Is the Earth in the wrong place in the Galaxy?
By now we have no doubt.
We are not at the center of the solar system, nor are we at the center of our galaxy...
Much less are we at the center of the universe...
One step back at a time, we have finally realized that our planet is just one of many, among
billions of billions of others.
Sure, a lucky planet, it seems...
favored by exceptional cosmic coincidences...
But are we really sure?
For example, are we really sure that the position where the Solar System was formed in the Milky
Way is just the absolute best?
Could we have found better?
Could we have found worse?
Agreed, we are not in a "privileged" position in the universe.
But perhaps the fact that we are not is in itself a special thing.
Who knows?
But we are not here to philosophize...
Our purpose is to try to understand whether Earth's position in the Milky Way may have
positively or negatively affected its own development.
We all know that what we call the Milky Way is nothing more than the galaxy that houses
our planetary system.
In this case, it is a spiral galaxy about 120,000 light-years in diameter (just to understand
what a boundless size we are talking about, one light-year is equivalent to about 63,000
times the distance between Earth and the Sun).
Its basic structure consists of a stellar component compressed into a disk about 1,500
light-years thick--a disk filled with hundreds of billions of stars, dust, and gas, mostly
arranged in irregular spiral arms.
Globular clusters surround the disk in a spherical swarm concentrated toward the center of the
disk.
This swarm of clusters, which also includes some single stars and scattered gas, defines
the Galactic halo.
According to current estimates, the Sun is about 27,000 light-years from the galactic
center, occupied as we shall see by a giant black hole.
The Sun, other stars, gaseous nebulae, and everything else in the galaxy revolves around
the galactic center moved by the gravitational attraction of a large number of stars concentrated
there, in the same way, that planets revolve around the Sun.
Okay, you will say… but we, with our negligible planet, where are we in relation to all this?
Well, from what we know today, the Sun is located near the inner edge of the so-called
Orion Arm, at an estimated distance of 27,000 light-years from the Galactic Center.
And about 50 light-years above the Galactic plane.
And is this good?
For the most part, we have gotten on well with the neighbors, the nearby stars, and
the interstellar clouds of gas and dust visible in the night sky.
Occasionally, however, some astronomical event has gotten in our face, incidents from which
life on Earth has always suffered.
It has been proposed, for example, that especially powerful stellar explosions of nearby stars
called supernovae may have been responsible for one or more of the mass extinctions that
have occurred over the history of the Earth.
Despite such astronomical events, complex life manages to flourish here.
But different regions of the galaxy have distinctly different features and properties.
Could we do so well if the solar system were located elsewhere in the galaxy?
Could such a richly diverse, life-supporting world as Earth have developed and survived
at the center of the galaxy?
Or in the bar of stars, gas, and dust that pierces the center of the galaxy?
Or above the spiral arms?
Or beyond them?
Follow me!
"Hey, guys, just a moment before we continue...
BE sure to join the Insane Curiosity Channel...
Click on the bell, you will help us to make products of ever-higher quality!"
At The Center Of Our Galaxy
We aren’t at the center of the galaxy, but could we be?
Activity in the center of our galaxy is dominated by a black hole with about four million times
the mass of our Sun.
A black hole is a region where matter has become sufficiently concentrated that its
gravitational force literally changes the shape of space so much that matter and most
energy that enter it cannot escape.
One exception to the rule of nothing escaping is the force of gravity created by the matter
and energy inside the black hole, which does extend out into the rest of the universe.
This black hole is not an anomaly.
Astronomers are discovering that such “supermassive” black holes, each with between a hundred thousand
and a few billion solar masses, exist in the nuclei of most galaxies.
The monumental gravitational attraction of the Milky Way’s supermassive black hole
is the main arbiter of activity in the galaxy’s center.
Among the objects we see orbiting it as rapidly as planets orbit, the Sun is a full-fledged
star.
Plausibly, planets could form around such stars.
But would these planets provide stable platforms on which life could evolve?
Contrary to the common belief that black holes are giant vacuum cleaners in space that suck
everything into them, weird effects only occur within a few radii of these objects.
For example, if a black hole is a million miles in diameter, then bizarre behavior of
space, time, matter, and energy occur only within ten million miles of the black hole,
which is a tiny distance on the scale of a galaxy.
Earth, for example, is ninety-three million miles from the Sun.
At the distances that the stars closest to it orbit the Milky Way’s supermassive black
hole, they only feel a normal gravitational pull from it.
Però c’è un problema.
Unlike the planets in our solar system, which all orbit in essentially one plane around
the Sun, the stars orbit in all directions or planes around the galaxy’s supermassive
black hole.
They have very elliptical orbits around that black hole, meaning that throughout their
orbits their distances from it vary tremendously.
It is not unusual for these stars to be ten times farther away at the most distant points
in their orbit compared to where they are closest to the black hole.
The change in distance between the black hole and a star means that the orbits of their
planets will also be continually changing due to the changing gravitational attraction
of the black hole.
Likewise, the other stars in close orbit around the black hole will pull on the planets orbiting
neighboring stars.
It is therefore unlikely in the extreme that any planets around stars in the nucleus of
the galaxy will orbit reliably in the Habitable Zone of their star.
Even planets whose average distance from their star is in the Zone would sometimes be in
regions that are too hot or too cold for water to remain liquid on their surfaces.
Plausibly, a passing star could rip the planet from another star completely out of orbit.
Even farther out in the galactic nucleus, beyond where stars are in tight orbits around
the black hole, planets will experience effects that are very likely to keep them uninhabitable.
All stars in this region would likely be on such chaotic orbits that they would frequently
pass close enough to each other to disturb comet clouds surrounding neighboring stars.
Each such event would greatly set back, if not stop, the evolution of life on an Earthlike
planet, making these worlds unlikely habitats for humanity.
In The Galaxy’s Bar
Moving a safer distance from the more dire effects of the supermassive black hole in
the Galaxy’s nucleus, we next encounter the bar of stars, gas, and dust (Figure 7.1).
The existence of this bar, suspected since the 1990s, was confirmed in 2005.
Its dimensions are roughly 24,000 light-years by 6,000 light-years.
The matter in the inner region of the bar flows perpendicular to it, whereas in the
outer region of the bar, stars, gas, and dust flow down the bar, turn around at the end,
and flow back.
Like a propeller, the bar itself also rotates about once every 15 million years.
Some of the gas and dust in the bar are apparently channeled down to the supermassive black hole,
providing raw material for star formation, as just discussed.
Several features of the bar suggest that few life-supporting planets could exist in it.
Consider a star and planets that just formed in the bar and are moving down it from one
tip to the other.
As this system nears the central region, it is likely to encounter stars that are moving
across the bar, perpendicular to its motion.
As discussed earlier, sufficiently close encounters will change the orbits of planets, taking
the potential Earth out of the Habitable Zone, or at least throw lots of comets toward the
planets.
These things are likely to occur eventually, even if this doesn’t happen on the first
hundred passes through the central region of the bar, long before advanced life evolves
on the planet.
Likewise, things get very hectic at the ends of the bar, where stars and clouds are turning
around.
A related problem in this inner region of the galaxy is the large quantities of interstellar
gas and dust in the bar that are being drawn down toward the black hole.
.
Above Or Beyond The Spiral Arms
Despite appearances, galaxies like the Milky Way are much more than just disks of stars,
gas, and dust with spiral arms.
There are things in the galaxy above and below the disk and things that exist far beyond
the visible limit of the disk.
Most notable among the galaxy’s constituents outside the disk are the globular clusters.
Containing up to hundreds of thousands of stars that are gravitationally bound together,
globular clusters swarm around the disk like mosquitoes in summer.
Globular clusters regularly pass through the disk, but they spend most of the time outside
of it in what is called the halo of the galaxy.
This is a spherical region centered on the supermassive black hole in the nucleus of
the galaxy, extending at least twice as far out in all directions as the spiral arms.
The halo also has billions of isolated stars that are so dim compared to the light from
globular clusters that we do not normally see them.
The problem in forming an Earthlike planet outside the disk in which we exist today is
finding enough gas and dust concentrated in one place out there to create the Sun and
planets.
Terrestrial planets form almost entirely from the debris of stellar explosions.
We know this because the matter in the universe when it first formed was only hydrogen, helium,
and lithium.
There is very little of any of these elements on Earth compared to iron, silicon, and numerous
other elements, all of which are formed by fusion in stars and stellar explosions.
A large fraction of the matter ejected when stars explode in the disk of the galaxy remains
in the disk, allowing this gas and dust to be recycled into new stars as it passes through
the spiral arms.
Our solar system must be at least a second-generation system because so much of the Earth’s content
must have been formed in a previous generation of stars.
The thing that prevents Earthlike planets from forming in the halo is that after the
first generation of stars formed out there when the universe was young, the gas and dust
ejected when these stars exploded did not remain sufficiently concentrated so that another
generation of stars could form from it.
Even in globular clusters, the gas ejected by exploding stars drifted away.
Astronomers, therefore, see very few, if any, new stars forming outside the realm of the
spiral arms.
In conclusion, based on what we have just said, we can divide our Galaxy into three
zones: that of the Center and Central Bar, the middle zone of the spiral arms, and the
outer arms and halo.
With these final considerations:
Within 13,000 Light Years from the center.
Too close to the galactic center.
Stars would receive intense X-ray radiation from the supermassive black hole at the center
of the galaxy, and it is very unlikely that life could develop in such an environment.
The center of the Milky Way is also much denser with stars than the outer regions.
Supernova explosions are much more frequent there, and the radiation would sterilize any
planet capable of harboring life.
The stellar density and fast elliptical motion of stars near the black hole would also lead
to continuous gravitational contention.
Between 13,000 and 33,000 Light Years from the Center.
This is the habitable zone by definition!
Planets have had sufficient heavy elements to form and initiate a biological cycle, as
well as a stable and safe environment that allows life to thrive for a long time.
Between 33,000 and 50,000 Light Years from the Center
This region is rich in hydrogen and helium while there is a very low ratio of heavy elements.
Hardly any planets within it will be rocky and therefore suitable for harboring life.
More so, the same can be said for the galactic halo, which is almost entirely devoid of stars
capable of producing heavy metals.
Of course, one cannot fail to notice that our solar system with our Earth is almost
exactly in the central part of the habitable zone.
A coincidence?
Is it really possible for us to have been so lucky?
And does this mean perhaps, that we should consider our existence almost unique?