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When I was a kid, I learned that our solar system had 9 planets, but today it doesn’t.
Pluto, which was discovered in 1930 was the 9th planet. But 76 years later, in 2006,
it was demoted to something called a dwarf planet, not a full-fledged member of the
elitist club of the main planets. The way Pluto was discovered
was that Uranus’s orbit, Uranus is the 7th planet, could not be explained
by the 1846 discovery of Neptune, the 8th planet, alone.
So an additional 9th planet beyond Neptune was hypothesized.
But even after Pluto was discovered, it was deemed not massive enough to explain
Uranus’s and Neptune’s orbits. So we are now back to square one,
the same place as almost 100 years ago. Something heavy seems to be causing Uranus
and Neptune to move in a different orbit than what we can currently account for with the
celestial objects we know of. Many scientists believe that planet nine is really out there,
and it’s much bigger than Pluto, hiding in the darkness of space.
Is planet 9 real? What might it look like if we discover it?
And how did it get so far out in space? That’s coming up right now…
To understand this,
let’s consider a simple universe where nothing exists except the sun and earth. In this case
Newtonian gravity would be perfectly sufficient to describe the orbit of the earth and the sun.
With Newton’s law of gravity and the knowledge of the mass of the earth and sun, we can determine
almost exactly how this two-body solar system would work. Thus, everything would be consistent.
Now imagine that there are two other planets that we haven’t yet seen or discovered: Mars, and
Jupiter. In this were the case, the orbit of earth and the sun would not add up. Our two planet
system, would not behave as a two body system. Newtonian physics would tell us that something
isn’t adding up, and that some other gravitational influence must be out there for us to discover.
So even without knowing that Mars and Jupiter existed, we would still be
able to tell that something was out there. The reason is that planets have mass,
and thus gravity. And gravity has an infinite range, so, these unknown planets would affect
the orbit of the earth around the sun. We would detect this as inconsistent
with how a two body system of just the sun and Earth would behave.
Furthermore, if we carefully plotted the observed orbit of earth, we could
likely approximate the location and mass of the hypothetical planets behind the inconsistencies.
We could then point our telescopes in the direction predicted by our calculations,
and perhaps discover Mars and Jupiter, to finally explain earth’s mysterious orbital anomalies.
But then we might realize that these newly discovered planets still don’t explain everything,
and that something else still doesn’t add up. This would force us to expand our theory,
to find that the culprit is yet more undiscovered planets in the solar system.
This is pretty much the way science works. And, how the
history of our solar system has been shaped. Mercury, Venus, Mars, Jupiter and Saturn were
discovered a long time ago, way back in the 2nd millennium BC by Babylonian astronomers because
we can see them with the naked eye. But Uranus, the 7th planet wasn’t
discovered until the 18th century because it is not so visible. But after its discovery
theoretical physics and telescopes had advanced enough, that it became apparent to scientists that
something was wrong with the orbit of Uranus, that something heavy must be affecting it.
And thus the 8th planet, Neptune, became the first planet to be mathematically predicted.
Scientists could even estimate its size and location prior to any observation.
And this led to its discovery in 1846. But calculations suggested that something
still wasn’t right with the orbits Uranus and Neptune. And so the hunt for the 9th planet began.
Then we discovered Pluto in 1930 and everyone thought, “Great,
mystery solved.” But it turned out to be too small and light to make the inconsistency go away.
Since then, we have kept looking but found nothing else.
How can this be, with modern telescopes all around the earth, Hubble, and supercomputers
processing pictures of objects billions of light years away, why haven’t we seen anything?
Well, there are a few reasons. First, maybe there is no planet 9,
and the reason for the orbital anomalies is due to a problem with our mathematical models.
This is unlikely because our understanding of orbital mechanics is fairly advanced,
and the movements of planets in our own solar system is well understood.
Some scientists like astrophysics professor Ann-Marie Madigan of Colorado University,
suggest that a large planet so far out from the rest of the solar system would violate what
we know about planet formation. We would need to explain how such a large planet got there.
She believes it is not just one large planet but a new belt of smaller celestial bodies
beyond the Kuiper belt. And these objects would be so small and so far away
that they are going to be difficult or impossible to detect with current technology.
And there are other far out theories too. Harvard astronomers have suggested that it may be a
primordial black hole the size of a grapefruit. This would have the approximate mass of Neptune.
But this not a favored theory because black holes that small and that old would have radiated away
all their mass due to Hawking radiation. I have a video on what Hawking radiation
is if you find this an intriguing idea. And of course, there are scientists who
don’t buy any of this and think that we have poor data or are misinterpreting the data,
and that the idea of a 9th planet is silly. But there are some compelling reasons why
planet nine is thought to exist. It is not just that it would have
an effect on Uranus and Neptune, but also on other distant celestial
objects in our solar system. These objects are known as Trans-Neptunian objects, or TNOs,
meaning objects orbiting beyond Neptune. These include not only Pluto but also other dwarf
planets we’ve discovered, and icy objects. On average they are about 250 times father
from the sun than the earth. So they are pretty far away. Observations suggests that these TNOs
are also not orbiting as expected. They all tend to cluster together in their approach to the sun
and their orbits are similarly tilted. In 2016, Caltech astronomers Mike Brown
and Konstantin Batygin proposed that a large 9th planet, four to ten times more massive
than the earth, but about half the mass of Neptune, orbiting 360-620 times farther away
from the sun than earth, could explain the similar orbits of at many of these TNOs.
Such a planet would have a shepherding effect on these distant objects. It would also explain
orbits of TNOs perpendicular to the plane of the inner planets, since Its gravity would
dominate the outer edge of the solar system. This 9th planet would be extremely far away.
To give you an idea, if the Sun was the size of a basketball, the earth
would be the size of small bead 2mm wide, and would be orbiting about 25 meters or 82 feet away.
Let’s picture this basketball sun and bead earth, sitting in the center of Times Square in NYC.
Neptune the 8th planet, would be 30 times farther out or about
750 meters away, or roughly half a mile away. But this is nothing compared to how far away
planet 9 would be – at its maximum orbital diameter, it would be about 15 km or 9
miles from our basketball sun. And it would be about the size of a small pea.
When an object is so far away, very few photons from the sun reach it, and given that it would
be the size of a pea, even fewer photons are reflected back to earth for us to detect.
Imagine trying to look for a faint pea 15 km away from where you are sitting
right now. It’s not like detecting stars which are generally large and bright.
Now if this planet exists, the question is how did it get there? Several possible
origins for Planet Nine have been proposed. Brown and Batygin proposed that it formed closer
to the sun like the other large planets of our solar system, but was ejected into an eccentric
orbit after a close encounter with Jupiter or Saturn close to the time of the formation of
the solar system. Supercomputer simulations seem to suggest that this is a realistic possibility.
Others have proposed that this planet was captured from another star during a
possible close encounter with another solar system billions of years ago.
Yet others suggest that planet 9 was once a rogue planet
wandering in the Milky Way galaxy and was captured by our Sun’s gravity.
Could it have formed organically at its current orbit? This is not completely out of the question.
This could have happened if the protoplanetary disk that formed our solar system
was more massive and more extensive than we realize. An outward drift of matter in this
large disk could have formed a narrow ring which formed into a planet over billions of years.
The probability of this occurring is pretty low though, and has been estimated to be about 10% or less.
Let’s just make this clear though. Until we discover the ninth planet, it’s existence is
conjecture. But let’s suppose it is there, what would it look like? What should we be looking for?
As I mentioned earlier, due to its extreme distance from the sun,
Planet 9 would reflect very little sunlight. It would be about 600 times fainter than Pluto,
making direct telescope sightings very difficult or near impossible.
Furthermore, since Planet 9 would take 10,000 to 20,000 years to make a full orbit around the sun.
Statistically, the planet is more likely to be at its aphelion or farther away
in its orbit from the sun. This is because objects in orbit
move much slower near aphelion than perihelion, as shown by Keppler’s second law.
There are three possibilities for what it could look like. First it could be a rocky planet like
the earth, but up to ten times more massive. Exoplanet data seems to suggest that rocky super
earths are fairly common in our galaxy. And the fact that we don’t have one in our solar system
appears unusual, so maybe Planet 9 is our super earth.
If so, it could be dark because of dried ancient lava flows, perhaps similar
to some parts of earth like Iceland. These lava flows could be active even today.
The problem with this theory is that such super earths are found close to their stars.
How such a planet could end up so far from the sun would need to be explained.
A second possibility is planet 9 formed from ice in the outer solar
system. The interesting fact if this is true is that an ice world that big would
have an internal ocean of liquid water, similar to the how Europa, one of Jupiter’s moons
has an internal ocean beneath the ice. Could that ocean harbor life?
The problem with this theory is that if true, it would have formed
like the other Kuiper belt objects. Pluto is the largest such object we know of.
But Planet 9 would be thousands of times more massive. Scientists don’t think there
was enough ice or other material out that far to form an object so large.
So if it didn’t form in the icy outer solar system or the rocky inner solar system, the
other possibility, perhaps more likely is that it is a gas giant, having formed in the gas rich part
of the solar system, just like Jupiter, Saturn, Uranus and Neptune. It could be a mini Neptune,
with a rock core, and a very thick atmosphere. Its atmosphere could be transparent or
semi-transparent such that it reflects very little light. This is possible because with
almost no energy coming from the sun which would just look like a bright star at this distance,
it would be extremely cold, only about 47 degrees above absolute zero, according to some calculations.
At these temperatures, all the heavier molecules in the atmosphere would drop to the surface,
leaving only the lightest molecules remaining in the atmosphere, like hydrogen and helium,
making it translucent. Faint light and thunderstorms could be present
due to solar winds from the sun and other stars. So it’s faintness could be the reason we haven’t
detected it, but another possible reason we have not discovered planet 9 is because
it’s hiding in the direction of the bright center of the Milky Way galaxy.
This is like trying to find a tiny firefly in front of a search light.
And given space is so huge, there are simply many places to look so
it’s like looking for a needle in a haystack. But with newer telescopes such as the James
Webb Space Telescope, we should have a better chance than ever of discovering it.
Maybe we have already seen it, but we just haven’t processed the data.
It may be there already on a computer image somewhere, and is waiting for an astronomy
student with a keen eye for detail to discover it. And if you have a keen eye and crave more detail
beyond what I just discussed in this video, I highly recommend a course available on Wondrium,
todays’ sponsor, called “A Field guide to the planets.”
It’s taught by Professor Sabine Stanley, planetary scientist at Johns Hopkins University.
She takes you on grand tour of the solar system including
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which makes it a lot of fun. It’s a detailed 24 lecture, college level course,
where you’ll experience among other things, what each planet is like and what it would
take for you to get there. She even reviews how the gas giant planets, potentially like planet 9,
can form in one part of the solar system buy migrate to another part.
You’ll find this and many other courses on Astrophysics as a member of Wondrium.
And they are taught by some of the best teachers carefully selected by Wondrium.
We are lucky to be living in universe where Wondrium is available. I’ve been a member
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Be sure to take advantage of this by going to wondrium.com/arvin. That’s
wondrium.com/arvin. And you’ll be supporting me when you sign up,
so I can’t thank you enough for that. Feel free to let me know what you thought about this video,
and whether you think Planet 9 really exists. I will see you in the next video my friend.