Why haven't aliens contacted Earth? New Fermi Paradox analysis suggests we're not that interesting yet

[Admin]

We have absolutely no evidence for any extraterrestrial civilization, let alone life. This is Fermi's great paradox: If life can happen, it should be common, and if it's common, we should already know about it. But we don't.

Why haven't aliens contacted Earth? New Fermi Paradox analysis suggests we're not that interesting yet : Read more

 

[rod]

Looks like a probability creation story, repeated again and again.

Why have aliens never visited Earth? Scientists have a disturbing answer

Aliens could be forced to give up cosmic expansion to avoid catastrophe. Why have aliens never visited Earth? Scientists have a disturbing answer : Read more

forums.space.com

 

[stormreforger]

Big deal.. besides,, fundamentalist creation not in harmony with the Bible which isn`t a science book nor meant to be and I just ignore what they say and here also..

 

[Helio]

I don't see that the "great filter hypothesis" offers something not already addressed. We know Earth is special in many ways, including its large iron core that provides us with an effective magnetic shield. Our large Moon gives us better obliquity stability. The Sun seems to be about the ideal star to host a planet with life since the red dwarfs can throw terrible tantrums too often.

The ~100 lyr. travel distance of our low-wattage AM broadcasts (~ 90 years for TV), greatly limits the no. of stars it has reached (~ 16k). Once you restrict it to the more likely star classes to host stable planets, this drops significantly. Then restrict the number to those stars that host HZ planets. Then estimate how many of those might host life. Then cut that to ones that might have intelligence. Then cut that to ones that have intelligence and have been around long enough to receive and recognize our signal. But this isn't new information.

SWAIM (So What Am I Missing)?

We are still in our infancy of understanding what planets are out there, and we still are new in finding which ones have the right planetary conditions to offer life, much less intelligent life.

It does appear that the red dwarfs (most numerous of stars) are poor hosts, so that is detrimental to the habitable count. This is somewhat new info., but time (more science) will tweak this.

My answer to the Fermi Paradox, *cough*, is that our neighbors have heard we orbit a yellow star, so they will likely never find us. [I know, lame. ]

 

[Unclear Engineer]

I don't get the statement that there are "up to 1 trillion potentially habitable planets in the galaxy (and even more if you include water-rich moons such as Europa and Enceladus)".

With ae estimated few hundred billion stars in the Milk Way, that implies several "potentially habitable planets" per star. That seems like an over estimate to me.

And, even if you get all the other requjirements for life, civilization, intelligence, technological development, etc., what about things like a clear sky so that they even realize that there are other worlds "above" their atmospheres? What about the larger rocky planets with thick atmospheres and very deep gravity wells that make space flight, even just satellites, unlikely to be achieved? What about "water worlds" with no place to develop air breathers who can discoverr fire, etc?

I think the "filter" is far more effective than this article describes.

And then, there is the noise of natural things. We can't see the part of the Milky Way that is on the other side of our central bulge and black hole, and probably not much of what is in the inner disk, at least with today's listening technology.

Plus, the idea that another species could permeate the entire gallaxy in a million years needs to somehow come up with a way for the life form to get to relativistic speeds, and also survive jurneys from star to star that would also still take huge amounts of time for life support systems to deal with. Frankly, I would expect to see rusty robots before seeing life forms coming our of the cosmos.

 

[murgatroyd]

Either the space.com article is poorly written and misses the point ... or the paper published to arXiv is lacking in basic research. By now, there are more answers to Fermi's question than there are atoms in the universe. Good luck finding yet another while sitting at your desk.

 

[rod]

Well, in all of this interesting discussion concerning probability creation stories, what happened to defining a star's abiogenesis zone? Ultraviolet light of certain wavelengths is needed to create prebiotic molecules in the latest and greatest for astrobiology. The December issue of Sky & Telescope has the report.

"What does stars' tempestuous activity mean for their planets' habitability? "TRAPPIST-1 At least three of the rocky exoplanets circling the red dwarf Trappist-1 lie in the star's habitable zone (a more generous definition ropes in a fourth world). But the little star puts out such a scant amount of ultraviolet light that none of the exoplanets receives enough radiation to trigger the photochemistry necessary to build up prebiotic molecules - unless the star's flares do the trick."

On page 40, "Dwarf Doldrums So which stars will prove the friendliest to life? Most astronomers suspect that it won't be red dwarfs - which would rule out about 75% of our galaxy's stars. Maybe we should instead focus our efforts on Sun-like stars, such as the solar twin Kepler-452 with its habitable-zone planet. Such exoplanets remain hard to study with current technology."

The report also states, "Where Life Might Arise" "It's not just a star's habitable zone that determines habitability. Scientists suspect that a planet also needs to receive sufficient ultraviolet radiation (specifically, in the 200 to 280 nm range) to trigger chemical reactions that make prebiotic molecules. If so, planets around red dwarfs are in trouble. These stars' abiogenesis zones lie far closer to the star than the habitable zones do. Powerful flares might help bridge the gap. But atmosphere matters, too: Modern Earth's ozone layer blocks these ultraviolet wave-lengths, meaning abiogenesis couldn't work the same way on Earth today as it might have billions of years ago."

Some identify HZ but so far, I see nothing on space.com about the abiogenesis zone around stars.

A Flare for the Dramatic, Sky & Telescope 144(6):34-40, 2022

 

[Helio]

I don't get the statement that there are "up to 1 trillion potentially habitable planets in the galaxy (and even more if you include water-rich moons such as Europa and Enceladus)".

Several times earlier in the article only "worlds" were stated, so this seems to have been a simple error.

Plus, the idea that another species could permeate the entire gallaxy in a million years needs to somehow come up with a way for the life form to get to relativistic speeds, and also survive jurneys from star to star that would also still take huge amounts of time for life support systems to deal with.

Right. If science continues to offer advances in travel, if improved telescopes find habitable planets, but without us having to terraform it.

Though astronomy has really achieved a lot in the last few decades, amazingly, yet there are still lots of "ifs and buts". As Dandy Don used to say, "If 'ifs and buts' were candy and nuts, we'd all have a merry Christmas!".

 

[Helio]

The report also states, "Where Life Might Arise" "It's not just a star's habitable zone that determines habitability. Scientists suspect that a planet also needs to receive sufficient ultraviolet radiation (specifically, in the 200 to 280 nm range) to trigger chemical reactions that make prebiotic molecules. If so, planets around red dwarfs are in trouble.

Perhaps UV activity on the asteroid's hydrocarbons play a role, not that I would know.

Red dwarfs do produce UV but primarily when they flare. They seem to flare quite often, perhaps so much that they are detrimental to life since the HZ is so close to them.

These stars' abiogenesis zones lie far closer to the star than the habitable zones do.

But abiogenesis is barely a hypothesis, regardless of how logical it appears. It's not logical to be able to determine just what exoplanet environments are required for the transformation of chemicals into life forms. Water (liquid) is certainly and excellent guess, hence this is the current factor to determine what is a habitable planet.

Powerful flares might help bridge the gap. But atmosphere matters, too: Modern Earth's ozone layer blocks these ultraviolet wave-lengths, meaning abiogenesis couldn't work the same way on Earth today as it might have billions of years ago."

Yes. More science should help resolve this.

 

[Helio]

No doubt, a lot of answers will come as we get closer to a respectable solution to the Drake equation.

 

[bwana4swahili]

We have absolutely no evidence for any extraterrestrial civilization, let alone life. This is Fermi's great paradox: If life can happen, it should be common, and if it's common, we should already know about it. But we don't.

Why haven't aliens contacted Earth? New Fermi Paradox analysis suggests we're not that interesting yet : Read more

Just possibly the simulation we live in doesn't have a need for extraterrestrial civilizations to accomplish its entertainment goals!?

 

[rod]

The Occurrence of Planets in the Abiogenesis Zone, The Occurrence of Planets in the Abiogenesis Zone - NASA/ADS (harvard.edu), March 2021. "Introduction: Precursor molecules to the building blocks of life such as ribonucleotides, amino acids and lipids could have been produced in an early, prebiotic Earth in which ultraviolet radiation induced the activation energy required to trigger photochemical reactions. For planets to be able to host these photochemical reactions and possibly originate life, they must be able to maintain liquid surface water. Therefore, planets must be orbiting within the Habitable Zone, which is generally defined as the area around a star in which a planet with an atmosphere could sustain surface liquid water [1]. (see Figure 1)."...
"Details for the Abiogenesis Zone. This zone approximately ranges in UV radiation from 200nm to 280nm, since this is the critical wavelength range for photochemical reactions as proved by Todd et al. [5]."

The December Sky & Telescope discusses the abiogenesis zone for stars and planets habitable zone. The abiogenesis zone now defined in astronomy in addition to the habitable zones, becomes a player in all that is discussed about the origin of biological life on exoplanets and in our solar system too. Science moves on, belief systems collapse. My observation, abiogenesis zones could be another indicator of a fine-tuning problem in the probability creation story commonly told the public.

 

[Helio]

Thanks Rod, that's interesting.

But I see it similar to the dumb ham joke, "If we had some bread, we could make a ham sandwich, if we had some ham." What do we know really is required to allow abiogenesis to take place?

The ocean bottom vents hypothesis may win the day, though it may have required surface activity to make that happen.

It was a safe bet that only certain star classes would be better than others, with an initial bias toward G2 stars, for some reason. We now know more about the fisty M-class stars, but more HZ planets (~1/2) are found in G-type stars, at least so far.

The premise in the above is a good guess, AFAIK, but I don't think it necessarily changes the HZ list, or does it?

 

[rod]

Thanks Rod, that's interesting.

But I see it similar to the dumb ham joke, "If we had some bread, we could make a ham sandwich, if we had some ham." What do we know really is required to allow abiogenesis to take place?

The ocean bottom vents hypothesis may win the day, though it may have required surface activity to make that happen.

It was a safe bet that only certain star classes would be better than others, with an initial bias toward G2 stars, for some reason. We now know more about the fisty M-class stars, but more HZ planets (~1/2) are found in G-type stars, at least so far.

The premise in the above is a good guess, AFAIK, but I don't think it necessarily changes the HZ list, or does it?

Helio, it may not change the HZ listed. The report in December Sky & Telescope on page 40 has the HZ defined for Trappist-1 system. Optimistic HZ, Conservative HZ, and because the host is a small red dwarf, the abiogenesis zone is much closer where flares and other activity from a stellar wind can cause problems, like what is uncovered now about Proxima Centauri b. The report did state (page 40), "The researchers delineated an abiogenesis zone around stars of different stellar types, based on the distance at which UV rays emitted by the star were strong enough to trigger the creation of prebiotic molecules on orbiting worlds. This zone can overlap with the liquid-water habitable zone; Earth, perhaps unsurprisingly, falls into both."

It gives astronomers something else to think about and define around different stars where we find exoplanets today. It seems to add to the complexity of the probability creation story for origins

 

[Helio]

Helio, it may not change the HZ listed. The report in December Sky & Telescope on page 40 has the HZ defined for Trappist-1 system. Optimistic HZ, Conservative HZ, and because the host is a small red dwarf, the abiogenesis zone is much closer where flares and other activity from a stellar wind can cause problems, like what is uncovered now about Proxima Centauri b. The report did state (page 40), "The researchers delineated an abiogenesis zone around stars of different stellar types, based on the distance at which UV rays emitted by the star were strong enough to trigger the creation of prebiotic molecules on orbiting worlds. This zone can overlap with the liquid-water habitable zone; Earth, perhaps unsurprisingly, falls into both."

It gives astronomers something else to think about and define around different stars where we find exoplanets today. It seems to add to the complexity of the probability creation story for origins

That’s logical. If you find a UV luminosity range, I can add it separately to improve a greater life probability selection. I suspect, however, that the sience is still too weak to give us a high probability of the HZ limits. The variation in the results from each model speaks strongly to this.

Also, further studies of known exo’s will tweak the data. In fact, today I crunched the exo HZ list and the count dropped by one (an M class exo).

 

[bolide]

We aren't going to last forever. The Sun will incinerate us one day, if we don't do it ourselves long before. Point is there's no reason to think any civilization will last forever. And the typical lifespan of intelligent life in any location may be only an eyeblink, in cosmic terms.

So even ignoring the distance problem, the universe might be full of advanced civilizations--winking into existence, and out again, in brief moments of time. The only way one would ever simultaneously know about another is if they both winked to a highly advanced level in the same moment, and within that moment conquered all the other difficulties mentioned here--including the distance problem.

And Earth? Sheesh, we've only even had radio for about 100 years!

 

[rod]

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I saw the movie Contact. Why doesn't science present this evidence as factual and confirmed to the public showing *alien spacecraft* are here on Earth? In the movie Contact, this was widely proclaimed. I would think the entire scientific community would be onboard here, loudly proclaiming we now have confirmed alien intelligence outside of Earth and our solar system demonstrated by *intact* alien spacecraft. I use the Galileo standard of science. Galileo could show others the tiny lights moving around Jupiter using his telescope. I use my telescopes and more than 400 years later, can still see these tiny lights moving around Jupiter.

Applying this simple test to the aliens are here and visited leaving behind the evidence, should be no problem and show the entire world that this is true.

 

[Spaceman Spiff4]

I’ll never say never… HOWEVER…. I'm going to play pessimist here. Life? Probably. Intelligent life like us on a scale capable of what our homo sapiens brains have become, or greater? Doubtful. Physics, chemistry, biology, and the formations of solar systems are pretty uniform across the universe as far as we know.

Here on Earth, there have been 12 extinction events and five of those were major ones. If any of those five don’t happen, we’re not here. If one of those other seven don’t happen, it’s also probable we’re not here.

BEFORE ALL THAT THOUGH... If there wasn’t the planetary collision that formed the moon that slowed down Earth’s rotation, we’re not here. If the moon didn’t generally settle at its current distance to create our current tide levels, thusly creating the inter-tidal zone where life began to move from the sea to the land, we’re not here. If we don’t have our axial tilt from that collision which created our seasons, we’re not here. In the early formation of the solar system, Jupiter made its way to the inner solar system but was pulled back out by Saturn. If there’s no Saturn to do that, Earth, if it wasn't destroyed, wouldn't settle in the habitable zone, and we’re not here. If Jupiter and Saturn weren’t out there to protect us from a bombardment of asteroids and comets, we’re not here. Yet if we didn’t have the right amount of those asteroids and comets hit Earth to bring us the right amount of water, microbes, and elements to start life, we’re not here. If we didn’t have that one asteroid that also got through and wiped out the dinosaurs (the last of the five major extinction events) leading to the evolution of mammals, we’re not here.

If our Hominid lineage didn’t split from chimps 6-7 million years ago, we’re not here. Even that lineage was still pretty ape-like for the next 3-4 million years. If even one of those subsequent Homo mutations/evolution events doesn’t take place, we’re not here. For us, it took 4.5 billion years to become the intelligent species we are, and we had to go through a GAUNTLET of if/thens to become the Homo Sapiens species we are. The odds of that happening again are slim at best. If Earth formed 24 hours ago, hominids appeared at 23:58. That’s not even Homo Sapiens which came much later. Dinosaurs only died out about 23:30.

Yes there may be millions, billions, trillions of “habitable” planets out there, and there’s nothing to say that intelligent life can’t or wouldn’t develop in other ways. It probably would. But when you look at what had to happen to Earth in those 23 hours and 58 minutes to become a “hospitable” planet for not only life, but intelligent life, along with the combination of the extinction and evolution events for us to develop/evolve into that intelligent life, the right and stable atmosphere to continually support developing life, and have the proper growth of land and sea plant life over 4.5 billion years to sustain that atmosphere which sustains said life, it doesn’t look promising that there is other intelligent life out there on our scale or greater no matter how that life is formed and subsequently develops.

 

[Pogo]

OK, so Drake Equation, ignorant earthlings, time overlap, and other things aside, here's the other problem we have in detecting other civilizations. Have they been here, no. That has been discussed in other threads, problems being travel time being kazillions of years, etc. But, could we detect radio emissions from them.?

Here's the problem. Despite what we think, our technology is still pretty primitive in this regard. The Voyager spacecraft is about a hundred and some AU from Earth transmitting data at maybe 23 watts of radio power in the Gigahertz range. We have a number of really large antennae, like huge dishes throughout the world listening at times for Voyager. They have to listen pretty much simultaneously to hear it, kinda turning the whole earth into one big antenna, sort of. α Centauri, the nearest star is about 4.3 LY from our solar system, that's 271,870 AU. The inverse square law says that a transmitter at α Cent would have transmit 739,000 times the power that Voyager does in order to detect it. And both the Earth arrays and Voyager use a very narrow angle high gain antennas, not omnidirctional. That means the α Cent transmitter would have to be pointing directly at us at the same time offset by 4.3 years that we listen in that direction. If aliens exist, it'll not be likely at α Centauri, it may be 50, 100, or 1000 LY away or even more. That would require humongous transmitters there and humongous receivers here to detect them. Not only that, if they were to transmit in radio bands like we commonly use here because it's easier (AM, FM, short wave, etc), we may not hear them because of local noise. Even the lower GHz range is used by millions of cellphones and the like, increasing the noise up in that range.

This would be sorta like listening for a cricket in a sports field full of cars, trucks, airplanes, air hammers, and people and other critters making noise, and you standing across town with a microphone.

 

[billslugg]

I have read an analysis of this and a few things stood out. AM/FM/TV cannot make it much past Jupiter due to their fairly low power, lack of directional antennas and the high level of radio noise in the Solar System.

About the only thing that could make it to the nearest star would be a military level radar, and antennas about the size of Arecibo on both ends.

I believe lasers would be a more efficient method than high frequency radio.

A super advanced civilization might even use xrays or gamma rays.

 

[rod]

In post #5, Unclear Engineer said, "What about the larger rocky planets with thick atmospheres and very deep gravity wells that make space flight, even just satellites, unlikely to be achieved?"

I see little or no discussion on this topic, especially when it comes to E.T. buzzing around and phoning home. The idea looks important. Consider an exoplanet of 2 earth mass with radii 1.3 earth radii. Mean density ~ 5 g cm^-3, so could be a rocky world like Earth. However, the surface gravity is 1.1832 earth gravity and escape velocity ~ 13.87 km/s. Some exoplanets could have escape velocities requiring high velocities as fast or faster than asteroids and meteoroids in our solar system seen. Do you really think NASA SLS would make it to the Moon with much higher escape velocities for Earth or Sputnik would be orbiting in October Skies in 1957? A 2.5 earth mass exoplanet with 1.3 earth radii, escape velocity jumps to 15.50 km/s.

 

[bolide]

In post #5, Unclear Engineer said, "What about the larger rocky planets with thick atmospheres and very deep gravity wells that make space flight, even just satellites, unlikely to be achieved?"

Consider an exoplanet of 2 earth mass with radii 1.3 earth radii. Mean density ~ 5 g cm^-3, so could be a rocky world like Earth. However, the surface gravity is 1.1832 earth gravity and escape velocity ~ 13.87 km/s. Some exoplanets could have escape velocities requiring high velocities as fast or faster than asteroids and meteoroids in our solar system seen. Do you really think NASA SLS would make it to the Moon with much higher escape velocities for Earth or Sputnik would be orbiting in October Skies in 1957? A 2.5 earth mass exoplanet with 1.3 earth radii, escape velocity jumps to 15.50 km/s.

I read somewhere that there is a theoretical limit to the thrust achievable with chemical rocket propulsion. Seems like there could be planets like yours where it's literally impossible to reach
escape velocity, or even orbital velocity. Explorers, also, had better think about where they are landing!

 

[billslugg]

The thrust in a rocket is made by the pressurized gasses pushing upward against the interior of the top side of the combusion chamber. The amount of thrust per square inch of that surface is determined by the pressure inside the chamber. Rocket engines run about 1,000 psi I believe. Higher thrust requires higher pressure which requires stronger chamber. At some point there is a strength of materials limit, but we are no where near it.

 

[rod]

The thrust in a rocket is made by the pressurized gasses pushing upward against the interior of the top side of the combusion chamber. The amount of thrust per square inch of that surface is determined by the pressure inside the chamber. Rocket engines run about 1,000 psi I believe. Higher thrust requires higher pressure which requires stronger chamber. At some point there is a strength of materials limit, but we are no where near it.

billslugg, are you saying here that the NASA SLS that sent Artemis with the Orion space capsule around the Moon and back would work just as designed on an exoplanet where the escape velocity about 14-16 km/s or would there be the need for much more engineering design changes and much more thrust ability?

The NASA exoplanet archive site shows at least 2706 exoplanets with radii 3 or less earth size and some or many will have even higher escape velocities from those exoplanets. NASA Exoplanet Archive (caltech.edu)

The average radius 1.898 earth radii. It does not take much to find exoplanets in that list with much higher escape velocities than Earth.

 

[billslugg]

Artemis would have to be re-engineered for a planet with a higher escape velocity than ours. This would entail reducing the payload mass, increasing the combustion chamber pressure or adding stages.

I just depends upon what the force of gravity is on the planet and how much weight you want to lift off.

A combustion chamber like ours, with 1,000 PSI inside it and, say, a ten square foot surface at the top has an upward force of 1,440,000 pounds. If the weight of the rocket is more than 1,440,000 pounds it won't get off the pad. If the weight of the rocket is 720,000 pounds it will acellerate upwards at 32 ft/sec^2.