ve never seen
before.
Click here to see exactly what New Horizons discovered on the surface of Pluto. Thanks
for watching Elderfox. Remember to subscribe and hit the bell to keep up to date with the
latest discoveries.