Is This Why We Are Alone?

Our giant galaxy is at least ten billion years old. It has 100 billion stars. Many of them second generation stars. It would seem most stars have planets, and many of these fall in the so-called habitable zone. In multiple star systems, some planets will fly away. Still, there may be 40 billion potential Earths in the Milky Way. Yes, and No.

However, ten billion years, over (more than) 100 billion planetary systems, gives more than plenty of time for a civilization to expand throughout the galaxy. Yes, throughout the galaxy. So aliens should have visited Sol, but all indications are that they did not: life on Earth is indigenous to Earth, or the Earth-Mars system.

Why We Are Alone: Earth Is Both Very Watery & Very Radioactive, Deep Inside. The resulting massive tectonic recycling creates a massive churning of carbon, preventing unbearably hot Earths, or snowball Earths escapades into conditions reducing life to bacterial life….

How did Earth get her enormous radioactive fission core? A mystery. It seems that Venus did not. Indeed, Venus has no self-generated magnetosphere (differently from Mercury, Jupiter, or Saturn; Mars is too small to have a churning metallic core, so has no magnetosphere, thus CMEs tear the Martian atmosphere away, in particular water). Instead Venus has an induced magnetosphere. A pathetic device that could not prevent Venus from being deprived of all its hydrogen, hence water, torn by the solar wind, especially during Coronal Mass Ejections CME).

The Sort of Comet from Hell On Top Is Venus. Earth, The Blue World, Is Below. Earth, With Her Complex Magnetosphere Created By Her Churning Liquid Metal Radioactive Core: Part Of The Reason Why Venus Melts Lead, & Earth Harbors Civilization. [Courtesy ESA.]

Travelling throughout the galaxy is already within our reach. Nuclear fission rocket engines were very successfully tested in the 1960s. They would allow us, should we decide to do so, to make spaceships going through the galaxy at 100 kilometers per second.

That’s 1/3000 of the speed of light. The galaxy is 100,000 light years across. So, with existing technology, primitive technology of the 1960s, we could cross the galaxy in 300 million years.

Now, of course, faster tech is perfectly imaginable, such as thermonuclear propulsion. (Thermonuclear propulsion may be easier to achieve than a contained nuclear fusion motor, because containment is the most major problem of controlled fusion; an uncontained engine would be half way between an H-bomb and ITER, and pure fusion is clean; actually NASA finances, all too modestly, such research.) Whereas the temperature in a fission reactor will be at most 3000 degree Centigrade, thermonuclear fusion could reach 100 million degrees, enabling an impulsion 10,000 times greater. Thus mastery of thermonuclear fusion would allow to cross the galaxy in a few million years.

So, if there was a civilization barely more advanced technologically than we are, it would have established a galactic empire in a few million years. It should have visited our blue planet, detectable from thousands of parsecs (with existing technology not deployed by the grotesque, imbecilic plutocrats who rule us through those countless obsequious greedy critters which most politicians are).

The definition of the traditional habitable zone is naive: it is only about temperature. Temperature has to be just right, so that there is liquid water on the surface. And, naively, that is thought to mean holding a particular distance relative to a star. However it takes more than temperature to keep a planet habitable. Mars had an ocean, but lost its surface water, a little bit at a time, from repeated Coronal Mass Ejections (CME). A CME consists in a super active flame jetting out from the sun. If a CME hit Earth now, much, if not most electric circuitry on Earth would fail.

Radioactive Elements, Being The Densest, Sink Towards The Star During The Planetary System Formation. (Observed So far.). Oxygen & Hydrogen, With 5% Average Nuclear Mass Migrate Out

Venus does not have a strong magnetic field either. Why? We don’t know. OK, Venus doesn’t rotate very much.

Earth, though rotates mightily, stabilized by her big Moon, and her high density (because of pressure, Earth is denser overall than Mercury, although the latter seems to be a planetary core…)

The Earth magnetic field is a shield, and it is created by the nuclear fission engine at the core of the Earth (it makes the surface of Earth’s core hotter than the surface of the sun! That makes a metallic iron ocean above churn violently, and generate the magnetic field… And also plate tectonics, which recycles carbon deep in the mantle, preventing it to sit in the atmosphere, as in the case of Venus!)

LIFE Giving NUCLEAR EARTH Reactor

Thermal hot spots are indirectly nuclear fission activated, but before three billion years ago, thermal hot spots driven directly by ionizing and mutagenic radiation from surface nuclear reactors, were probably ubiquitous. Thus, to have the four billion of years needed to develop ADVANCED native life, with all the mutations it entails, one needs to have both a nuclear reactor inside a planet AND the planet enjoying surface water for four billion years.

A tough, and rare call. Moreover, life has to escape crashing dwarf planets, migrating Super-Jupiters, super stellar Coronal Mass Ejections (all the more frequent in Red Dwarfs), close encounters with passing stars, gamma ray bursts, supernovae, colliding black holes, super stars exploding, central galactic black hole eruption (many of these catastrophes were very recently revealed, and did not percolate yet to We The People). To let life develop over four billion years, we need an extraordinary confluence of circumstances. (And when I say four billion, that’s generous: the “Cambrian explosion” when animals appeared in a great number of types of species, was only 540 million years ago. Moreover, life on Earth may have been accelerated by being started on Mars, at a time when Earth was still way too hot for sophisticated chemistry, and then transported by meteorites (that this strange method of life transportation could still be done to this day, has been demonstrated, by carefully analyzing meteorites of Martian origin).

Life, Sustained Long Enough For Advanced Animals? Unlikely. The Milky Way Is Ours. And Oblivion Watches Over Us, Not Tenderly.

So expect life to be very frequent in the galaxy, and habitable planets to be many. As long as one is talking about bacterial life functioning on DNA like system (the details will be different, as post-DNA life was already synthesized in the lab!)

But little green men, Kzins and Doctor Spock? Not a chance. If they happen some day, they will be our descendants. That we are alone changes the stakes. We are not just threatening civilization, with our childish, yet lethal and atrocious antics, but we are threatening to annihilate the one and only crown of creation.

Patrice Ayme’

Where Is Everybody?

Where Is Everybody? This is the question Enrico Fermi asked in the 1950s, referring to the little green men who were supposed to inhabit the galaxy. This question is still with us. Efforts have been made to search for extraterrestrial life. Science Fiction books are full of worlds peopled by clever creatures.

It would be reassuring to know that the galaxy is full of clever creatures. After all, if we, human beings disappeared, well, we were just not good enough, no big deal: those out there, better than us, would pursue the mission we are named after, wisdom (sapiens in Latin).

Yet I see plenty of reasons why Earth’s advanced biosphere is unique. I go even further. In my opinion, the fact Earth has a radioactive, high density metallic core is crucial. I will reinforce this argument today (in light of just published research).

Weirder Planets Than We Ever Imagined Are Out There. Yet, Weirdest Of Them All Could Be The Earth

Fermi, Nobel for discovering the neutrino (“little neutron” in Italian), discovered the Fermi principle and statistics (which posits that matter does not collapse because Fermions refuse to be in the same “state” in the same place at the same time). Fermi, who had fled from fascist Italy, was also the scientific head of the Manhattan project.

Fermi was both a theoretician and an experimentalist. Thus immensely clever, but yet down to earth. He obviously found that the obvious absence of civilization out there in space was a striking fact. And it is.

The idea that the stars had little Earths orbiting them, graced with little green men, with their own little green Christs, came all the way back to another Italian, Giordano Bruno. Bruno had lectured in universities around Europe, and was a friar. However, to suggest the world was not exactly as Christianism described it, was a capital crime in places ruled by Christianism.

To punish Bruno’s mental exuberance, the Vatican imprisoned and tortured him for seven years. Then the Vatican and its horrid Fundamentalist Jihadists stripped Bruno naked, pierced his palate with iron (so that he could not address the public), and burned him alive after he refused to submit to infamy. I am still waiting for the excuses from the institution at fault, the world’s oldest, the Catholic Church.

Yet, although he was a genius, Bruno was probably wrong about the little green men. Why? Where is everybody? Indeed. I argued that the nuclear reactor at Earth’s core has been crucial for plate tectonics, and preserving Earth from the runaway greenhouse which destroyed venus as a potential biosphere. I even argued that said nuclear reactor may have generated the Moon, by far the largest satellite in the Solar System relatively to the size of the planet it orbits around. (The Moon is larger than Jupiter’s third largest satellite, Europa.)

To generate a large radioactive core to a planet, one needs, first of all, metals. Actually metals enable to make very complicated molecules central to the wealth of biology. Hemoglobin carries iron which is used to transport oxygen.

Where do metals come from? Supernovae.

Which type of planets do we expect to observe in the Habitable Zone?

Vardan Adibekyan, Pedro Figueira, Nuno C. Santos

(Submitted on 8 Sep 2015)

“We used a sample of super-Earth-like planets detected by the Doppler spectroscopy and transit techniques to explore the dependence of orbital parameters of the planets on the metallicity of their host stars. We confirm the previous results that super-Earths orbiting around metal-rich stars are not observed to be as distant from their host stars as we observe their metal-poor counterparts to be. The orbits of these super-Earths with metal-rich hosts usually do not reach into the Habitable Zone (HZ), keeping them very hot and inhabitable. We found that most of the known planets in the HZ are orbiting their GK-type hosts which are metal-poor. The metal-poor nature of planets in the HZ suggests a high Mg abundance relative to Si and high Si abundance relative to Fe. These results lead us to speculate that HZ planets might be more frequent in the ancient Galaxy and had compositions different from that of our Earth.”

So the (empirical) argument is that, if a planet has metal content similar to Earth, it orbits so close to its parent star that it will be too hot for life. Reciprocally, planets which orbit in the Habitable Zone are found to be metal poor.

Planets are built from the same elements as their stars. Most of the properties of planets of different types strongly depend on their host stars’ chemistry, and chemistry varies. It seems Habitable Zone planets were formed long ago. After supernovae formed and exploded, spewing heavy metals such as Iron and Uranium, second generation stars such as the Sun formed, and were metal rich. However, the observations on hundreds of planets tend to show that metal-rich stars like our sun have large rocky planets wrapped in huge gaseous envelopes (caveat: it may still be a bit of a statistical fluke, at this point!).

If not a fluke (and that’s a big if), it gives a new reason to doubt that Earth-like planets are frequent in the galaxy: I argued that not just metal, but the very heaviest metals, the ones which have such large nuclei that they fission, are indispensable for life. Now it turns out that Earth orbits a metal rich star, but at a respectable distance.

So it may well be that Earth is a very special case. Maybe some day the Galactic Human Empire will be able to colonize habitable planets in various Habitable Zones, because thanks to human technology, humans, or, rather, transhumans, will capable of synthesizing metals, as needed (one could do this, if one had a mastery of accelerator technology to fine-tuned nuclear fusion as needed).

Philosophically, this rarity of Earth’s circumstances tells us, once again, that life is more precious that we ever imagined. It’s not just our unborn great grandchildren who will suffer from the holocaust of the biosphere we are engaged in. It’s the universe itself, because we may well be that unique.

Patrice Ayme’

40 BILLION EARTHS? Yes & NO.

Abstract: Habitable does not mean inhabited. And habitable does not even mean inhabited by bacteria. Earth’s advanced life barely survived many disasters. Advanced life on Earth may well be a miracle, as so many dangers lurk in the galaxy. So a profusion of water bearing planets and satellites ripe for colonization by humanity does not imply a zoo of little green persons out there. The evidence points in the opposite direction. 

Up to twenty years ago, a reasonable opinion among scientists was that there might be just one solar system. Ours. Scientists like to project gravitas; advocating the potential existence of a profusion of planets with little green men all over didn’t look serious. Cardinals held the same opinion, five centuries earlier, and burned Giordano Bruno for suggesting that stars had planets with intelligent life on it, as the shocking concept seemed to compromise the special relationship between humanity and divinity.

However, studying delicately the lights of stars, how they vary in intensity, how they doppler-shift, thousands of planets have been found… most of them very strange (maybe because of our present observational methods). Solar systems seem ubiquitous… But NONE of them like ours… And our own Sun is gloriously alone, not in a cluster. Astronomers reported in 2013 that there could be as many as 40 billion habitable Earth-size planets in the galaxy. Suddenly wisdom had gone to the far side. However, as I said, our solar system is special, in more ways then one, as far as we can see, so far… And consider this frightening sight:

A really big bang deleterious to life: Centaurus A: Lobes Of Tremendous Black Hole Explosion Are Fully Visible

Yes, that’s the center of a galaxy, and it has experienced a galactic size explosion from its central black hole. Giant  black holes at the core of giant galaxies regularly suffer giant explosions, making probably the central zone of giant galaxies unsuitable for animal life.

One out of every five sun-like stars in our galaxy has a planet the size of Earth circling it in the Goldilocks zone, it seems — not too hot, not too cold — with surface temperatures compatible with liquid water. Yet, we have a monster black hole at the center of our giant galaxy,  just like the one exploding above.

The Milky way’s black hole is called Sagittarius A*. It exploded last two million years ago. Early Homo Erectus, down south, saw it. The furious lobes of the explosion are still spreading out, hundreds of thousands of light years away.

We are talking here about explosions potentially stronger than the strongest supernova by many orders of magnitude (depending upon the size of what’s falling into Sagittarius. By the way, a large interstellar cloud is just heading that way, and astronomers hope to see fireworks soon, just like an avalanche makes sparks, but on an immensely greaters scale).

Such galactic drama has a potential impact on the presence of advanced life. The richer the galaxy gets in various features with potentially catastrophic developments, the lower the probability of advanced life to flourish. Reciprocally, the metallic content of stars has to be enough to develop life. Metals are found closer to the center of galaxies. Hence the galaxy itself is endowed with a potentially inhabited-with-indigenous life zone (generally labelled by the misleading “habitable” adjective; the main point of this present essay is the distinction between planets that humanity could colonize, and planets harboring advanced indigenous life; anything with a brain is advanced).

The profusion of potentially habitable planets is all the more remarkable, as the primitive methods used so far require the planet to pass between us and its star.

(The research, started on the ground in Europe, expanded with dedicated satellites, the French Corot and NASA’s Kepler spacecraft.) Sun-like stars are “yellow dwarves”. They live ten billion years. Red Dwarves live much longer (giving more time for life to evolve)

From that, overlooking the distinction between “habitable” and “inhabited“, the New York Times deduced: “The known odds of something — or someone — living far, far away from Earth improved beyond astronomers’ boldest dreams on Monday. “

However, it’s not that simple.

Primitive bacterial life is probably frequent. However advanced life (animals) is probably very rare, as many are the potential catastrophes which would set life backwards. The case of Earth shows that one needs billions of years to go from primitive life to the creation of animals.

After life forms making oxygen on Earth appeared, the atmosphere went from reducing (full of strong greenhouse methane) to oxidizing (full of oxygen). As methane mostly disappeared, so did the greenhouse. Earth froze, all the way down to the equator:

When Snowball Earth Nearly Killed Life

Yet volcanoes kept on belching CO2 through the ice. That CO2 built up above the ice, caused a strong greenhouse, and the ice melted. Life had survived. Mighty volcanism has saved the Earth, just in time.

That “snowball Earth” catastrophe repeated a few times before the Earth oxygen based system became stable. Catastrophe had been engaged, several times, but the disappearance of oxygen creating life forms had been avoided, just barely.

Many are the other catastrophes we have become aware of, that could wipe out advanced life: proximal supernovas or gamma ray explosions.

Cataclysmic eruption of the central galactic black hole happen frequently. The lobes from the last one are still visible, perpendicularly high off the galactic plane.  The radiation is still making the Magellanic Stream simmer, 200,000 light years away. Such explosions have got to have sterilized a good part of the galaxy.

In 2014 when part of the huge gas cloud known as G2 falls into Sagittarius A*, we will learn better how inhospitable the central galaxy is for advanced life.

Many of the star systems revealed out there have surprising feature: heavy planets (“super Jupiters“) grazing their own stars. It’s unlikely those giants were formed where they are. They probably swept their entire systems, destroying all the rocky planets in their giant way. We don’t understand these cataclysmic dynamics, but they seem frequent.

Solar energy received on Earth fluctuated and changed a lot, as the Sun itself changed. Maybe the energy received from the Sun doubled over the last five billion years. But, as it turned out, just so, that Earthly life could survive. Also the inner nuclear reactor with its convective magma and tectonic plates was able to keep the carbon dioxide up in the air, just so.

This looks like a double miracle, and maybe, it’s all what it is. If it’s really a double miracle (that is the square of two very low probabilities), advanced life could be very rare, indeed.

The Goldilocks zones, the habitable zones, that astronomers presently consider seem to be all too large to allow life to evolve over billions of years. They have to be much narrower and not just for those around red dwarves.

Red Dwarves are (by far) the most frequent and long living stars. But they seem prone to X Ray flaring, inimical to life as we can imagine it, and possibly eroding planetary atmospheres.

One of our Goldilocks, Mars, started well, but lost its CO2 and became too cold. The other Goldilocks, Venus, suffered the opposite major technical malfunction: a runaway CO2 greenhouse.

Mars’ axis of rotation tilts on the solar system’s plane enormously: by 60 degrees, over millions of years. So Mars experiences considerable climatic variations over the eons, as it goes through slow super winters and super summers (it’s imaginable that, as the poles melt, Mars is much more habitable during super summers; thus life underground, hibernating is also imaginable there).

Earth’s Moon prevents this sort of crazy hyper seasons. While, differently from Venus, Earth rotates at reasonable clip, homogenizing the temperatures. Venus takes 243 days to rotate.

It is startling that, of the four inner and only rocky planets, just one, Earth has a rotation compatible with the long term evolution of advanced life.

Earth has also two striking characteristics: it has a very large moon that store much of the angular momentum of the Earth-Moon system. Without Moon, the Earth would rotate on itself once every 8 hours (after 5 billion years of braking by Solar tides; the braking by Moon tides is much stronger).

The Moon used to hover at least ten times closer than now, when Earth’s days were at most 6 hours long.

The tidal force is the difference between gravitational attraction in two closely separated places, so it’s the differential of said attraction (which is proportional to 1/dd; d being the distance). Hence the tidal force is inversely proportional to the cube of the distance.

Thus on early Earth tides a kilometer high were common, washing back and forth every three hours. a hyper super tsunami every three hours, going deep inside the continents. Not exactly conditions you expect all over the universe.

Hence biological material fabricated on the continental margins in shallow pools  would get mixed with the oceans readily. That would guarantee an accelerated launch of life (and indeed we know life started on Earth very fast).

Acceleration may imply existence, though: life, once started for real, is very adaptable. So the tremendous mixing of the kilometer tides may be a necessary condition for life!

If such is the case, it goes without saying life is exceedingly rare. (I’m exaggerating the point a bit here; yet, it’s entirely possible that the tides made the difference between bacterial life in a few dozens of million years, and having it in a few billions. Incubators where life starting processes are engaged work better with mixing.)

The theory of formation of the Moon is wobbly (recent detailed computations of the simplest impact theory do not work). All we know for sure, thanks to the Moon rocks from Apollo, is that the Moon is made of Earth mantle materials.

Somehow the two planets split in two. (Fission. Get it? It maybe a hint.)

Another thing we know for sure is that Earth has, at its core, a giant nuclear fission reactor, keeping Earth’s  core hotter than the surface of the sun. An unimaginable liquid ocean of liquid iron deep down inside below our feet undergoes iron weather. Hell itself, the old fashion way, pales in comparison.

Could the Moon and the giant nuclear reactor have the same origin? This is my provocative question of the day. The Moon, our life giver, could well have formed from giant nuclear explosions, of another of our life givers, what became the nuke at the core. I can already hear herds of ecologists yelp in the distance. I present the facts, you pseudo-ecologists don’t decide upon them. It’s clear that nuclear fission is not in Drake equation: if nothing else, it’s too politically incorrect.

All the preceding makes this clear:

Many are the inhabitable planets, yet few will be inhabitated by serious denizens.

This means that the cosmos is all for our taking. The only question is how to get there. The closest stars in the Proxima, Beta and Alpha Centauri system are not attainable, for a human crew, with existing technology.

However, if we mastered clean colossal energy production, of the order of the entire present energy production of humanity, we could get a colony there (only presently imaginable technology would be fusion).

Giordano Bruno, professor, astronomer, and priest suggested that there were many other inhabitated systems around the stars. That insult against Islam Christianity was punished the hard way: the Vatican, the famous terrorist organization of god crazies, put a device in Giordano’s mouth that pierced his palate, and having made sure that way that he could not tell the truth, the terrorists then burned him alive. After seven years of torture.

The horror of truth was unbearable to theo-plutocrats.

Now we face something even worse: everywhere out there is very primitive life. It is likely gracing 40 billion worlds. But, if one has to duplicate the succession of miracles and improbabilities that made Earth, to earn advanced life, it may be just here that civilization ever rose to contemplate them.

Congratulations to India for launching yesterday a mission to Mars ostensibly to find out if there is life there (by finding CH4; while life is presently unlikely, Mars has much to teach, including whether it started there). That’s the spirit!

The spirit is to have minds go where even imagination itself did not go before.

If we sit back, and look at the universe we have now, from Dark Matter, to Dark Energy, to Sagittarius, to the nuclear reactor below, to billions of Earths, to a strange Higgs, to Non Aristotelian logic, we see a wealth, an opulence of possibilities inconceivable twenty years ago.

Progress is not just about doing better what was done yesterday. It’s also about previously inconceivable blossoms of entirely new mental universes.

***

Patrice Ayme

Fragile Earth Syndrome

Abstract: The Earth is already all too close from being getting all too hot, from its astronomical position at the interior edge of the Sun’s Habitable Zone.

The Greeks viewed Gaia, the Earth, as the Mother Goddess from whom all other gods sprang. Yet, discoveries they made later showed that this metaphysics was misleading. In truth, habitable planets, far from being all powerful, are confined to narrow zones around their stars (and these zones move, and are under continual threat, as I describe below):

Sun Like Stars Are Most Hospitable.

Vertically the masses, going up, the unit being the mass of the sun; as stars gets bigger, they get hotter, thus they change color, covering the entire black-body spectrum, from brown dwarves to blinding ultra violet hot “Blue Stragglers”.

Horizontally, the distance from the star; the graph gives only a rough idea of the notion of Habitable Zone; in truth the whole point of this essay is that Earth is at the edge of Sol’s Habitable Zone, within 1% of boiling; Habitable Zones narrow as the stars get smaller, and get much larger, far out, around bigger stars.

The life of Earth on the edge has got more dicey in the last 400 million years. Thus the risk of hyper warming is greater than in the Carboniferous Era. By pumping into the atmosphere the equivalent of 100 million tons of CO2, every single day, we are, literally, playing with fire. (A first counter-measure would be to outlaw, through regulations, those gases that warm up the air a lot, and are not indispensable; for example leaks in the pipelines of the USA allow 4% of the CH4 to escape!)
***

The two close calls by large space rocks were a reminder that this is a serious, not particularly friendly universe. Something to meditate carefully.

Those who play apprentice sorcerers with the climate and planetary ecology should pay attention.

For reasons having to do with the periodic table, the frequency of elements and the chemical characteristics of carbon, namely its ability to form many liaisons, it seems likely that life in the universe will have to resemble life on Earth. That is being water, carbon and oxygen based. (Believing that life does not have to be carbon-centric may sound cute, but it’s unreal.)

Thus the habitable zone is the zone around a star where it is neither to hot, nor too cold, and a planet can support water.

Not all stars can have an habitable zone: the greater the mass of a star, the more fiercely it burns. A star with five times the mass of the sun will typically have 625 times the luminosity of the sun.

Why? In small stars, the part of the core hot and dense enough to sustain thermonuclear fusion is relatively small. In large stars, it becomes enormous, and embraces much more of the thermonuclear fuel tank.

For Stars, Mass Is Everything.

Thus, the larger the mass, the shorter the lifespan of the entire system orbiting the star. A star with 60 solar masses will shine only 3 million years before running out of hydrogen. At that point it will run hotter and hotter as it burns heavier elements until it explodes as a super nova. A star of five solar masses will live longer, but still only 100 million years or so. Long enough to make it a tourist destination, not long enough to evolve life (all the more as the habitable zone will migrate out fast, as the stellar furnace gets hotter, fast).

Even a star with only 50% larger mass than the Sun will live only three billion years. On Earth, after that duration, the first oxygen making organisms were appearing, and the atmosphere was going to change completely, from reducing to oxidizing. That would bring the “Snowball Earth” episodes, 600 million years ago, or so, when most of the planet froze, before enough CO2 could be generated to reach the appropriate greenhouse effect.

Clearly, for evolving advanced life, more than a billion years is needed. Thus planets with indigenous life will be restricted to red and yellow dwarves (the sun is one of the latter, with an estimated lifespan of ten billion years before turning ephemerally into a red giant).

The 2012 sci-fi (silly) movie “Battleship” has it right on that point: most of the habitability is found cuddling next to red dwarf stars, so that aliens would be blinded by our sunlight is likely. This also means that life out there has a good probability to have evolved in what, for us, would be rather dim circumstances. Indeed most stars are red dwarves and those are the longest living stars, easily going on for 15 billion years (they use their thermonuclear fuel conservatively).

Some red dwarves could have evolved life, in our Milky Way galaxy, when our sun, a mighty yellow dwarf, did not exist yet. Such stars, with their habitable planets, could still be around.

Being in the habitable zone is necessary for life, but it’s not sufficient.

For example, any planet orbiting too close to its star will lock its orbital rotation and its diurnal rotation (as the Moon has with the Earth). Thus the planet will have one side too hot for life, and the other too cold.

That means that when red dwarves become too small, their habitable zones, get too close, and would-be habitable planet lock down. (Venus, although 100 million kilometers from the Sun is nearly locked: it rotate on itself slower than it does around the Sun.)

The Earth is totally exceptional. She is endowed with a huge satellite that stabilizes her inclination on the orbital plane (Mars’ inclination on the elliptic varies wildly, causing wildly fluctuating super-seasons). This resulting, constant and mild inclination allows the poles to not get too cold, and the tropics, not too warm: it spreads the goodness of sunlight around.

Earth is also a mighty nuclear reactor, providing with the shield of a powerful magnetosphere (Venus does not have any, so its upper atmosphere is scorched by the solar wind), and plate tectonic (allowing for a complex recycling mechanism involving CO2 and long term climate stability).

The present, sort of official, habitability zone theory is 20 years old. It showed that Earth was within 5% of receiving too much warmth from her star. What has been found by the latest study is even more disturbing: Earth is within 1%, 1.5 million kilometers of inhabitability (5 times the Earth-Moon distance).

Earth is, astronomically, at risk of getting too hot, and of suffering a run-away greenhouse, as Venus did.

Long ago, Venus may have been in the habitable zone. However, general main sequence star theory, and observation, show that the Sun has warmed up. Its power output has increased by at least 25% since it got started. So the habitable zone in the Solar System has been slowly moving outward.

Why did the Earth cool over the last 100 million years, if the sun is slowly warming up? It probably has to do with non linear effects related to the geometry of the continents: the continents migrated north, and shallow tropical seas disappeared. The migration of land towards the north augmented the albedo of the Earth (as land stays frozen in summer more easily than sea, ice and snow keep reflecting more sunlight back to space, even then; that’s the core of the two centuries old glaciation theory).

So, as Earth should have warmed up, by a miracle, a sun shade, the glaciated North, was put in place, just in time!

Not all the coolness is due to ice and snow. Earth, before very recent human interference, had long been endowed with a cool climate. It seems that clouds make the difference (the effects clouds bring are too complex to be taken into account in computer programs of habitability at this point).
It’s a double edged sword. Water vapor may bring more clouds, but it is also a mighty greenhouse gas.

Still the point remains that all the objective data show that, our planet is not far, astronomically speaking, from a runaway greenhouse. By keeping on pumping a witches’ brew of greenhouse warming gases in the atmosphere, we are, literally playing with fire. Every day we add nearly 100 million tons, in CO2 alone, in our apparent urge to mimic Venus.

Pumping 450 million years of carbon into the air all of a sudden is not smart: Earth has had plate tectonics from the start, so much of this carbon was sequestered. Now we are freeing huge quantities of it… and in a geological, and biological, snap.

All other things being equal, the Earth is closer to inhabitability through warming than it was 400 million years ago (when the CO2 was very high). Having the same CO2 in the air as in the Carboniferous Era would result in a warmer planet.
To make things worse, there are no plausible technological fixes to too much CO2 in the atmosphere (with existing science and technology; and contrarily to disinformation from the fossil carbon burning fanatics).

In between the high- and low-mass stars lie stars similar to our own Sun. They make up about 15% percent of the stars in the galaxy. Such stars have reasonably-broad Habitable Zones, do not suffer from hard UV irradiation, have lifetimes of the order of 10 billion years. They are the best candidates for harboring planets with indigenous life.

Intriguingly, the three stars of the Alpha Centauri system may harbor life. The system is made of two main yellow dwarves, one slightly bigger, one slightly smaller than the sun. They come as close to each other as Saturn is from the Sun (not close enough to affect each other Habitable Zones directly).

A planet was just detected, grazing the .9 solar mass Alpha Centauri B. (We have the means to find out if the system supports life, but NASA and the Congress of the USA, shut down the projects, in an apparent fit of obscurantist anti-science rage; one of them called the Terrestrial Planet Finder; Alpha Centauri would be reachable with nuclear propulsion.)

The stability of orbits (hence of the Habitable Zones) in the Alpha Centauri system has been debated. Many a stellar system has been found where giant planets have progressively swept the entire system. And we are always one giant comet away from extinction. That could happen in 6 months. And we don’t know, because we are apparently not interested to find out. (Although the mightiest nuke could solve that problem, that would require some preparations.)

Life exists in the cosmos, everywhere, but it’s fragile. Everywhere. Including on so far invincible fortress Earth. Invincible, but still so fragile.

3,000 years after the Greeks elaborated their mythology, we find out that, contrarily to what they guessed, Earth is far from the mother of all what is divine. There are greater powers out there… The worst of them being, potentially, ourselves.

As a star goes up the main sequence, its Habitability Zone moves out. So we should be careful to think we can reconstitute the conditions of the Carboniferous Era, by pumping as much CO2 in the air as there was then, and prosper.

Everything indicates that we will punch straight trough.
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Patrice Ayme