Why are stars and planets spherical?
Why do all large celestial bodies take a spherical shape? Are there any large
celestial bodies that are not spherical? Celestial objects can take even bizarre
and irregular shapes, as long as they are small enough. However, there is a limit,
dictated by mass, beyond which every object tends to the spherical shape.
Above this limit, in fact, the force of gravity overcomes the cohesive force of the larger
surface structures, and any significant prominence is drawn toward the center
of gravity with enough force to destroy it. As the mass increases, and with it the surface
force of gravity, the shape of the celestial body, therefore, tends to be more and more spherical. Of
course, there will always be small disuniformities because the sphere is a geometric abstraction.
But is that really all there is to it? Let's find out together!
How many planets are there in the Universe? A lot. It is estimated about one sextillion. Do
you know how much an octillion is? Quite a lot. A number so large (a billion trillion)
that it is impossible to make a mental picture of it. Try looking at it this way,
just to get an idea: the planet Earth weighs about an octillion times more than a walnut...
Yet all this infinity is made up of objects that have one thing in common:
they are all spherical. There is not a single cuboid, dodecahedral, or heart-shaped one, to say.
But why? First of all, it should be noted that we
are talking about planets, not "celestial objects. "The word "celestial bodies" includes, in general,
a myriad of objects with different characteristics and with shapes that, for various reasons,
can also be very different from the spherical one. Consider, just as an example, our Galaxy,
the Milky Way, which within a so-called "halo," consisting of stars distributed
in a more or less spherical shape, possesses a disk-shaped structure rich in gas and stars.
On the other hand, if by "celestial body" we denote individual objects, such as stars
or planets, it is correct to say that the predominant shape is approximately spherical.
The reason why is also quite intuitive... All the most massive planetary objects,
be they stars or planets, are formed under the action of gravitational force,
which is the force that leads the elements of matter to be attracted to each other,
and then to the center of the growing body. And if myriads of particles are attracted to
a common point of aggregation, in the end they can only lead to a spherical agglomeration!
And the larger the mass that is being composed, and the lower its density,
the more the final result will be similar to that of a perfect sphere.
The physical principle involved is similar to that which makes soap bubbles spherical,
or fat bubbles in broth circular: nature always chooses the shape that minimizes the
energy involved. In fact, for the same volume, the sphere is solid with the least surface
area. For this reason, the soap bubble tends to assume a perfectly spherical shape. It, in fact,
tries to organize itself into the shape that uses the least amount of energy to sustain itself.
Leaving soap bubbles and returning to planets, we can say that very intense gravity prevents any
surface roughness from exceeding a certain height. For example, it can be shown that mountains higher
than about ten kilometers cannot exist on Earth because then they would slowly sink
in on themselves to a smaller size. Therefore, the Earth, with its mass, could never have a rougher
shape than it has... except for the short time needed to level out the irregularity.
On neutron stars (stars whose very high mass and density make the surface gravity a hundred billion
times more intense than what we experience on Earth), it is estimated that the highest surface
roughness cannot exceed half a centimeter in height... Relating the dimensions to the
Earth's scale, it would be as if there could be no "mountains" on our planet higher than 5 meters...
The "level" that keeps the shape of an object possessing the physical requirements to be
called a planet almost strictly spherical is the so-called "hydrostatic equilibrium."
A condition that is achieved when within it the gravitational force that draws
matter toward the center is offset by the thrust due to the compression of materials.
The concept of hydrostatic equilibrium also makes it possible to determine whether a
celestial object is a planet, a dwarf planet, or a minor object, such as an asteroid, comet,
etc... In fact, let's not forget that according to the new definitions released in 2006 by the
International Astronomical Union for our Solar System, planets and dwarf planets must mandatorily
have a spherical shape and have reached hydrostatic equilibrium to be declared as such.
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But how massive does an object have to be to achieve hydrostatic equilibrium?
It depends on what it is made of, basically the density of its upper layers...
The fact is that gravity is actually a surprisingly weak force. An object
has to be really big before it can exert a gravitational pull strong enough to overcome
the resistance of the material it is made of. This, incidentally, is why you need not fear
that your bodies will collapse into a spherical shape at any moment!
Anyway, let's try to put some order into it... Although there are many factors involved in
determining the shape of an object, in the end, there are only three types that frame the problem.
Objects around 400 km in diameter If such an object has a "snowball" density,
that is, about one gram per cubic centimeter, then it will be able to achieve a spherical
shape. Mimas, for example, with a diameter of 396 kilometers is the seventh satellite
of Saturn and the 21st in the solar system in size. And it is the smallest celestial body
known to have a spherical shape, thanks to a very low density (1.15 grams per cubic centimeter).
This example illustrates an important distinction: the difference between
being "spherical" and being in "hydrostatic equilibrium." The impact that created the huge
crater that makes it resemble the "Black Death" left a scar that Mimas' gravity, cannot erase.
Sphericity alone, therefore, is not a sufficient condition to make you a planet, there must also
be the achievement of hydrostatic equilibrium! Objects of the same size, but with higher density,
will show more or less irregular shapes. Such as Pallas and Vesta,
for example... the two large asteroids in the main belt, which although they have diameters
even greater than 500 km, because of their very high density show a shape far from spherical.
Objects with diameters around 1,000 km
This size usually ensures a spherical shape but does not guarantee hydrostatic equilibrium.
If the object is dense and is affected by a major distortion event, such as an impact,
its gravity will probably fail to "close the wound," and its overall shape will suffer.
Remarkably, The largest body known to be not in hydrostatic equilibrium is Saturn's bizarre moon
Japetus, with a diameter of as much as 1470 km! Despite its low density, which should
make it easier to achieve a spherical shape, its silhouette is in fact "defaced" along the equator
by an unbroken mountain range. Unbroken and very mysterious--since its origin cannot be explained.
On the other hand, do you know what the smallest spherical object in the solar
system is in hydrostatic equilibrium? It is the largest asteroid, namely the dwarf planet Ceres,
which has a diameter of 940 km! Other spherical objects of this
size in the hydrostatic equilibrium appear to be the transneptunian objects Orcus (916 km),
Salacia (846 km), Quaoar (1120 km), Gonggong (1230 km), and Sedna (995 km).
Objects around 1,500 km in diameter
Objects approaching or exceeding this size, except Japetus, all turn out to be perfectly round and
balanced. In this category fall all the large moons of Jupiter and Saturn, certainly Titania
and Oberon of Uranus, Neptune's moon Triton, Pluto, and Eris... And of course our own Moon!
In short... Including the Sun, there are about thirty spherical objects and also in compliance
with the balance requirement. Then there are (literally) millions of small bodies more akin to
flying mountains, with extraordinarily different shapes: lumps of boulders assembled in the most
rambling of ways, as comet Churyumov-Gerasimenko visited in 2015 by the Rosetta probe,
or like the small asteroid Itokawa, reached in 2005 by the Japanese probe Hayabusa...
Not to mention objects whose shape eludes any attempt at classification. And I'm thinking
of the incredibly ominous silhouette of the interstellar visitor Oumuamua,
capable of stirring up endless theories about its origin as it traversed our solar system... Or the
inexplicable "flying saucer" shape of Pan, a tiny moon of Saturn nestled among its rings.
All clear then! Practically, is all the merit and demerit of gravity...
But gravity is not everything. While gravity conspires to make planets
spherical, the speed of their rotations simultaneously seeks to flatten them.
The faster a celestial body rotates, the more disproportionate its equatorial bulge becomes.
This is why there are no perfect spheres in our solar system--only oblate spheroids.
Thus, Jupiter would be perfectly spherical if it did not rotate ... but "unfortunately" it rotates
quickly on itself in about ten hours, and thus its equatorial diameter results 10% larger than
its polar diameter. Likewise Saturn...while Venus, which rotates very slowly around its axis in 243
days, is the most spherical planet of all! Even the Sun, due to its immense gravity and
relatively slow rotation speed of 25 days, can be considered a perfect sphere,,,, but a significant
percentage of stars in the sky rotate much faster and swell considerably at the equator.
The case of Achernar, the ninth brightest star in our sky, is emblematic: its rotation period
is two days, compared with about 27 for our Sun. To make matters worse, the star
is much larger and more massive, producing an equatorial velocity of at least 250 km/s,
which is 125 times faster than the Sun. Faster than that, and the star would fall apart!
This extreme speed generates an intense centrifugal force, which flattens the star,
making its equatorial diameter (which is 11 times that of the Sun) 56 percent larger than the polar.
In terms of flattened stars, however, it is in good company: peculiar rotating stars are many,
and among them, we also find Regulus, Altair, and the very famous Vega!
Red supergiants such as Betelgeuse and other stars of immense proportions may
have a roughly spherical shape but with considerable surface
alterations due to the convective rise of hot gas that generates huge,
irregularly shaped bubbles on their surfaces. And I want to end by mentioning galaxies again...
They, too, in fact, sport very varied shapes due to their rotation:
if their global rotational motion were zero, they would take the shape of globes as is the
case with spheroidal elliptical galaxies and as is normally the case with globular clusters.
In short, there is never a dull moment in space. Don't you agree?