Why do all planets orbit in the same plane?
Why do the Sun, Moon and planets all chase each other on the same orbital plane?
We noticed it quite early, we humans.
Of what?
Well...of the fact that there were fixed stars in the sky, and then other stars that were,
however, constantly changing position, always chasing each other along a celestial path
that seemed unchanging over the centuries.
A path also followed by the Moon, and which apparently seemed to follow the Sun as well,
before it was realized that our star was stationary and that it was the Earth that was turning
along with all the other known planets.
Basically, the ancient observers were witnessing a strange thing...The fixed stars filled the
sky throughout, free to shine wherever they wished, while the "wandering stars," i.e.,
the "planets," seemed to be forced to move all within that very thin, imaginary band
that enveloped the entire celestial sphere A belt that came to be called the "ecliptic"
because only along its path could an eclipse occur between the Sun and Moon; and with which
was immediately associated the so-called zodiac belt, in which the 12 constellations crossed
by the planets, Sun and Moon were identified.
The ecliptic was of great interest to early sky watchers, as it was obviously imbued with
meanings related to astrology and divination based on the position of the stars.
When the mysticisms of astrology gave way to science, it became clear that there was
indeed meaning in the ecliptic, but only of an astronomical nature.
It suddenly became clear, after Kepler with his laws and then Galileo with his telescope
established once and for all the real nature of the wandering stars, that it could not
be an accident that the planets orbited the Sun all in the same plane...
In fact, what could be expected was that the orbits were randomly oriented, since gravity,
the force that keeps the planets in these stable orbits, works the same way in all three
dimensions.
So it would have been more normal to observe an arrangement more like that of a swarm buzzing
around a beehive rather than a nice, orderly series of near-perfect circles - in fact,
much more than normal since someone took the trouble to calculate that the odds of having
a solar system with 8 planets orbiting such similar planets are about one against 30 billion!
That oddity, clearly, had to depend on something related to the origin of the solar system
itself.
But from what, exactly?
The answer to this question is quite complex and involves several important physical concepts
that we are about to learn about together.
Are you ready for a plunge back in time billions of years?
A long time ago, when the star formation rate of the Milky Way had practically reached its
maximum, something very normal for those times, but at the same time very special for us,
began to happen in one of its innermost arms; in fact, what we now consider our celestial
condominium was about to take shape: the Solar System.
A construction that, as we shall see, would not last a day, or even a century - but even
a decade or so of millions of years.
The time it took for our Sun to form and begin to shine as a reassuring main sequence star.
Ten million years on the human time scale may seem like an infinite amount of time,
but in fact, it represents only one-thousandth of the age of the universe at the time.
Then again, what we are about to describe are all slow, very slow, albeit immense processes;
phenomena in comparison with which our most terrible and earth-shattering earthquakes
or the most disastrous tropical hurricanes are in no way comparable.
Our tale begins 4.6 billion years ago, amid the faint veils of a cloud orbiting lazily
at a distance of 27,000 light-years from the galactic center.
It is an immense mass of gas by our standards: extending for 50 or perhaps 100 light-years,
although the size represents only a thousandth of the diameter of the entire Galaxy.
Its density, typically of a hundred molecules per cubic centimeter, could rival the most
driven vacuum we can get here on Earth today; and its temperature is terribly low, only
a few degrees above absolute zero.
It may seem strange, but the origin of our Solar System lies entirely in this icy cosmic
void floating in darkness for the time being.
A scenario so dark that it brings to mind certain passages from Fred Hoyle's novel "The
Black Cloud," if not even the words of Genesis "...darkness covered the abyss and the spirit
of God hovered over the waters."
But this is a video that is mostly about astronomy, so we must clarify right away that the "spirit
of God" was actually composed of 73% molecular hydrogen and 25% helium, with a remaining
2% carbon, nitrogen, oxygen, and silicon compounds...
As indeed were thousands of other nebulae that covered the disk of the Galaxy at that
time.
At a certain moment, however, something disturbed the tranquility of the cloud, and regions
with higher densities began to collapse under the pull of gravity.
A "something" that according to the most accepted theories may be represented by a "close" encounter
with a massive star, shock waves triggered by the
radiation of nearby newborn stars or by the explosion of one or more supernovae.
In fact, the cloud is in a state of dynamic equilibrium between the thermal energy of
its particles, which tends to disperse them, and the force of gravity, which instead tends
to keep them close together.
If due to internal turbulence or an external cause this equilibrium is broken, with the
creation of instability, in some parts of the denser cloud the force of gravity may
prevail over internal pressure and trigger a gravitational collapse capable of leading
to the birth of something very different from a cold cloud of gas...
Whatever its origin, in fact, the effect of the perturbation is to fragment and thicken
the cloud into hundreds or perhaps thousands of small globules, each about a tenth of a
light-year in size, more than a hundred times the current size of the Solar System.
And each of which will then give birth to a star.
It should be known that stars are not born in solitude, but come into the world in clusters
of hundreds of individuals…
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Within the globule that interests us most closely (obviously the one in which the Sun
is forming), gas continues to collapse and the denser region to heat up, while around
it a kind of dusty cocoon forms formed by the material of the primordial cloud not
yet involved: atoms, molecules, and dust composed of heavier elements.
Slowly the center of the globule begins to compact and show a rotation more and more
pronounced.
The collapse, in fact, does not occur simultaneously and with the same intensity throughout the
cloud.
The initial cloud is so large that it admits different falling velocities in its various
regions, so that at some point, as in a downward air current, a preferential rotation is formed.
As a result, the cloud begins to rotate and crush faster and faster.
This phenomenon is governed by the important principle of Conservation of Angular Momentum,
according to which, in the absence of external forces, angular momentum, that quantity that
measures the intensity of a body's rotation about its axis, does not change over time.
Are you familiar with an ice skater?
The closer her arms are to the body, that is, to its axis of rotation, the more she
increases her rotational speed, and her angular momentum remains constant.
Thus, although our primordial cloud possessed a very slow initial rotation at first, as
the mass becomes more concentrated and the volume decreases, it speeds up its rotation.
Because of the centrifugal force then due to the rotation, the matter does not all fall
toward the center where the Sun is to be ignited, but also spreads all around, forming an accretion
disk of at least 100 Astronomical Units in radius.
The latter then fragments into smaller parts, each of which begins to collapse and rotate
on itself independently, taking with it some of the overall initial angular momenta.
Collapsing and aggregating thus form the so-called "planetesimals," solid bodies a few kilometers
in size, which in turn give rise to the "protoplanets," that is, the progenitors of the planets we
know today.
This is the big picture, but there are aspects of the issue that need to be much better defined.
For example - when we say that the planets orbit the sun in an approximately common plane,
what do we mean by that "approximately"?
Well, actually the alignment is not perfect - in absolute terms, the difference between
the orbit most inclined is the least inclined is only 6 or 7 degrees--and its exact value
depends only on the reference system we use to count those differences.
And here we learn that the ecliptic line, besides being entirely imaginary, is also
only a convention!
The ecliptic plane, in fact, is nothing more than the orbital plane of our planet!
And it is from this fictitious plane that we begin to calculate the inclinations of
the other orbits!
From this, it follows that the inclination of Earth's orbital plane is by definition
equal to zero degrees!
Mercury's consequently will be 7°.
That of Venus--3.4°.
Mars' 1.85° Jupiter's 1.3°
Saturn's 2.5° Of Uranus 0.77°
And Neptune's 1.77°.
Okay, but these are values we use only for computational convenience, as they do not
correspond to a reality based on the physics of planetary formation.
Actually, we are talking about a pure convention, as we Earthlings like to be in the center
of every unit of measurement and calculation: if we were to calculate the inclinations of
the orbital planes on the equatorial plane of the Sun things would change a lot!
In fact, the Sun's axis of rotation is not perpendicular-as one would expect-to the orbital
planes of the planets, but is definitely tilted.
Until now no one had advanced a convincing explanation for this deviation.
Now an explanation may come thanks to the hypothetical existence of a large planet that
some astronomers believe may orbit, tilted 30°, a few hundred astronomical units away
from the Sun.
Basically, what everyone is calling Planet Nine, and which all astronomers have been
trying to locate, since early 2016-when its existence was hypothesized based on gravitational
anomalies observed for objects beyond Pluto, could have acted over billions of years on
the arrangement of the orbital planes of other planets.
As an aside, we were saying that if the inclinations of the orbits were calculated with respect
to the equatorial plane of the Sun, then everything would change.
Earth's orbital plane would become the most inclined, with 7.1°.
That of mercury would be 3.4° That of Venus 3.9°
That of Mars 5.7° That of Jupiter 6.1°
Saturn's 5.5° Uranus' 6.5°
And Neptune's 6.4°.
Really strange, isn't it?
But of course, all this applies to planets and all other celestial objects formed in
accretion disks; such as the large satellites of gas giants, for example, which lie almost
perfectly in the plane of rotation of the planet to which they belong.
Then there are smaller bodies, such as asteroids and comets, which in the most extreme cases
show very high orbital inclinations.
Or as also the dwarf planets Pluto and Eris, which although definitely large, move on orbits
inclined 17° and 44° respectively.
All objects evidently had very different births or gravitational fates than their larger siblings.