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Thread: Can an Earth-like planet be bigger than Earth itself

  1. #1
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    Can an Earth-like planet be bigger than Earth itself

    Can an Earth-like planet be twice or more as big as Earth itself

  2. #2
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    I don't why Earth is the maximum size for habitable worlds. A larger planet would have more gravity, so we might have problems adapting. Or it could have a lower density and maintain one gee. Dunno. I've often wondered this myself...

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    Quote Originally Posted by ChesleyFan
    A larger planet would have more gravity
    so the creatures on those planets would be more bulkier but smaller right?

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    Far from an expert on this, but my understanding is that Earth is habitable more due to its distance from the Sun than its size. Although I think the size is related to the Earth's ability to maintain the atmosphere.

    However, we're assuming that we're discussing life as we know it. On this planet alone, life has shown a tremendous ability to survive in conditions that we would deem completely inhospitable. The only constant that I'm aware of is the existence of water. Given enough protection from radiation, I would think that life would be possible on just about any size planet.

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    Well what exactly do you mean by twice the size? do you mean twice the diameter or twice the volume? or something else? What type of mass would be added? Is it possible to increase the mass simply by adding water ocean?

    When we talk about earth-like planets for supporting life, perhaps we should bear in mind that earth is not a single planet, but the larger of a binary system. The moon is a major factor in earth's climate, and therefore, related to life as we know it.

    I've often wondered how stable an earth size planet would be if orbiting a larger planet like jupiter. How about orbing a binary star, or locked in a lagrange point of a binary star, or star & large planet.

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    Also not an expert but I would guesstimate that there is indeed an upper limit on the size (diameter) of a world that is habitable as we know it. Humans may not be able to live on a planet too much larger(more 2x to 5x) but that doesn't mean nothing can.

    Edit: My 2x to 5x numbers are guess's.

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    The Earth might be unusually dense for a planet of this size; this is due to the collision event which created the Moon.
    During that event the Moon was formed from the lighter crust of the Earth; the Earth is therefore formed from the denser remnant ,with proportionately more core material (iron, nickel) making it more dense.

    A planet such as Earth which maintained the same density as the early Earth could be considerably larger, and still have an Earth-like gravity because of the lower density.

    It might be the case that such a world would have more water on its surface than our Earth, because much water may have been lost on our world when we lost our crust to form the Moon.

    Alternately if our would was large and wet it might have lost water through u-v dissociation at some point in its history bfore life formed; once ozone was present in the atmosphere a biosphere could form.

    So we could end up with a planet with as much as twice Earth's surface area, yet a similar level of gravity to Earth;
    depending on the impact history and distance from the local star it could be covered in a deep ocean, be Earth-like, or even be a dry world like Mars.

    Until we start looking with extremely long baseline telescopes or actually sending probes, we won't know for sure.

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    That's true I guess. :-? I think though that anything with the right chemicals and temperature to do whatever it was that created the first organisms has a fair chance of creating life. It'd be different years later when that life is trying to colonise the land perhaps but I don't see why cells couldn't form in the ocean at any size really.

    Of course, then we have questions like "What is a "fair chance" exactly?" I wonder if there's anything neat on Europa.

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    I think the current main theory on planetary formation states that the distance to the star has to be below some certain limit for terrestrial planets to form. The ideal is that heavy materials should be able to condense into particles, while lighter materials should be too hot for this to happen. Since (as usual) we only have one planetary system to observe, the truthfulness of this theory is somewhat doubtful.

    The other main factor for a habitable planet is its size (I think that the escape velocity is the key figure). Too big, and you get a surface gravity that will attract lighter gases, producing a too-thick atmosphere; too small, and the gases necessary for life won't stay.

    To illustrate, it might be possible to terraform Mars or the Moon, but we'd have to replenish the atmosphere every now and then to compensate for losses.

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    If we are talking "Earth like" as in it has liquid water I can imagine life existing in quite high gravity. Life here evolved to 1G but the small stuff life evolved from arn't that effected by gravity so it can start in quite high G and just evolved to adapt.
    Rather than bulky creatures I would imagine it would be light ones that would flourish. Animals that have efficent power to weight ratios like ants etc.

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    In his book "A Scientist at the Seashore", physicist James Trefil states that if the Earth's mass were 10 percent greater, it would have released so much carbon dioxide during its first few hundred million years that a serious greenhouse effect would have been the result.

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    thanks for the replys

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    Quote Originally Posted by chaotica
    In his book "A Scientist at the Seashore", physicist James Trefil states that if the Earth's mass were 10 percent greater, it would have released so much carbon dioxide during its first few hundred million years that a serious greenhouse effect would have been the result.
    And if it wa a little farther out from the Sun, it would have needed the extra warming!

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    Quote Originally Posted by chaotica
    In his book "A Scientist at the Seashore", physicist James Trefil states that if the Earth's mass were 10 percent greater, it would have released so much carbon dioxide during its first few hundred million years that a serious greenhouse effect would have been the result.
    If the earth had a different density would that also be a factor in the greenhouse effect?

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    Yes, sometimes; the lower density might mean an abundance of carbon in the crust of the planet, leading to the possibility of a thick CO2 atmosphere;
    (here is an imaginary planet of mine fitting that description; it was similar to Venus in many ways, although quite a bit larger, until terraformed).

    Other planets might have thicker silicon crusts than our world, so might develop an earth-like biosphere spontaneously. However Volcanism and plate tectonics might be less active on such a world, leading to many obvious differences to Earth conditions.

    In fact I am of the opinion that few worlds will have exactly the same surface conditions as the Earth, even ones of the same approximate size.

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    The Kepler Mission has assigned some arbitrary parameters for what it considers Earth-like parameters. The following info describes the mission's limits that it gives for Earth-like planets as well as the maximum limits for terrestrial planets as a whole (from SolStation.com):
    The Kepler Mission is defining the size of an Earth-type planet to be those having between 0.5 and 2.0 times Earth's mass, or those having between 0.8 and 1.3 times Earth's radius or diameter. The mission will also investigate larger terrestrial planets that have 2 to 10 Earth masses, or 1.3 to 2.2 times its radius/diameter [note from ZS: a terrestrial planet with 2 times Earth's mass and 2.2 times its diameter would have about two times Earth's surface gravity]. However, larger planets will be excluded because they may have sufficient gravity to attract a massive hydrogen-helium atmosphere like the gas giants (Jupiter, Saturn, Uranus, and Neptune). On the other hand, those planets -- like Mars or Mercury -- that have less than half the Earth's mass and are located in or near their star's habitable zone may lose their initial life-supporting atmosphere because of low gravity and/or the lack of plate tectonics needed to recycle heat-retaining carbon dioxide gas back into the atmosphere (see Kasting et al).
    However, I think that the range of Earth-like planets might be a bit wider than what the Kepler Mission assigns. After all, when it comes to terrestrial planets of suitable size within a solar star's habitable zone, we have only 3 planets to study--Earth, Venus, and Mars--with us being the only planet known has life (Of course, there could also be life on Europa). There may be many more types of terrestrial planets with conditions that we may not expect, including smaller (Mecury- or Mars-mass) planets or giant (10x Earth-mass) terrestrial planets that ended up with suitable conditions to generate a life-sustaining environment similar to Earth's, including large bodies of water and a nitrogen-oxygen atmosphere that is not too thick or thin. Who knows what kinds of geological and atmospheric conditions we could find in other solar systems?

    Also, it all depends on what you define as "habitable." We know that simple organisms on Earth can survive conditions that would kill most complex vertebrates. Considering that extraterrestrial life, if it does exist (I think some form of life, not necessarily intelligent, though, is likely to exist in relative abundance), may evolve in ways wildly different than on Earth, including such drastic differences like the possibility of the development of genetic material that doesn't resemble DNA, we could find complex organisms on planets that wouldn't be suitable for humans or other inhabitants of Earth. While it is prudent to set certain limits on what we define as as a habitable Earth-like planet, we should also not overlook that we may find such worlds where we might not think they would be.

    Of course, this is all my opinion. I could be wrong.

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