Updated April 10, 2024.
Why We are Alone in the Galaxy | Marc Defant | TEDxUSF
EDIT: I noticed TED has "flagged" Dr. Defant's video, apparently due to questions about some of his scientific claims. Here are links to the three topics he speaks to:
The creation of the solar system, and dating due to calcium–aluminium-rich inclusions in the Allende meteorite:
Allende meteorite
Calcium–aluminium-rich inclusion
Aluminum-26 production from a stellar evolutionary sequence
That said, Dr. Defant is incorrect when he says "all" of the elements in the periodic table beyond hydrogen and helium were generated by supernovas. Some scientists believe lithium was created prior to star formation, and most scientists believe medium weight elements up to iron and nickel were created in stars. The process that creates the elements from beryllium to nickel is called stellar nucleosynthesis. This is also called the "slow process" or the "s-process." However, some scientists believe the initial phase of a supernova (the collapse of the star) creates conditions to fuse iron with other elements to create some of the elements heavier than iron. This is called the "rapid process" or the "r-process." The supernova explosion then releases the elements into the void of space.
Stellar nucleosynthesis
The creation of many elements including iron and copper are heavily dependent on Type Ia supernovas, a supernova of a white dwarf star which is part of a binary star system. Other elements are dependent on dying low-mass stars, which become red giants, then dissipate into planetary nebulas. Many scientists now believe very heavy elements like platinum, gold and heavier could only be generated in a neutron star. That a supernova collapse is not powerful enough to cause the r-process, and instead the intense gravity of a neutron star is able to do so. However, given the high gravity of a neutron star, the only way these elements could be released is by the collision of two neutron stars (a kilonova). This most likely occur in a binary neutron star formed for the collapse of a binary star system. The possibility of a single wandering neutron star colliding with a single nearby neutron star, given the average diameter of a neutron star is about 12 miles, seems an almost infinitesimal probability. However, neutron stars are the remnants of a core of a star that went supernova, so perhaps Dr. Defant consolidated these topics for the sake of time.
The creation of many elements including iron and copper are heavily dependent on Type Ia supernovas, a supernova of a white dwarf star which is part of a binary star system. Other elements are dependent on dying low-mass stars, which become red giants, then dissipate into planetary nebulas. Many scientists now believe very heavy elements like platinum, gold and heavier could only be generated in a neutron star. That a supernova collapse is not powerful enough to cause the r-process, and instead the intense gravity of a neutron star is able to do so. However, given the high gravity of a neutron star, the only way these elements could be released is by the collision of two neutron stars (a kilonova). This most likely occur in a binary neutron star formed for the collapse of a binary star system. The possibility of a single wandering neutron star colliding with a single nearby neutron star, given the average diameter of a neutron star is about 12 miles, seems an almost infinitesimal probability. However, neutron stars are the remnants of a core of a star that went supernova, so perhaps Dr. Defant consolidated these topics for the sake of time.
The following article talks about the r-process:
How are heavy elements formed in neutron star collisions?
Regardless, if the medium weight elements were generated in stars and release via supernovas, and there is no consensus on the supernova collapse creating heavier elements, we also needed nearby dying low-mass stars, a nearby white dwarf that was part of a binary star system, and a nearby binary neutron star systems that collided to give our solar system the elements from copper to uranium. We know a kilonova seeded our stellar nebula, because we have very heavy elements from silver to uranium in the Earth's crust. When you look at the abundance of elements in Earth's crust, while iron is plentiful, once you move above iron in atomic weight, the rest of these heavier elements make up less than one-half of one percent of the elements in the Earth's crust.
How are heavy elements formed in neutron star collisions?
Regardless, if the medium weight elements were generated in stars and release via supernovas, and there is no consensus on the supernova collapse creating heavier elements, we also needed nearby dying low-mass stars, a nearby white dwarf that was part of a binary star system, and a nearby binary neutron star systems that collided to give our solar system the elements from copper to uranium. We know a kilonova seeded our stellar nebula, because we have very heavy elements from silver to uranium in the Earth's crust. When you look at the abundance of elements in Earth's crust, while iron is plentiful, once you move above iron in atomic weight, the rest of these heavier elements make up less than one-half of one percent of the elements in the Earth's crust.
Visualizing the Abundance of Elements in the Earth's Crust
(Source: Visual Capitalist)
Copper and tin are incredibly important elements. They are the key elemental metals used to form the alloy bronze, and as such were key to the bronze age. Copper is created from the s-process or Type Ia supernovas. Tin is created either by a dying low-mass star or the r-process. Bronze was the basis for hard metal tools like nails, spearheads, plows, etc. The problem is copper and tin are rare compared to iron. Iron is one thousand times more prevalent in the Earth's crust than copper. Iron is more than 25,000 times more prevalent than tin, antimony, or arsenic. It makes sense to believe there may be planets which have iron, but have no tin, antimony, or arsenic, meaning no ability to make bronze. But iron requires considerably more heat to shape and purify (through heating and hammering). The earliest bronze dates to 4,650 BCE, while the earliest shaped iron dates to about 2,000 BCE. The historical "Bronze Age" is pegged to 3,300 BCE to 1,200 BCE, which gave way to the "Iron Age" in 1,200 BCE. Regardless of how you measure it, for about 2,000 critical years, the only metallurgy that existed in human society was done with copper and tin.
It seems highly possible there could be planetary systems created with only the medium weight stellar elements, and few heavier elements. But if there was no tin, antimony, or arsenic, any intelligent life would have to go from a stone age to an iron age, and have to figure out how to smelt iron. Without iron, anything requiring hardness would need to use stone or pottery. Copper alone is too soft. There would be no opportunity to create metal nails or spikes until iron was mastered. It is possible an intelligent society would develop enough skills in an extended pottery age to transition to an iron age, but this seems less likely, because the uses of pottery and iron are very different, unlike the transition from bronze to iron. There is a significant chance an intelligent society without tin, antimony, or arsenic, and the opportunity for a bronze age would never perfect iron smelting, and instead remain stuck in a stone age.
But back to the not one, but four cosmic events, a nearby supernova of a massive star, a nearby low-mass star death, a nearby Type Ia supernova of white dwarf in a binary star system, and a nearby collision of two neutron stars (which could be remnants of the same nearby massive star supernovas that contributed the lighter elements), to Dr. Defant's original point, that would be incredibly rare. Even if there as a nearby binary star system, with nearly simultaneous supernovas, each leaving behind a neutron star forming a binary neutron star, that then merged and released heavier elements, while this is an easier explanation, it is also rare. And it still likely needs a dying low-mass star and a nearby Type Ia supernova. All of this is far more rare than just a nearby supernova.
The next point Dr. Defant makes is the rise of the mammals being a direct result of the extinction of the dinosaurs:
The Rise of Mammals
And then there is the East African Rift directly triggering the evolution of bipedalism in great apes and rise of pre-humans:
Did tectonic rift push apes and monkeys apart?
How a changing landscape and climate shaped early humans
But wait, there's more!
Where is Everybody? Why Haven't We Found Extra-Terrestrials?
A little more on this. The Earth and the Moon are a system. The Moon does not neatly orbit around the center of mass of the Earth. Instead, the center of mass of the Earth, and the center of mass of the Moon, both orbit around the center of mass of the pair (the barycenter). This wobbling makes it much harder for an object being pulled towards the center of mass of the Earth-Moon system to follow a straight line. All of that extra mass out there is pulling an object away from the center of mass of the Earth-Moon system.
SkyMarvels™ EARTH-MOON BARYCENTER (celestia celestia4all)
As an example of how this works, look at the crazy, almost half-century trip of Apollo 12's third stage. It was fired out into a heliocentric orbit around the Sun, but kept getting pullled back into the Earth-Moon system's gravity well into a geocentric orbit around us, then ejected out into a heliocentric orbit again, and then back into geocentric orbit. It is like a piece of driftwood bobbing along a tideline in the ocean, sometimes getting pulled away by one current, sometimes by the other.
But wait, there is even more!
Jupiter shepherds large transient objects (asteroids) into a belt outside the orbit of Mars. The very large mass of Jupiter, well outside of the inner Solar System, is a unique and very good thing for us fragile creatures on the Earth. Notice the Asteroid Belt is not a neat circle in between the orbits of Mars and Jupiter, it is very different. That will be explained in the post that follows.
The asteroids of the inner Solar System and Jupiter
Jupiter 'shepherds' the asteroid belt, preventing the asteroids from falling into the sun or accreting into a new planet.
Now, the above video is beyond cool, because it illustrates four of the five Lagrange points (L1, L3, L4, and L5).
In addition to shepherding space objects, Jupiter "Hoovers" these objects just like Earth's Moon.
Here Joe Scott of "Answers With Joe" explains the "Rare Earth Theory", which includes the role of Jupiter at the 9:09 mark:
Joe's video goes much further than this blog post on the habitability of planets within the Milky Way galaxy, and the role of Earth's magnetic field. Check it out.
Basically, the "Habitable Zone" is protected by Jupiter, we have Earth at an ideal distance, with the right chemical composition, and we have our own Moon as a terminal defense system.
There are two additional important considerations.
NASA has discovered many exoplanets. Some of these orbit cool stars, such as "Red Dwarf" within a "habitable zone" which is much closer to the star. This arrangement can lead to tidal locking, which is problematic.
Another consideration if a planet is close to a cool star is the planet could be exposed to high high levels of radiation. Both tidal locking and high radiation are factors in the TRAPPIST-1 plantary system discovered in 2015.
The topic of radiation brings up the final point of a planet capable of supporting life. Of the solar system's three inner planets in or near the "habitable zone" (Venus, Earth, and Mars), only Earth has a magnetic field. Earth's magnetic field deflects harmful high-energy cosmic rays coming from the Sun away from the Earth. Mercury has a magnetic field, but is too close to the Sun to be habitable. Jupiter has a magnetic field, but as a gas giant is not considered habitable.
[Edit: Added 11/26/19]
Most of this post has dealt with what it takes to form a stable solar/planetary system capable of supporting life. However, life is complicated, and no scientist has figured out abiogenesis (life from lifelessness). But even with that spark of abiogenesis, how does complicated and sophisticated live emerge? The assumption has been Darwinian Evolution. Indeed Darwin's theory is the nexus of what today is called "neo-Darwinism." However, some believe neo-Darwinism has breached the limits of mathematics. This excellent discussion with David Berlinski, Stephen Meyer, and David Gelernter, which was triggered by David Gelernter's article "Giving Up Darwin", is worth a listen:
I often like to joke if life were easy, we would all have green thumbs. Anyone who has had a tried to keep a house plant or a goldfish alive should realize even in a stable biosphere, life is fragile.
And all of this first presumes a stable universe, where all of the key physical forces are balanced so that atoms form and matter condenses into stars and planets.
I am not suggesting any conclusion from all of these comments and videos except one: Life is incredibly rare and precious. Far rarer than many believe. It is certainly possible someday we will find evidence of primitive life on Mars, but Mars was unsustainable. It is certainly possible someday we could terraform Mars, but the terraformed Mars will not last. Because its gravity is low and it has no magnetic field, its atmosphere will dissipate, and in a hundred, thousand, or ten thousand years, it will be back to what it is today. Besides, if we ever have the technology to terraform a planet, we will be able to fix any problems on Earth.
[Edit: Added 12/22/22]
Having further researched some of the early cosmological events, I now think the Earth, with a very small amount of critical heavier elements which could only be manufactured in a neutron star, and released by a collision with another neutron star, and the critically important nature of copper and tin in metallurgy and the rise of civilization, is even more rare than I first thought. It is very likely there are planetary systems with no elements above nickel in their crusts. There may be intelligent life on these planets, but they would likely be trapped in a stone age forever. On the bright side, they would never develop nuclear weapons, and likely not even develop cannons or firearms, so they could have less chance of destroying themselves, but it would also be harder for them to feed themselves as they never would have bronze plows. Certainly, without copper, they likely would never be able to build a radio transmitter to send a signal SETI could pick up.
[Edit: Added 3/22/24]
I updated several links due to some dead links and some URLs changing. No content was changed.
[Edit: Added 4/10/24]
I added an additional depiction of the asteroid belt to show the main belt as well as those shepherded by Jupiter.
