An exoplanet seen from its moon (artist's impression). Via the IAU.

Bayesian Analysis Rains On Exoplanet Life Parade

Article written: 26 Apr , 2016
Updated: 5 May , 2016
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Is there life on other planets, somewhere in this enormous Universe? That’s probably the most compelling question we can ask. A lot of space science and space missions are pointed directly at that question.

The Kepler mission is designed to find exoplanets, which are planets orbiting other stars. More specifically, its aim is to find planets situated in the habitable zone around their star. And it’s done so. The Kepler mission has found 297 confirmed and candidate planets that are likely in the habitable zone of their star, and it’s only looked at a tiny patch of the sky.

But we don’t know if any of them harbour life, or if Mars ever did, or if anywhere ever did. We just don’t know. But since the question of life elsewhere in the Universe is so compelling, it’s driven people with intellectual curiosity to try and compute the likelihood of life on other planets.

One of the main ways people have tried to understand if life is prevalent in the Universe is through the Drake Equation, named after Dr. Frank Drake. He tried to come up with a way to compute the probability of the existence of other civilizations. The Drake Equation is a mainstay of the conversation around the existence of life in the Universe.

The Drake Equation is a way to calculate the probability of extraterrestrial civilizations in the Milky Way that were technologically advanced to communicate. When it was created in 1961, Drake himself explained that it was really just a way of starting a conversation about extraterrestrial civilizations, rather than a definitive calculation. Still, the equation is the starting point for a lot of conversations.

But the problem with the Drake equation, and with all of our attempts to understand the likelihood of life starting on other planets, is that we only have the Earth to go by. It seems like life on Earth started pretty early, and has been around for a long time. With that in mind, people have looked out into the Universe, estimated the number of planets in habitable zones, and concluded that life must be present, and even plentiful, in the Universe.

But we really only know two things: First, life on Earth began a few hundred million years after the planet was formed, when it was sufficiently cool and when there was liquid water. The second thing that we know is that a few billions of years after life started, creatures appeared which were sufficiently intelligent enough to wonder about life.

In 2012, two scientists published a paper which reminded us of this fact. David Spiegel, from Princeton University, and Edwin Turner, from the University of Tokyo, conducted what’s called a Bayesian analysis on how our understanding of the early emergence of life on Earth affects our understanding of the existence of life elsewhere.

A Bayesian analysis is a complicated matter for non-specialists, but in this paper it’s used to separate out the influence of data, and the influence of our prior beliefs, when estimating the probability of life on other worlds. What the two researchers concluded is that our prior beliefs about the existence of life elsewhere have a large effect on any probabilistic conclusions we make about life elsewhere. As the authors say in the paper, “Life arose on Earth sometime in the first few hundred million years after the young planet had cooled to the point that it could support water-based organisms on its surface. The early emergence of life on Earth has been taken as evidence that the probability of abiogenesis is high, if starting from young-Earth-like conditions.”

A key part of all this is that life may have had a head start on Earth. Since then, it’s taken about 3.5 billion years for creatures to evolve to the point where they can think about such things. So this is where we find ourselves; looking out into the Universe and searching and wondering. But it’s possible that life may take a lot longer to get going on other worlds. We just don’t know, but many of the guesses have assumed that abiogenesis on Earth is standard for other planets.

What it all boils down to, is that we only have one data point, which is life on Earth. And from that point, we have extrapolated outward, concluding hopefully that life is plentiful, and we will eventually find it. We’re certainly getting better at finding locations that should be suitable for life to arise.

What’s maddening about it all is that we just don’t know. We keep looking and searching, and developing technology to find habitable planets and identify bio-markers for life, but until we actually find life elsewhere, we still only have one data point: Earth. But Earth might be exceptional.

As Spiegel and Turner say in the conclusion of their paper, ” In short, if we should find evidence of life that arose wholly idependently of us – either via astronomical searches that reveal life on another planet or via geological and biological studies that find evidence of life on Earth with a different origin from us – we would have considerably stronger grounds to conclude that life is probably common in our galaxy.”

With our growing understanding of Mars, and with missions like the James Webb Space Telescope, we may one day soon have one more data point with which we can refine our probabilistic understanding of other life in the Universe.

Or, there could be a sadder outcome. Maybe life on Earth will perish before we ever find another living microbe on any other world.

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9 Responses

  1. BlackWolfStanding says

    I’ve stated this before, our galaxy is a big place. Even if the first radio signal could be detected, it still wouldn’t have been detected by the majority of our own Galaxy. If another advanced civilization was just 1/10th the radius of our own Galaxy away from us, they still would think the Earth was uninhabited. Which means if they are just as advanced as us, we would think their planet was uninhabited.

    We had a lot of set backs on life here on Earth. one nearly ended all life. Yet, those same set backs are responsible for human life. Maybe other planets have various degrees of set backs. Maybe life was going to be intelligent and then a comet killed that life off. Maybe a planet had life but not advanced because the planet never had any set backs.

    Life is probably plentiful in the Universe. Even Intelligent life. But in the end, the Universe is an awful big place.

  2. Member

    Wow… you ever see the Peter Fonda movie, Dirty Mary, Crazy Larry? It’s basically one of those feature length 70’s car chase flicks that follow two ex NASCAR racers and a token female as they rob a small town grocery store in the hopes of using the score to fund their dream of having another racing team together… Anyway, long story short, we follow them for 90 minutes through ups and down as they play cat and mouse with a local Sheriff until they finally get to a huge orchard complex that has many roads winding through it. Once inside, they’re basically scott free as there’s something like 28 different ways out. Pretty much impossible to track. After some bumps in the road they finally get to where they know they’ve eluded the final effort of the Sheriff to thwart their dream funding scheme. As they’re celebrating with cheers, flying down the road… they cross a railroad crossing JUST as a train races through… DEAD. END. Out-of-freaking-NO-where! This story reminds me of that movie.

    “Maybe life on Earth will perish before we ever find another living microbe on any other world.”

    http://www.sadtrombone.com/

  3. sangos says

    Too many ifs and buts. Maybe a simple child like reasoning would uncomplicate matters. Ok it is known that the distances between stars are humongous and also we do not have the technology yet of enough resolution to peer at exoplanets. So essentially we are like our ancestors who did not know if there were people on the next far off continent. And people keep harping on Fermi’s paradox as if aliens are supposed to walk by our front street huh!? Even a child can see through that one.

    • Ja says

      Hi, i believe we should not get so pessimistic. Rome was not built in a day, and we have just begun to acquire the technology which enables us to actively search for life, and the main thing is; as someone said, the galaxy is a damn big place. Just looking in the wrong direction at any one time is like a stranded sailor keeping watch for a boat to the south, when a ship passes him by from the north. So give us time and don’t be so defeatist about the chances of humanity surviving. We will.

    • jasonalpha says

      very well put.

  4. jasonalpha says

    Hi, i believe we should not get so pessimistic. Rome was not built in a day, and we have just begun to acquire the technology which enables us to actively search for life, and the main thing is; as someone said, the galaxy is a damn big place. Just looking in the wrong direction at any one time is like a stranded sailor keeping watch for a boat to the south, when a ship passes him by from the north. So give us time and don’t be so defeatist about the chances of humanity surviving. We will.

  5. mortimer zilchcMy says

    life on Earth is too unlikely for us to assume there would be life out there. If life on Earth weren’t so unlikely it would be more reasonable to assume it would be out there too. But it is absurdly unlikely – a total freak happenstance of near impossibles. It’s more likely we are the only ones, the fertile ovum.

  6. mortimer zilch says

    life on Earth is too unlikely for us to assume there would be life out there. If life on Earth weren’t so unlikely it would be more reasonable to assume it would be out there too. But it is absurdly unlikely – a total freak happenstance of near impossibles. It’s more likely we are the only ones, the fertile ovum.

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