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Shortlist of Possible ET Addresses
An anonymous reader writes "Yahoo News is reporting that Astronomer Margaret Turnbull of the Carnegie Institution has released a 'top 10' list of potential inhabitable star systems. NASA is planning on using this top 10 list as the targets for their Terrestrial Planet Finder a 'system of two orbiting observatories scheduled for launch by 2020.'"
The Terrestrial Planet Finder has been cancelled:2
http://www.spaceref.com/news/viewnews.html?id=109
So this list seems redundant. To bad, as it was NASA's most exciting project IMO.
But there is still ESA's Darwin, an essentially identical project, which is still scheduled for a 2015 launch as far as I am aware.
As Michael Griffin explains in Griffin Builds Hopes For Terrestrial Planet Finder And Hubble Rescue Missions.
The short reason is that the Crew Exploration Vehicle takes priority.
Here or here, a very nice article on the project, "Margaret Turnbull and Jill Tarter have a new list, called HabCat: A Catalog of Nearby Habitable Stellar Systems." (2003) Interview included.
Interesting that starting with the Hipparcos catalog of 120,000 stars and skipping all with major problems for life ("cataclysmic, eruptive, pulsating, rotating, or X-ray stars", low metal content systems, rotating too fast or too much UV or bad size or composition), left 1 star in 6 still potential life bearers.
Wiki on HabCat and Turnbull. The Turnbull page has a link to a PDF, which is a very interesting scientific paper about how the list of habitable stars was made.
Wiki article on the Terrestrial Planet Finder, which uses Turnbull's list of 5000 candidates within a 100 light year radius. List of Top 100 candidates. Note 18 Scorpii at 46 light years is number 62 in the list, and 37 Geminorum is not listed.
The highest ranked 2 candidates in that list are just 4 ly away from Earth, at Rigil Kentaurus, and then Tau Ceti at 12 ly. There is one at 3 ly and some others at 19, 20, 24 ly too.
Allen Telescope Array
Turnbull's top 10 list includes 51 Pegasus, where in 1995 Swiss astronomers spotted the first planet outside our solar system, a Jupiter-like giant.
Others include 18 Sco in the Scorpio constellation, which is very similar to our own sun; epsilon Indi A, a star one-tenth as bright as the sun; and alpha Centauri B, part of the closest solar system to our own.
That's what I'm saying. I'd be highly suprised if alien life used precisely the same biochemistry, however I'd be equally suprised if it didn't use nucleic acids, amino/acid/proteins, sugars/polycarbohydrates and lipids. These grouping are too useful and easily available for them not to be used.
We can even reasonably be a bit more precise about it. With proteins of the 20 amino acids in prime use a good dozen of them could be expected to turn up in an alien biochemistry just because they're the simplest that do the job. With the carbohydrates many are also a dead certainty - glucose, fructose etc. and polymers like chitin are certain to be just as useful to alien biochemistry as they are to ours. On the lipid front, wll lipids are lipids and our biochemistry uses just about everyone going anyway so there's certain to be major overlap.
Nucleic acids are more interesting though. I'd lay a bet on RNA just because the ribonucelic acids tend to form easily in prebiotic conditions. DNA is more suspect, particularly as life here can get along without it just fine. Nevertheless it's the next simplest step up from RNA so may be favoured against other varients. Of course which nucelic acids are actually used is open to chance, although it's noticable that the ones we have are among the simplest.
Beyond these broad categories though indeed it gets more speculative. Even so, some assumptions seem probable. For example if there is RNA/DNA then a triplet genetic code is likely, because as has been observed, a doublet code doesn't give you enough combinations to work with (but there is evidence that our early genetic code was doublet and we evolved the triplet later) whereas a quad code would be inefficient needing 33% extra DNA to code and more error prone.
Other things that might also be expected to turn up. For example porphorins (the building block of haem, chlorophyll and many other useful molecules).
Unfortunatly I guess we'll never know, unless we strike lucky on Mars or Europa.