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How do we define 'Earth like planet'

  1. Dec 9, 2011 #1
    And not just define but be able to detect the essential components of our definition.

    I find sentences like the above quoted from here, to be annoying because from what I understand it is misleading and trivialises science for the general populace who on the whole have no conception of distances as it is, never mind about the physics of planets.

    I do not think that any reasonable definition of an Earth like planet would qualify for the one referred to in the quote.

    The article even mentions a three essential characteristics none of which were detected. In fact the planet is not even known to be solid.

    As one of the essential characteristics for an Earth like planet must surely be protection from deadly cosmic rays, a liquid metal core would seem to be a must have item.

    Therefore while we can detect a slight dimming as a planet transits a relatively nearby star, would it ever be possible in the foreseeable future for us to be able to detect whether one of these transit events that are hundreds or thousands of light years away, has a liquid metal core?

    It doesn't seem like it to me but I could be wrong.

    and this quote here...

    That is also not true. How has it been shown?
  2. jcsd
  3. Dec 9, 2011 #2


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    Well, as I recall, an estimate that is considered VERY conservative puts the number of planets in a galaxy way up in the tens of millions (if not billions) and in the observable universe there are billions of galaxies, so this comes out to something like way more than 10E15 planets.

    You reckon ours is the only one of those that's habitable?
  4. Dec 9, 2011 #3
    That's just a part of Drake's equation which can give anything from Billions or one chance in 10 (Muller: physic for future presidents).

    You have avoided my question. You have not given me a definition of 'habitable' so your question to me makes no sense.

    If we go by the definition as 'Earthlike' which is suggested from the quote, then my consternation is how are they able to say that atronomers have confirmed

    We can confirm that there are stars that are capable of having Earthlike planets. But we have not even the slightest hint that there are planets out there that would be habitable. In fact the evidence in our own Solar system shows that all the planets and moons are so very different and are highly unhabitable, every last one of them bar the apparently unique Earth

    You see the essence of my complaint is that the article suggests that astronomers can actually confirm an Earthlike planet exists. Obviously it's possible but being possible is very different to 'confirming', then I'd like to know how they intend to confirm these so called habitable planets.

    I don't think it's very scientific to say, "gee whiz there's so many stars and it's all so very big, how can there not be ...(fill in the blank)
  5. Dec 9, 2011 #4


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    SETI defines 'habitable' as a planet orbiting at a distance that permits liquid water to exist at its surface. That appears to be reasonable. I fail to see how there cannot be many such planets in our own galaxy, much less the universe.
  6. Dec 10, 2011 #5
    What other things are we able to detect about planets orbiting the right star, that confirms that there would be liquid water and a solid surface.

    Would a planet without an atmosphere or a planet without a magnetosphere be considered habitable, even if it was in the habitable zone?
  7. Dec 10, 2011 #6


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    My bet is that a very significant fraction of stars (as in, over 10%) have conceivably habitable planets. Hopefully we'll be able to do some real statistics on this sort of thing within 5-10 years.
  8. Dec 10, 2011 #7
    Habitable by whom? Humans? Intelligent life other than humans? Any life?

    It's a pointless bet because we aren't going to know about any planets outside the Galaxy, and I can't see how we are going to get to know enough information about any planets that are more than a hundred light years distant.

    So if we cannot ascertain for sure that a planet is definitely habitable within our realm of examinability, then we have no business extrapolating our imaginary worlds over the rest of the Galaxy, never mind about the rest of the Universe.

    I mean how could we tell that a planet 100 light years away that is in the so called habitable zone, even has a solid surface, or a magnetosphere let alone liquid water.

    Why talk about billions of galaxies. It's not like in the 25th Century we are going to have the technology for a round trip to the Andromeda galaxy, seeing as it's a 10 million year journey at half the speed of light. Don't wait up.

    Even in our own Galaxy, you can rule out 2/3rds of all the stars because there ain't gonna be any habitable planets around a binary star system. Then you have to eliminate all the unsuitable stars that are left from the remaining 1/3. Then just forget about all the stars that we do not have the technology to investigate to the point where we can probe it sufficiently to make these highly misleading claims.
    Last edited: Dec 10, 2011
  9. Dec 10, 2011 #8


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    As in roughly earth-sized and within the habitable zone of the star. Upon detailed examination, only a fraction of these planets will be genuinely habitable by carbon-based life. Most life out there is probably going to be microbial in nature.

    As for alternative life chemistries, nobody knows if they're possible or not. So best, for the moment, to focus on the life chemistry we know works: carbon-based.
  10. Dec 10, 2011 #9
    We've gone off the Earth like track, which I should have picked up earlier. OK habitable then means in the habitable zone but habitable zone is a bit of a misnomer because there is a sort of sly implication that a planet in the habitable zone has a good chance of being a habitable planet, which is very far from the case.

    I'd like to get right back on topic as per the thread title, the article I link to talks about Earth-like planets. Earth like is a very broad spectrum description but they must surely mean about the same size and density of Earth and certainly with a magnetosphere.

    So my complaint is how can they possibly give the impression that they are able to confirm an Earth like planet (that is the words they use) , when clearly they cannot confirm an Earth like planet. Are astronomers really able to tell that a star that shows some darkening reveals a planet that has a liquid iron core? I think not.
  11. Dec 10, 2011 #10


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    Instead we'd be looking for chemical signs of life. Basically, we'd be looking for out-of-equilibrium chemistry. Here on Earth, for instance, that means O2 in the atmosphere.
  12. Dec 10, 2011 #11

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    That is your fault. You are the one who focused in on "habitable" in post #3 where you said
    You have avoided my question. You have not given me a definition of 'habitable' so your question to me makes no sense.

    I suggest you drop the attitude you are displaying in this thread.

    More or less the same size: Correct.
    More or less the same density: Also correct.
    Magnetosphere: Incorrect.

    An Earth-like planet is a terrestrial planet, aka a rocky planet, that is indeed a "planet". This rules out gas giants, dwarf planets, and asteroids. What you are missing is that just because an Earth-like planet has been found in a star's habitable zone does not mean it harbors life.
  13. Dec 16, 2011 #12
    I agree with Chalnoth's analysis and views here.

    I have wondered from time to time if it would ever be possible to construct a space telescope with enough resolution to see these planets directly or if there is simply no signal left above their stars interference to be able to resolve them?
    Last edited: Dec 16, 2011
  14. Dec 16, 2011 #13
    Wouldn't an 'Earthlike' planet also include a relatively and apparently rare oversize moon, as well as a big Jupiter like sweeper planet to clear up the inevitable debris. Not to mention nice almost circular orbits so they don't muck each other up. All of these are necessary to give life the hundreds of millions of years of stability that it requires.

    And look how we got our moon, a real throw of the dice that one. Does anyone think we'd be here now reading this forum if not for the precise and fortuitous crash that caused the moon to be blown into a stabilising orbit.

    In short, could we reasonably be able to detect a real Earth like planet, with our current technology?

    'Earthlike' meaning either somewhere close enough to colonise, or one we cannot colonise but at least can pretend that we are not alone in the cosmos.
  15. Dec 16, 2011 #14

    What a nice post. It is nice to see intelligent life forms in this planet.

    Just kidding, I agree 100%.

    Is Earth a unique planet?
  16. Dec 16, 2011 #15

    D H

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    No, no, and no.

    You are reading far, far much into the term "Earth-like". As far as astronomers are concerned, Venus, the Earth, and Mars are all Earth-like planets. Think of Earth-like as meaning that life (life as we know it) might be possible. Might be possible, not is. It is not a guarantee.

    As far as a large Moon, a magnetosphere, a big Jupiter, and all of those other arguments that rare Earth proponents use: We don't know if those features are critical for life. Those who espouse this point of view are extrapolating from a sample size of one. They same is true for those who argue that life is common. We don't know. Extrapolating from a sample size of one is meaningless.

    What the Kepler mission has done is to give us some candidate Earth-like planets that we can look at in more detail.
  17. Dec 16, 2011 #16

    Ok, I'll accept that to non lay people then 'Earth like' has a technical meaning which is correct within their definition. However when reported in the popular press, to the lay person 'Earthlike' popularly means the sort of place that Captain Kirk always seems to find. You know rivers, streams plants, nicely breathable atmosphere.

    Recently I tried to explain to someone who brought the matter up, how big space is, and how long it would take just to barely go anywhere. He said, 'but what about wormholes', in a triumphant 'don't think I don't know about science' sort of way. As if wormholes are like signposted expressways throughout the universe just waiting for a warp drive ship to come through the toll gates. Or by golly, we'll just build one.

    Science projects like the LHC recognise their duty of care to try and make their science accessible and explain what they are doing (even if most people could care less)

    DH, don't you think it odd to call Venus, probably the most inhospitable planet in this neck of the woods, 'Earthlike'. Surely 'Earthlike' cannot but convey something very specific to the average lay person and to anyone else there must be a better, more accurate term. Maybe we need to invent a new word.

    All I can say is that I'm glad I don't live on an Earth like planet [wink]
  18. Dec 16, 2011 #17


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    What's your point?

    I don't know about DH, but I don't agree at all. Anyone with just an inkling of curiosity can ask what it means and get a specific answer. People aren't separated into "The average lay person" and "Everyone else". Within my group of friends, acquaintances, and co-workers there are many different personalities and aptitudes. Some will understand, some will not. Any way you put it in a simple news article is going to grossly simplify it and leave out the technical details. It is up to the individual to get more information if they want to.
  19. Dec 16, 2011 #18
    My point is that physics and cosmology are utterly astounding and it's not beyond the reach of almost everyone to understand. The facts are more astounding than pop sci, that's all.

    In answer to the second point I think that the planet referred to was not even known to be solid, therefore it was simply incorrect to refer to it as Earth like even by the scientific definitions, and we are talking about the BBC not some hokey news organisation.
  20. Dec 16, 2011 #19


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    Did you mean they are or aren't beyond the reach of almost everyone? This doesn't seem to make any sense the way it is written unless I'm misunderstanding you.

    Perhaps. It all depends on what is meant by "Earth-like".
  21. Mar 30, 2012 #20
    An interesting small article suggesting Billions of planets with life:

    Any thoughts?

    http://news.yahoo.com/billions-planets-life-074000332.html [Broken]
    Last edited by a moderator: May 5, 2017
  22. Mar 30, 2012 #21


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    Well, the title says that. I don't think the article suggests it.

    Nevertheless, it is entirely conceivable. I don't think anybody knows how likely it is, as we don't know how common life is, but it isn't unreasonable that there are a billions of planets holding life within our own galaxy.
    Last edited by a moderator: May 5, 2017
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