Oct 5, 2010

365 Days of Astronomy Podcast: Life in the Universe: Odds and Expectations

Hi everyone,

Today (yesterday actually, 4th October) my podcast was transmited worldwide through the  365 Days of Astronomy Podcast website (http://365daysofastronomy.org/). The title is "Life in the Universe: Odds and Expectations".

(!you can listen to it in the link above!)

Hi, I am André Gonçalves from Vieira do Minho, Portugal. Today I will talk about life on Earth and the possibilities of life arising in other worlds far away from ours, as well as the basic conditions a planet must have in order to support life. I will try to reach several topics and make you think, wonder and even speculate about extraterrestrial life.
The most accepted theory for the origin of life on Earth is called “The Primordial Soup Theory” which suggests that life began in an ancient ocean filled with organic compounds and a primitive atmosphere mainly constituted by hydrogen and nitrogen. The heat, the ultraviolet radiation and the abundant lightings triggered chemical reactions that formed amino-acids, the building blocks of life that make up proteins.
RNA is generally assumed to be the earliest self-replicating molecule, which eventually led to the first unicelular organism. Several millions of years later, photosynthesizing cyanobacteria evolved and the concentration of oxygen in Earth’s primitive atmosphere rose. The origin of multicellularity may have occurred from symbiosis of single celled organisms, each with different roles in the colonies.
But these were only the first steps. Then, life exploded on Earth: plants, animals, fungi, etc emerged and evolved for millions of years until today.
Nowadays, our ‘blue planet’ is full of life and millions of different species inhabit our planet. Why Earth is really good for life? Well, there are lots of reasons but I’ll try to focus on the most important ones. Liquid water is obviously an important condition, and almost all the life forms on Earth depend on it. Also, it is a solvent and allows the interaction of organic molecules (in ice the molecules are trapped and can’t interact with each other; in water vapor the molecules are far away from each other and the interaction is difficult). Polarity is another property of water which makes it an “universal solvent”. Water can dissolve salts, acids, sugars, as well as gases. Most cell components including proteins, polysaccharides and DNA dissolve in water making it the basis of life.
Every planetary system has an “habitable zone” where a planet can maintain liquid water on its surface. Earth is within this zone as well as Venus and Mars, but these three planets are very different from each other! This leads us to one of the most important conditions for life: a life-friendly atmosphere. Venus has an extremely dense atmosphere which consists mainly of carbon dioxide, thus the pressure at the planet’s surface is about 92 times that at Earth’s surface and it generates (along with other greenhouse gases) a strong greenhouse effect, creating surface temperatures above 450 ºC. In the other hand, Mars has a thin atmosphere and no greenhouse effect turning the planet in a big cold desert. Because Mars’ mass is only 10% the mass of the Earth, it makes atmosphere retention difficult and the constituent molecules are more likely to be lost to space when buffeted by solar wind. Here on Earth, the atmosphere has just the right amount of CO2, keeping the temperature suitable for life.
Earth’s magnetic field protects us from solar wind and cosmic radiation which damages our cells. It is caused by electric currents in the liquid outer core which combined with the planet’s rotation creates a dynamo responsible for the magnetic field.
Now we know that atmosphere, mass, a liquid outer core and a right amount of planetary rotation are also major conditions for maintaining life.
However, there are organisms capable of living under extreme conditions such as high/cold temperatures, high levels of radiation, high salinity, acidity, etc… These organism are called ‘extremophiles’. This is amazing because it tells us that life can thrive even in planets with harsh conditions and perhaps life is not so rare in the Universe as we may think at first glance.
Most extremophiles are microbes like bacteria and archaens (primitive bacteria-like organisms) and are thought to be the first organisms on Earth and they are the most likely to survive to an extinction caused by a catastrophic event.
Astrobiologists are very interested in extremophiles, as many organisms of this kind are capable of surviving in environments similar to those known to exist on other planets or moons of our Solar System. Some people speculate that Mars may have regions that could harbor communities of extremophiles as well as in the hypothetical subsurface ocean of Europa (a Jupiter’s moon). Titan, Saturn’s largest moon (and the second-largest moon in the Solar System), has a dense atmosphere mainly constituted of nitrogen and trace amounts of methane and other hydrocarbons. Cassini spacecraft and the Huygens probe found evidence of lakes of liquid methane as well as clouds and even rain! There has been some speculation that methane may have a role similar to the importance of water here on Earth and maybe some bacteria could live on Titan’s surface.
Some people believes that life exists throughout the Universe, in asteroids or other small space junk, in a dormant state until it encounters a suitable environment. This is know as the “Panspermia hypothesis”. Panspermia proposes that forms of life like extremophiles become trapped in debris that was ejected into space after a collision between a planet which harbor life and comets or asteroids. This kind of organisms may travel dormant for an extended amount of time before colliding with other planets. If the new planet has suitable conditions, the bacteria becomes active and the process of evolution begins. Of course, the bacteria would have to resist to high pressures, high temperatures, radiation, etc. The mechanisms proposed for panspermia are hypothetical and currently unproven and most scientists remain skeptical about this.
But the hundred million dollar question is: extraterrestrial life exists? Life could have emerged in other planets? The truth is that we never found alien life or even evidence of it, neither in the Solar System nor in exoplanets. But this means that we are alone in the vast Universe? Our little planet is that special and unique for life and there isn’t another planet capable of support life in the big dark ocean out there? I don’t think so.
Why? Well, our galaxy has thousands of millions of stars and there are thousands of millions of galaxies throughout the Universe, so the total number of stars in the cosmos is unimaginable! With the discovery of exoplanets, it is not hard to image that many of those stars have planetary systems which can have planets capable of harbor life. In fact, it would be improbable for life not to exist somewhere in the Universe due to the astronomical numbers of galaxies, stars, planets and moons.
With that in mind, Frank Drake formulated his equation in 1961 as a way to focus on the factors which determine how many intelligent civilizations may exist in our galaxy. Drake’s equation depends on factors such as: the total number of stars in the Milky Way, it depends on the fraction of those stars that have planets, it depends on the average number of planets that can potentially support life per star that has planets, it depends on the fraction of those planets that actually go on to develop life at some point, it depends on the fraction of inhabited planets on which intelligent life emerges, it depends on the fraction of those worlds where intelligent beings evolve to be able to communicate and finally, it depends on the fraction of a planet’s lifetime that is graced with a technological civilization.
I strongly advise you to see episode 12 “The Encyclopaedia Galactica” of the famous TV series “Cosmos” presented and written by Carl Sagan. In that episode, Carl Sagan obtained from Drake’s equation that there are 10 civilizations capable of communicate in the Milky Way.
Of course, no one knows those parameters exactly and they give us only a hint of the number of technological civilizations in our galaxy. But remember that the Universe has lots of galaxies! So, even if the outcome of Drake’s equation is a very small number, that number multiplied by the estimated number of galaxies in the observable Universe will probably yield a considerable number of advanced civilizations in the whole cosmos! And the number of non-intelligent life has to be even greater!
We saw that life is possible outside planet Earth, but how we will find it? The distances are so great that we can’t send spacecrafts or probes to other worlds. But we can try to listen.
There are projects, like SETI, that use radio-telescopes to survey the stars, looking for radio-signals or messages from another intelligent beings. Now, you may be wondering how we will understand their messages, because no one expects that they can speak English or any other human language. But there is a universal language: mathematics. They can send a signal with a series of prime numbers, or the Fibonacci sequence, or something else that any intelligent civilization will understand as a message, clearly distinct of any other natural radio source.
I hope you enjoyed this podcast. Thanks for listening and you can now wonder beyond the horizon, beyond planet Earth.

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