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Thursday, March 24, 2011

Pandora and the Comet Empire

Our third and final candidate in the Alpha Centauri star system is Alpha Centauri B, a K1-class orange dwarf star just a bit smaller than our sun. It also happens to be the star of the moon Pandora in James Cameron's movie Avatar.


Alpha Centauri B is slightly less "sunlike" than Alpha Centauri A. However, computer modeling of accretion patterns from the protoplanetary disks results in an earth-sized rocky planet neatly inside the liquid-water habitable zone of Alpha Centauri B on more modeling runs than not. This is not true of Alpha Centauri A, although it is certainly possible that a planet from another orbit might have been captured into a habitable zone orbit of A.

The first salient question which arises is whether or not a planet within, say, 50% of earth's mass in either direction, actually exists in the habitable zone of Alpha Centauri A or B (or Proxima, for that matter).

We don't know if any such planets exist, yet. We do know what does not exist. There do not appear to be any planets in the Alpha Centauri system larger than five times the earth's mass. No gas giants like Jupiter or Saturn, no ice giants like Uranus and Neptune, around any of the three Alpha Centauri stars. Sorry, Mr Cameron. At the very least, the absence of any giant planets in the system means that the orbits within the habitable zones of each of the three stars are available for earth-sized rocky planets, should they happen to exist.

One possibility that we cannot discount yet is that there are no planets in the Alpha Centauri system at all. Given that each of the three stars are distant enough from each other to have had independent protoplanetary disks, and given the high metallicity of each of the three stars ("metal", to an astronomer, is any element heavier than hydrogen or helium), the simple law of averages would seem to indicate that there are planets around at least one of the Alpha Centauri stars, if not all three. But right now we don't know that for sure, because we do not yet have the means of detecting planets as small as we're hoping to find. NASA had a mission well underway to build and launch an interferometry telescope called SIM for specifically this purpose, but it's funding was cut in 2008 and the program was canceled. Hopefully Russia, China, India, the European Union or some private interest will pick up the ball that the United States has dropped and run with it, because a lot of decision making is going to depend on that information.

But, the best computer modeling we currently have for planetary accretion shows that earth-like planets should have formed around Alpha Centauri B, and Jupiter-like and Neptune-like planets should not have. Observation seems to confirm the modeling regarding giant planets; whether it will also confirm the modeling regarding small, rocky planets, only time will tell. Here is the research of Ji-Wei Xie, Ji-Lin Zhou, and Jian Ge of Nanjing University and University of Florida at Gainesville, respectively, regarding planetary accretion around Alpha Centauri B:

http://arxiv.org/PS_cache/arxiv/pdf/1001/1001.2614v1.pdf

Given that it is possible for earth-sized worlds to exists within the habitable zone of Alpha Centauri B, the next question is, "how habitable"?

Alpha Centauri A and Proxima would likely serve the function of clearing the majority of asteroids and comets out of the way, so that a world around Alpha Centauri B would not be constantly pummeled. On the other hand, at least some of earth's water came from comets, so some cometary activity is probably useful. As it stands, we really don't know how most of the water on earth (or Europa or Enceladus or Mars or...) got there, so a better understanding of the role of cometary hydration would go a long way toward understanding how the unique trinary gravitational profile of the Alpha Centauri system is going to affect the hydrographic percentage of its worlds.

New technology is needed to help us image the Alpha Centauri system before sending even robot probes four and a half light-years away. The new technology is almost ready to implement. More on that, soon. With luck and perseverance we should know within the next few years whether or not there are planetary candidates worthy of further exploration around the stars of Alpha Centauri.

Or, you know, three-meter tall naked blue people.

7 comments:

  1. Cameron's use of Polyphemus (the gas giant planet in Avatar) was stupid. It did nothing to advance the storyline, it was scientifically unsound on many levels, and Lucas had already done it with Yavin.

    Also stupid is the fact that we're even still asking these questions. SIM would have answered many of them. What a loss.

    Do please compile this series into a book, either online or in paper. Charge money for it. You're doing good work here, and your voice and perspective are unique and needed. But it would be awesome to be able to easily find your work here without sifting through twenty pages of Seattle weather reports.

    Make this a book. People will buy it.

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  2. Hello,

    Just wrote a lengthy and well-thought-out response to you comment, and Blogspot ate it when I hit "post comment". Will try again.

    Yes, I will be posting soon about SIM. Did you work on the project? Would love to pick your brain on that, it is a shame that we lost funding for it. I'll also be talking about Jim Kasting's work on habitable exoplanets.

    Regarding the book; yes I'll compile everything into a more usable format once the series is finished. But actually publishing for money doesn't seem likely. First off, most of the illustrations I've used were just ganked from the internet, tracking down their creators for copyright purposes would be a nightmare.

    More to the point, space science is just a hobby for me. I have no credentials whatsoever in this field, I dropped out of college to join the Navy when I was 20. I'm just a boat driver. Nothing in these posts has been in any way original research, most of everything I've written here could be found with a simple Google search. I know enough about positional astronomy to be able to teach a navigation class to another boat driver, but that's about the extent of it. I'm a sailor, not a scientist.

    That said, I greatly appreciate the very positive feedback I've gotten from yourself and others who ARE space scientists. I will admit it scared the hell out of me when I realized that there were real scientists reading my blog, which was really intended for other sailors. But, I trust that when I get things catastrophically wrong you guys will keep me from embarrassing myself too terribly much.

    Okay, had the sense to copy this into a text file this time before posting. Here goes nothing--

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  3. I don't know what's going on with your Blogspot, but it ate my earlier post as well.

    Your experience on ballistic missile nuclear submarines in addition to your knowledge of astronomy is all of the street-cred you need for this. You have hands-on experience with nuclear power, nuclear weapons, medium-lift rocketry and long-term habitation in enclosed environments. Few NASA types can claim the experience you have. You are a scientist. You happen to be an applied scientist rather than a theoretical scientist or a research scientist. I'm an applied scientist as well. No, I didn't work on SIM, that was a JPL project; my work is more in rocket engines.

    Nobody cares that you don't have a PhD. Einstein was a patent clerk. Do you need a doctorate to feel that your voice is worthy? Fine. Stand back from your computer screen, here it comes! ThuuuuuuuhWACK! There, your very own Doctorate of Applied Astrophysics from USC (the University of Space Cowgirl).

    The "real scientists" get what you're doing with this blog. I started reading Strait of Magellan when you were writing about Felisa Wolfe-Simon's work at Mono Lake. Relax. Your posts are well-researched, well-written and entertaining. But they're written by and for the sort of people who will actually be crewing the missions to Mars, Europa and Alpha Centauri. As opposed to the sort of people who sit in a lab all day thinking about missions to Mars, Europa and Alpha Centauri.

    You said it yourself, NASA PhDs are singularly ill-equipped to communicate with the general public. Meaning the taxpayers who fund projects like SIM. And Orion. You provide an information bridge between NASA and the maritime industry, who are precisely the kind of people who will actually volunteer to drive a ship to Mars and beyond and have the temperament and mind-set to accomplish that.

    Teach them to yearn for the vast and endless sea, Captain.

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  4. Saint-Exupéry. Touché.

    For the benefit of anyone else following this thread, the full quote is from Citadelle:

    Quand tu veux construire un bateau, ne commence pas par rassembler du bois, couper des planches et distribuer du travail, mais reveille au sein des hommes le desir de la mer grande et large.

    "If you want to build a ship, don't drum up the men to gather wood, divide the work and give orders. Instead, teach them to yearn for the vast and endless sea."

    I hadn't thought about it, but this series does kind of come down to that. The "ship" is Orion, and the sea in question is more vast and endless than even Saint-Exupéry envisioned. But the meaning is the same.

    As always, thank you for your perspective.

    And for the PhD. Does USC count as one of those "distinguished unaccredited universities" I keep getting spam emails about? ;)

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  5. @ Space Cowgirl--

    Okay, now I feel really stupid. I had forgotten that I had the Saint-Exupéry quote on my profile page.

    Ironically, when I put that there it had nothing to do with space travel. I had recently worked with a group of college students who had restored an old square-rigged sailing ship and were about to sail it to Hawaii and then Fiji. They had basically been given this boat which was about to be scrapped, and they worked their tails off and actually rebuilt the thing.

    Saint-Exupéry seemed singularly appropriate to their endeavor. But their labors have had me thinking a lot about an independent volunteer venture to build Orion, or something like it. I really think it can be done.

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  6. I think it can be done, too. I also think that taking the project away from governments which are beholden to taxpayers and corporations which are beholden to shareholders is critical. This needs to be a long-term, slow-burn project that builds on itself. It won't be cheap, but if many people get involved it won't be exorbitantly expensive either.

    Regarding the Saint-Exupéry, yes, I pulled it from your profile page. I only know The Little Prince, which you've alluded to here before as well. I'd never even heard of Citadelle before now. I did assume the passage was intended to mean your work on Orion, and that promoting Orion was the real not-so-hidden agenda of your blog. It's a good agenda. The survival of our species just might depend on it one day.

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  7. There has been a recent discovery of an Earth size planet orbiting Alpha Centauri B. The European Southern Observatory announced the discovery on Tuesday, October 16, 2012.

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