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

Radio telescopes point skyward at sunset.
Radio telescopes point skyward at sunset. (Image credit: sharply_done/Getty Images)

If life happened here, then it likely happened elsewhere. But as far as we can tell, we're totally alone. So where is everybody? 

A new analysis proposes an alternative solution to this conundrum, known as the Fermi paradox: Perhaps we're just being impatient. Maybe, with a multitude of worlds to potentially study, alien civilizations would likely wait for one to start broadcasting their presence before sending a probe. So, if we wait a few hundred or a few thousand years, somebody might come knocking.

As famed physicist Enrico Fermi purportedly said during a casual lunch conversation in 1950, "Where is everybody?' Intelligent life arose here, on Earth, but we are certainly not the only planet in the universe. Our galaxy, the Milky Way, likely contains up to a trillion worlds, and it is one of up to 2 trillion galaxies in the entire cosmos. Sure, most planets are completely inhospitable to life, but physical processes don't really happen just once. If nature can make life here, with the sheer number of other planets, it should happen elsewhere.

Related: Why are we still searching for intelligent alien life?

And presumably, some of those intelligent critters would start building spacecraft and exploring their neighborhood. Given enough time and effort, they could spread themselves or robotic emissaries far and wide. Even if they averaged only a tiny percentage of the speed of light, they could essentially swamp the entire galaxy in only a few million years.

Even if most intelligent civilizations fail in the attempt (or simply move on to other things), the fact that our galaxy has existed for over 10 billion years means that at least one civilization should have already visited our solar system, or at least left some sign of its existence.

And yet, nothing. 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.

Interstellar roadblocks

Over the decades, astronomers have proposed many solutions to this puzzle. One idea, called the rare Earth hypothesis, posits that perhaps life really is special and unique on the cosmic scale. In this scenario, life is so incredibly rare that we may be among the first creatures or any kind to arise in the Milky Way. In other words, the circumstances that led to the emergence of life on Earth are so special that even with trillions of other worlds, life happened essentially only once.  

Another proposed solution, known as the great filter hypothesis, postulates that perhaps life is common but intelligent life is difficult. After all, life appeared relatively early in the history of our planet, but it took billions of years for intelligence to arise, This means that perhaps we are incredibly lucky to have the brains we do. And even then, the introduction of nuclear weapons and climate change put the future of our species in doubt. So, while we may find bacteria or other simple organisms throughout the galaxy, we're unlikely to meet anybody capable of conversation.

Now, a new paper written by Amri Wandel at the Hebrew University of Jerusalem and published in the preprint database arXiv (opens in new tab) puts forward a new explanation: Because we have only recently arrived on the cosmic scene, in the sense of being able to broadcast our presence through radio transmissions, maybe we just need to wait a bit.

The construction site of China's Solar Radio Telescope is seen on November 12, 2022 in Daocheng County, Garze Tibetan Autonomous Prefecture, Sichuan Province of China. (Image credit: VCG/VCG via Getty Images)

A game of patience

Wandel argues that while we can't imagine the technological capabilities of advanced alien civilizations, their powers aren't infinite. They still have to deal with mundane issues, like energy capture and storage, waste heat, information processing, and a finite amount of time. With up to 1 trillion potentially habitable planets in the galaxy (and even more if you include water-rich moons such as Europa and Enceladus), it seems reasonable to assume that these alien civilizations wouldn't be able to send active probes or messages to every single one.

However, it's much easier to build large, sophisticated listening stations than active probes, so the aliens would probably wait. Eventually, some intelligent civilization will arise in the galaxy and figure out the magic of radio. Inadvertently or otherwise, that civilization will start broadcasting their presence through unambiguous artificial signals. 

If the aliens were to receive a signal, they would spring into action, crafting a message of their own or even a probe to visit their new friends.

But all this takes time. A lot of it. We've been broadcasting for less than a century, meaning our "radio bubble" is less than 200 light-years wide, compared with the 100,000-light-year width of the entire Milky Way. So it may take hundreds or thousands of years for our signals to reach an alien civilization. If they respond with a signal of their own, we could get it in another few thousand years — that is, if we see it at all, because we would have to be looking in the right direction at the right time to capture it.

If the aliens decide to send a probe, it will have to crawl along the interstellar depths at a fraction of the speed of light, so it will take even longer to get here.

So maybe we're not alone after all and our galaxy is home to many other advanced civilizations. They just haven't answered our call — yet.

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Paul Sutter
Space.com Contributor

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute in New York City. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy, His research focuses on many diverse topics, from the emptiest regions of the universe to the earliest moments of the Big Bang to the hunt for the first stars. As an "Agent to the Stars," Paul has passionately engaged the public in science outreach for several years. He is the host of the popular "Ask a Spaceman!" podcast, author of "Your Place in the Universe" and "How to Die in Space" and he frequently appears on TV — including on The Weather Channel, for which he serves as Official Space Specialist.

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

    https://forums.space.com/threads/why-have-aliens-never-visited-earth-scientists-have-a-disturbing-answer.55892/
    Reply
  • 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..
    Reply
  • 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.
    Reply
  • 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.
    Reply
  • 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.
    Reply
  • 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
    Reply
  • Helio
    Unclear Engineer said:
    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!". ;)
    Reply
  • Helio
    rod said:
    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.
    Reply
  • Helio
    No doubt, a lot of answers will come as we get closer to a respectable solution to the Drake equation.
    Reply
  • bwana4swahili
    Admin said:
    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!?
    Reply