Scientists are supposed to have extrapolated that in the Milky Way alone there are billions of exoplanets (planets outside of our solar system) that exist in habitable zones (in temperatures where liquid water is sustainable). The implication, of course, is that in our galaxy alone there are billions of exoplanets that might support life. Per the article: 


....By analysing these planetary systems, researchers from the Australian National University and the Niels Bohr Institute in Copenhagen have calculated the probability for the number of stars in the Milky Way that might have planets in the habitable zone. The calculations show that billions of the stars in the Milky Way will have one to three planets in the habitable zone, where there is the potential for liquid water and where life could exist....The results are published in the scientific journal, Monthly Notices of the Royal Astronomical Society.

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The problem is getting to one of them.  Maybe most of them do have life, and they are out there looking at us with the same thoughts.

Can anybody remember the drake equation? Astronomers put the bar so high for the chances of extra-terrestrial life it seemed a proverbial miracle it ever got going on earth. Cosmologists used to cite everything from the ratio of the various elements to the axial tilt of the earth to explain the unlikelihood of even microbial life. Everything changed with the confirmation of earth- like planets orbiting stars, all the old scepticism disappearing overnight, it's almost as though all that's needed now is to pop over and say hello. We're probably miles out, future generations will grant us a sympathetic ' how were they to know'?.  

I have a very vague recollection of the Drake Equation. Anyway I looked it up in Wikipedia and learned that there was a lot of criticism of it. According to Wikipedia:

"The Drake equation has proved controversial since several of its factors are currently unknown, and estimates of their values span a very wide range. This has led critics to label the equation a guesstimate, or even meaningless." 

I don't think we know enough about what causes life to estimate what the probability is of it occurring on a habitable exoplanet. Right now what we do know is that there appears to be billions of exoplanets in our galaxy that might be habitable.

Even if some habitable exoplanets do not support life it would be good to learn about them because if humankind survives long enough some people might have to be evacuated to them (if possible) due to lack of resources here and contamination of our environment. This will be especially true if science discovers ways to very significantly extend our lifespans as this might result in an overpopulation of the planet.

That's the impression that I've always gotten about the Drake Equation.  What's the point of estimating the odds of something, when you could be off by a couple orders of magnitude on one variable alone.

Although extremophiles are thought to possibly exist on mars or Europa, orbital distance, or the goldilocks  zone as it's called, is probably first base if your searching for ET's. I didn't know there were so many exo-planets in the habitable zone, a couple of years ago you could count them on one hand. The one planet we know is smack dab in the goldilocks zone harbours life, so if it happens once, as they say, it's compulsory. 

We look for carbon based life supported by water on exoplanets. Is some other form of life possible?

I can remember about 15 years ago when we were all wondering if we'd ever get the technology to detect inter- Galactic planets. Now it all seems so run of the mill.

The reason why few people consider other bases for life is probably that were carbon chauvinists. There has to be chances of other life forms.

From the scientists I've heard from, carbon is by far the most probable because It can combine with so many things.  

Yup, carbon is the molecular slut.  Silicon is too, but carbon is about 7 times as common per unit of mass, 16 times as common in terms of atom count.

Thermodynamics, Disequilibrium and Evolution (TDE)
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Co-Chairs: Laura M. Barge (Jet Propulsion Laboratory, USA) and Eugenio Simoncini (Max Planck Institute, Jena, Germany)

The TDE Focus Group is intended to bridge the gap between researchers working on the theory and experimental aspects of the Origin of Life and astronomers and remote sensing researchers planning future space missions and deciding on future targets for the search for extraterrestrial life. It will concentrate on the entropy and energy requirements for life and planets and how they inform our selection of potentially habitable planets and environments in the cosmos.

Earth displays abundant life that uses solar and chemical energy, the latter partly produced from geothermal energy. The convective mass transfer of heat driven by one or more of these forms of energy, from the very core of our planet through to the upper atmosphere, eventually conduces the interfacing of the chemical tensions appropriate to the nurturing of life. Disequilibrium on a cellular scale is made possible by the cell membrane, which maintains redox and pH gradients that are put to work by the cell. On a supra-cellular scale disequilibrium conditions are created most visibly by colony-formers such as stromatolites and corals, which are multi-cellular (often symbiotic) aggregates on a local scale. On a planetary scale, biological processes such as photosynthesis can establish and maintain disequilibrium conditions. With this understanding of the fundamental character of life – that it depends upon disequilibrium in the environment as a source of energy, that it maintains a state of disequilibrium as a condition of life, and that it can mark its existence in an environment through the presence of disequilibrium – the TDE Focus Group seeks to integrate the astrobiology community around this paradigm to inform our search for life in the universe.


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