How many planets REALLY are there in the Solar System?
If I asked you to imagine our solar system, you would think about the big posters that
we are used to looking at since we were little kids, wondering about the mysteries of the
universe.
You might think about Jupiter, with its orange and brownish colors and its intriguing features
on the surface, or you might think about Saturn with its beautiful rings, or Earth, with its
blue oceans.
But what if...
I asked you how many planets are there in the solar system?
The straightforward answer would be 8, of course.
I mean, those posters are quite clear.
We go from Mercury to Neptune.
But turns out that the planets are eight...only because of the formal definition of the planet
that we adopt.
For example, if we go back to 2005, Pluto was still considered a planet, therefore posters
before that date would present the solar system as formed by 9 planets, instead of 8.
It's crazy if you think about how our understanding of the universe modifies the semantics we
use, and how the semantics in turn change the vision we have of reality.
For example, if we defined planets as any object with an almost perfect triangular shape,
the number of planets in the solar system would be...zero!
Anyway, we were saying that, by definition, in the solar system there are only 8 planets.
By definition, there are three rules to define a planet:
1.
It should have a mass such that its surface is a sphere.
In fact, gravity acts upon objects with a given mass, and if the object is massive enough
it is going to become spherical in shape, gravity being the same in all the directions
and pushing inwards towards the center of the body.
An object that's too small to become a sphere, cannot be classified as a planet.
However, this is not a so-sharp definition, because the variety of objects and masses
and shapes is kind of continuous in our universe, and therefore there might be a “grey zone”
in terms of mass, where it is difficult to tell if the object we are facing can be classified
as a planet in these terms or not.
For example, there could be some objects really similar to planets, but with a slightly less
amount of mass than is needed for gravity to give it the needed spherical shape.
So, in simple terms, if an object exists that orbits around the sun, but is not enough massive
to have a spherical shape, I'm sorry to inform you that..it can't be classified as a planet.
The second rule is: that in order for an object to be a planet, it should be orbiting around
the Sun.
2.
Of course, we are talking about our solar system, but, more in general, an object could
be defined as a planet if it orbits around its host star.
In this case, however, we would be talking about exoplanets, like the ones that have
been studied and discovered by the Kepler mission, and not about PLANETS.
So as you can understand, the only real PLANETS are the eight ones in our solar system.
3.
The third requirement is that the planet must be dominating its neighborhood.
This basically means that it should be the biggest object along its own orbit, but the
things here are a bit complex.
For example, Pluto doesn't satisfy this last requirement, because, at a certain point along
its orbit, it crosses the orbit of Neptune, and therefore it is no longer the main object
dominating its own orbit.
It is all matter of orbits and laws of physics.
And if this pisses you off, go talk to the celestial mechanics' police!
But the worst thing about this definition is that, if you took Earth and placed it where
Pluto is, the same would happen: the Earth would cross Neptune's orbit as well, thus
being declassified from its privileged status of a planet.
Basically, this third condition seems a bit weird because it is not based on what the
object is, but it is more geography-based and on the position of that specific object.
So on one hand, it can be useful to characterize the relative positions and roles of objects
in the solar system, but on the other hand, it is useful to have it, because a different
definition, maybe a geological definition, could be the wrong choice because we would
have to exclude all the planets that don't match that specific geological definition.
Just saying, we could also think about eliminating the second rule, and say that a planet is
an object massive enough to be spherical in shape, but then we would end up including
stars as well, so we have to modify it by saying that: a planet is an object massive
enough to be spherical in shape, but not massive enough to start the nuclear fusion in its
nucleus.
At that point, we could also say: that objects that look like potatoes are asteroids, objects
with a tail are comets... and so on.
This would be a super simplified version of the definition.
In this simplified version, we could define planets as objects massive enough to be spherical
in shape but not massive enough to shine because of nuclear fusion.
Then if we wanted to, we could also include the second rule: planets are the ones orbiting
around the sun.
If we adopted this version of the definition, we would have 13 planets in our solar system.
In fact, we would have to add 5 of them to the 8 already known:
Pluto, Ceres, Makemake, Haumea, Eris.
These are classified as dwarf planets.
So as you can see, a little modification in the semantics, in the definitions, could have
a huge impact on our vision of the universe, and our posters would change drastically.
"Hey!
If you are still here watching the video, it means you are passionately curious about
the planets and dwarf planets!
We constantly strive to make videos that excite a curious person like you, so be sure to subscribe
now and press the bell notification"
But are we sure about the number of dwarf planets in our solar system?
How many of them actually exist?
This is complex.
We have a lot of them!
They are objects that we have seen, but we have really poor data about them because they
are way further than Pluto.
We know that their size can span a range between 400 and 1000 km in radius and if they are
composed of rocky ice and water – as we think since they are really far away from
the sun - considering their density and their size their mass should be enough for them
to be spherical in shape.
In this case, a lot of dwarf planets would be present in our solar system, waiting for
us to classify them better, and add them to our posters.
Some of them are, for example, Gonggong, Orcus, and Sedna.
New space telescopes, such as the JWST, could give us more data about them so that at the
end of this decade we would have a better understanding of the universe we are living
in.
Another thing to take into account is of course the possible presence of planet 9.
Astronomers observed some deviations in the path of some small objects, meaning that their
orbits are under the influence of some unknown perturber.
According to the observed data, this perturber would have a mass of 6-10 Earth masses and
it would be a really far away object, in the interstellar space.
So, if such a planet exists, we would see it as a really really dim source, orbiting
around the sun, along an orbit with a huge semi-major axis.
Several possible origins for Planet Nine have been examined including its ejection from
the neighborhood of the known giant planets, capture from another star, and in situ formation.
In their initial article, scientists Batygin and Brown proposed that Planet Nine formed
closer to the Sun and was ejected into a distant eccentric orbit following a close encounter
with Jupiter or Saturn during the nebular epoch.
The gravity of a nearby star, or drag from the gaseous remnants of the Solar nebula,
increased its perihelion, leaving it in a very wide but stable orbit beyond the influence
of the other planets.
The odds of this occurring have been estimated at a few percent.
Had it not been flung into the Solar System's farthest reaches, Planet Nine could have accreted
more mass from the proto-planetary disk and developed into the core of a gas giant.
Instead, its growth was halted early, leaving it with a lower mass than Uranus or Neptune.
Or maybe Planet Nine could have been captured from outside the Solar System during a close
encounter between the Sun and another star.
If a planet was in a distant orbit around this star, three-body interactions during
the encounter could alter the planet's path, leaving it in a stable orbit around the Sun.
A planet originating in a system without Jupiter-massed planets could remain in a distant orbit for
a longer time, increasing its chances of capture.
Astrophysicists Amir Siraj and Avi Loeb found that the odds of the Sun capturing Planet
Nine increase by a factor of 20 if the Sun once had a distant, equal-mass binary companion.
So maybe we could look for a detached old companion of our Sun!
Being this far and dim, it is really difficult to spot.
Even JWST would have a hard time looking for planet 9 because Webb is designed to observe
objects with a known position in the universe, but we don't really know where planet 9 could
be in space.
It would be like looking for your smartphone in your house, using a magnifying glass.
It would be much better if you can just use your own eyes, and it would be less expensive
in terms of time.
Good luck with that!
In any case, I would point out the fact that, if we rely on the definition of a planet as
some object orbiting around a star, we would be missing out.
For example, what if I told you that there are objects big enough and spherical in shape,
that have oceans on their surface and that could also host life, but they are not orbiting
around any star?
It is the case, for instance, a Saturn's moon-like Titan, which has its own hydrologic cycle
– just like the Earth, only based on methane rather than water – could be classified
as a planet.
In this case, we would say that planets are defined as all objects big enough to be spherical
in shape and that orbit around other planets.
We could call them “solar planets” if they are orbiting around the Sun, we could
call them “satellite planets” if they are orbiting around another planet or object,
we will then have exoplanets if they orbit around another star.
In this case, our Moon would be classified as a satellite planet and our posters would
picture something like 108 planets in the solar system.
As you can see, the definition of a planet is a bit controversial, and as soon as it
changes, our understanding of the universe change.
Ah!
If only we could go back in time and adopt the Greek definition!
It would be much easier!
In fact, the Greeks decided that everything moving in the night sky (with the exception
of comets) was a planet.
The only thing is...our night sky is filled with artificial satellites that orbit and
move during the night.
Should we refer to them as planets at that point?
Thanks for watching everyone!
What do you guys think about this video?
Let me know in the comments below, be sure to subscribe, and I'll see you next time on
the channel!