Where Would Earth-Like Planets Find Water?

astroengine writes "The term 'Earth-like worlds' is a vastly overused and hopelessly incorrect term that is popularly bandied about to explain some recent exoplanet discoveries. Although some of the distant small worlds being discovered by the Kepler space telescope may be of Earth-like size, orbiting their sun-like star in Earth-like orbits, calling those worlds 'Earth-like' gives the impression these alien planets are filled with liquid water. It turns out that we have only a vague idea as to where Earth got its water, and it will take a long time until we have any hint of this life-giving resource on worlds orbiting stars thousands of light-years away."

Wish they would just knock it off with "earth-like

[elrous0]

The whole nonsense of even using the term "earth-like" is a joke, born of the press and PR-minded astronomers. Calling a planet "earth-like" implies way more than correlation with earth's size and it's orbit around the sun. There are so many characteristics which may well make the earth a very unique planet. It's not just the presence of water, either--it's also our magnetic field, the presence and effects of our moon, the nature of our core, etc. It could very well be that true earth-like planets are VERY rare in the universe. Though the shear size of the universe suggests it's likely there are other planets out there like ours and other life out there, it's probably a LONG way to our nearest earth-like neighbor--and likely a much longer way than even that to the nearest planet with similar intelligent life living coincidental with us.

Much as I hate to say it, having grown up on space dreams and science fiction, the more I learn about space the more I've become convinced that, for all intents and purposes, we're basically alone on this little blue ball. When I used to dream otherwise, I really had no real appreciation of just how vast and empty space really is, for one thing. I think the popular perception is that the next solar system begins close to the edge of our own (I certainly thought so when I was a kid watching sci-fi movies). In reality, every solar system is a tiny isolated island in a giant lonely ocean. A space probe that takes 9 years to go from earth to Pluto would take over 100,000 years to get to even our closest neighbor, a mere 4.2 light years away. And that's in a universe that's 15 *billion* light years across. It's a big place, with an unimaginable number of other planets. But mostly it's just a giant, empty void.

So there are probably indeed other earth-like planets out there. But barring some incredible technological advances (probably thousands of years worth) and a complete overthrow of Einsteinian physics, no human is ever going to see them or even be able to communicate with them.

This is usually the part where I make a joke, but somehow I just feel lonely and sad now.

Re:Easy

[Marxist Hacker 42]

That was my response as well. Whereever Oxygen and Hydrogen exist, the problem is NOT creating water. In fact, it's very likely that the largest source of water outside of the Earth in our Solar System is orbiting Saturn.

Re:Alien life would need water?

[Zocalo]

It's not so "presumed" as it is believed to be the most likely basis for complex, multi-cellular, life by a considerable margin due to carbon's versatility in forming the huge number of chemical forms with other elements that necessary for the required biological processes. That said, it's definitely not the only option [wikipedia.org], silicon, nitrogen and phosphorous based biochemisties all being seen as theorerically viable, although silicon is most often seen as the most likely alternative. Here's a (somewhat old) link to Lou Allamandola, an NAI astrobiologist, discussing [nasa.gov] the various merits of silicon- versus carbon-based life.

Re:Alien life would need water?

[v1]

Once life gets going and has managed to develop evolutionary mechanisms such as sex and dna, (neither of which are specifically required) life tends to become highly adaptable and resilient to changing conditions. The problem is getting there in the first place. That first "spark of life" collection of molecules that can reproduce has to happen from an incredibly good stroke of luck.

The odds of that incredibly rare event happening are made possible by and improved on by favorable conditions. Liquid water, atmosphere, a water cycle, abundant energy, and a magnetic field are all part of that "thumb on the wheel", improving the odds of genesis occurring here on earth.

But they're not required. The only thing that is probably actually required is a liquid cycle of some sort, to provide a circulation of materials because original life was almost certainly not capable of locomotion, and an abundant source of energy. I've read several papers on a plausible genesis based on a liquid methane cycle.

Several conditions on earth are probably not even optimal. The low temperature and pressure of our atmosphere for example - someplace more like Venus has an edge on Earth in that respect. Part of why people tend to think of water/carbon as necessary is they are assuming earth's low pressure and temperature. Molecules get a lot more flexible under those different conditions. If you have "water tunnel-vision" you may completely discount a place like venus where liquid water can't really exist in any quantity.

I think it's fair to argue that some combination of a liquid cycle where the liquid is at a reactive temperature and pressure are probably almost required for genesis. I hesitate to flat out say "required" because a sufficiently lucky turn of events can lead to genesis even in the most apparently unfavorable conditions imaginable. But we can't really get anything accomplished unless we set some constraints on things and try to look at more "reasonable" scenarios. Even though the number of exoplanets in existence is nearly infinite for our practical purposes, it is a finite number, and odds must come into play. Just because there's a ton of planets out there doesn't mean a bunch of them have life. Without any control point of reference it's hard to argue that even just earth in the universe having life was anything but a stroke of incredible luck. We're probably a lot more special than any of us can possibly imagine.