A myth to be revised: Jupiter is not Earth's protective shield!
We know it well...
One of the greatest existential threats to life on Earth is the treacherous occurrence
of a cosmic impact...
Whether caused by an asteroid, comet, or interstellar intruder, a sufficiently energetic collision
with Earth-typical of objects a few kilometers in size or larger-could easily cause a mass
extinction event, completely sterilizing a living world and ending a chain of life billions
of years in the making.
Such events, judging from the photos sent back by interplanetary probes, have occurred
almost everywhere in our solar system, leaving the surfaces of moons and rocky planets looking
like cratered battlefields.
And it has also happened here on Earth ... where, however, the surface of our planet, which
is fortunate enough to be continually reshaped by endogenous and atmospheric forces, soon
manages to erase almost all traces In fact, the most famous example of what remains
of our personal cosmic war is that of the Chicxulub crater, witness to the event that
66 million years ago wiped out in an instant almost all the living creatures of the Cretaceous
period ... which remained hidden until a few years ago.
Despite the painful remnants of cosmic impacts, people in popular astronomical literature
and pop culture accepted the hypothesis that the Earth owed its existence as a Garden of
Eden to the presence of Jupiter.
In fact, it was said that the gas giant, with its disproportionate mass, capable of generating
an enormously extended gravitational field, acted as a lightning rod for all the cosmic
projectiles that came from the depths of space and tried in some way to hit our beautiful
planet.
In short ... without Jupiter acting as a "cosmic vacuum cleaner" sucking up these dangerous
objects, there would be so many catastrophic impacts that life on Earth would probably
not have evolved and we would not be here today.
At least that was the generally accepted "Vulgate".
And we stuck with that belief almost to this day, until someone wanted to test it out...
With what results?
If you want to know, just keep following us...
To begin our story, let's take a trip back to the night of June 14, 1770, when the famous
astronomer Charles Messier discovered a faint comet in the constellation of Sagittarius.
It was not the first, nor would it be the last, comet the French astronomer spotted,
but it would prove to be one of the most interesting and dangerous ever seen in Earth's skies.
In fact, over the next few nights, the comet rapidly grew larger and brighter until it
was visible to the naked eye, and on July 1, moving across the sky at an unprecedented
speed, it passed within 2.2 million kilometers of Earth (as it would come to be known).
The closest approach of a comet to our planet, still an unbeaten record!
It later faded, but remained visible for several months - it was Messier himself who made the
last recorded observation of the comet in early October.
Once they greeted the unexpected cosmic guest, European observers immediately began poring
over the collected data, trying to figure out what kind of orbit the comet had followed.
And at a time in history when astronomers were still trying to understand the nature
of these strange-tailed celestial creatures, they thought of a very long period comet - like
Halley's comet - that had come from who knows where and then was destined to return to the
depths of space.
The Swedish astronomer Anders Lexell was the first who was able to calculate its orbit,
and it came as a surprise to learn that the comet revolved around the Sun with a period
of only 5.6 years.
This immediately raised a question: if its orbit led it to return near the Earth every
few years, how come it had never been sighted before?
Also, why has the comet never been seen since?
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The solution to the mystery became clear when astronomers extrapolated the comet's path
back and forth in time and discovered that the one responsible for everything, both the
sudden appearance and the equally sudden disappearance of the comet could only have been the most
massive planet in the solar system, viz: Jupiter!
In fact, it was discovered that, in 1767, the comet--which was most likely part of a
population of objects we now call Centaurs: icy cometary nuclei orbiting in the outer
solar system between Jupiter and Neptune--had approached Jupiter while moving on a much
longer orbit--following a path that could never have taken it in the direction of Earth.
But as it passed near the solar system's largest planet, Jupiter's gravity drew it just enough
to push it into a much smaller orbit that would dangerously cross our planet's trajectory
a few years from now.
But why did the comet then get lost, never to visit us again?
Well, as far as the prodigious Celestial mechanics experts were later able to calculate in the
following decades, it seems that just twelve years after its initial encounter with Jupiter,
the comet approached the giant planet once again, and may even have been hurled outward,
away from the inner solar system.
Where exactly it ended up is a mystery: our observations of its 1770 appearance are not
good enough to determine exactly what its fate might have been.
But one thing was certain: the comet had been projected into an orbit that could no longer
threaten Earth.
So it could be said that, at least in this case, Jupiter had played the role of both
villain and heroic savior of Earth.
But if it was already understood a few centuries ago that Jupiter could have this dual nature
of shield and sling towards the Earth, what was it that led astronomers to support the
"impenetrable shield" theory of Jupiter?
It is not easy to find the origins of the "Jupiter as a shield" theory.
Searching for traces in the astronomical literature, only a 1994 paper signed by geophysicist George
Wetherill mentions the possibility that in the absence of a massive planet like Jupiter
at the right distance from the Sun, the inner Solar System would have been threatened by
a comet flux from the Oort Cloud a thousand times greater than what we actually observe
today.
But the original idea had to go back even further because already in the 1980s this
was taken for granted in numerous publications.
A very quaint and romantic hypothesis, but not at all supported by calculations.
Those were the years when the only possible explanation for the apparently large difference
in the cratering rates of the two lunar hemispheres (the far and the near) was that the far hemisphere
was completely exposed to space, from where asteroids and comets could strike the lunar
surface unhindered, while the near hemisphere was "protected", so to speak, by the ponderous
presence of the Earth.
Even then, the theory went around for several years until, as we also explained in our video
"Why are the two sides of the moon so different", it was realized that the difference lay in
the thickness of the lunar crust, which was thinner in the Earth-facing hemisphere.
In short, if there is a lesson to be learned from these misadventures, it is that assumptions
based on common sense and the suggestion of the moment can often fail miserably.
The solution?
Well, the usual one: apply the scientific method and look for supporting evidence.
Not that Wetherill didn't try.
In fact, his thinking was based on the results of computer simulations, but at the time,
computing power was not available.
In recent years, the situation has improved greatly, and several teams of astronomers
have been busy putting Wetherill's beliefs to the test by reconstructing different versions
of our solar system on the computer: one with and one without Jupiter, and three with planets
of sizes corresponding to one-quarter, one-half, and three-quarters of its mass, respectively.
The virtual system also simulated the presence of 100 thousand objects, divided into asteroids
located between Mars and Jupiter, centaurs orbiting between Jupiter and Neptune, and
long-period comets from the Kuiper belt and the Oort cloud.
The results obtained after tens of millions of virtual years disproved Wetherill's theory
by showing that the presence of Jupiter in the solar system, far from protecting us from
cosmic catastrophes, actually leads to an overall 30% increase in collisions with Earth!
This, we repeat, considering the totality of the 100,000 objects tracked in the simulation...
As for the Jupiter effect on asteroids in the main belt, the surprise reached its peak,
because it was possible to understand that the presence of the gas giant has an even
more catastrophic effect, increasing the number of objects aimed at our planet by as much
as 72 percent!
Only for the long-term comet population did the simulation show no change in the frequency
of impacts...
As if to say that it is only in the case of comets from the outer solar system - much
fewer in number than the asteroids it could instead influence at will - that Jupiter's
presence manages not to harm the Earth...
All this brings us back to the age-old question: is Jupiter a friend or foe of Earth?
Or just indifferent?
Based on what little we know and what we have just said, it is reasonable to conclude that
most of the asteroids that have struck the Earth over time, including most likely the
one that posed as far back as the empire of the great reptiles, would never have made
it to the mark had it not been for Jupiter's malevolent ability to disrupt their orbits
until they intersected with the orbit of our planet.
In short...
Contrary to what was once thought, Jupiter must be considered an enemy of Earth...
But simulations have also shown that the story is much more complicated than it seems.
Not least because there is another point of view to consider...
Which one?
Well...
Let's put it this way...
Impacts on our planet may have played a key role in transforming a newly formed barren
planet into the wonderfully habitable world we know today.
Without the objects hurled by Jupiter from the asteroid belt and the outer solar system,
Earth might have remained a barren world, a desiccated shell.
Ultimately, the gas giant planets, along with billions of years of relentless bombardment,
would have brought billions and billions of tons of water mixed with organic molecules
to the surface of a young Earth.
An enormous welcome gift.
Although in the great game of the cycle of life and death, it would be the dinosaurs
who would later pay the price of the ticket.
Something we will never forgive Jupiter for!