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17B Earths in the galaxy?

  1. Jan 8, 2013 #1

    "there are 17 billion other Earths in our galaxy, the Milky Way."

    and that's only this galaxy. How many there are in the entire universe?!
    Also, does any one out there know we are here? Are they counting us a part
    of some alien planet as we count them?
  2. jcsd
  3. Jan 8, 2013 #2


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    arXiv link
    A fascinating result. Earth-sized planets are a common phenomenon.
    Depends on the unknown size of the universe, but you can simply multiply it with the number of galaxies in whatever region to get a good estimate. The observable universe has some 100 billion galaxies.

    I think it is unlikely, but this is based on several speculative assumptions.

    Keep in mind that "earth-like" is not the same as "earth". Mars and Venus can be considered as "earth-like", as they have a similar mass, size and distance to our sun.

    In addition, all those 17 billion years are not earth-like in terms of their orbital period:
    Our earth has an orbital period of 1 year, or about 365 days.
    Last edited: Jan 8, 2013
  4. Jan 8, 2013 #3


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    Mfb got the key point but just to reiterate it: Earthlike does not mean a planet with oceans, average temperatures not to far above zero degrees C, 100kPa at sea level, home to life etc etc etc. There is still no reasonable way to figure out the likelihood of truly Earthlike planets.
  5. Jan 9, 2013 #4


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    Gomar, I notice you have been a member here on Physics Forums since July, 2010. I would have expected that by now you would know the rules here, but your post above indicates that you have not read them.

    Specifically, when you asked “does any one out there know we are here?” and “Are they counting us a part of some alien planet as we count them?” exactly who are you referring to? Do you have some evidence, as yet unknown to the general public, indicating there are some other intelligent life forms in our Galaxy or in the Universe?

    According to our Rules, “Personal theories or speculations that go beyond or counter to generally-accepted science” are not allowed.

    These completely fantastic speculations are not generally-accepted science. Certainly there must be other places on the internet to post this mythopoetical narrative. Will you please help maintain the integrity of our Physics Forums and abstain from posting science fiction? Let us all continue enjoying real science and reach our mission goal:

    “Our mission is to provide a place for people (whether students, professional scientists, or others interested in science) to learn and discuss science as it is currently generally understood and practiced by the professional scientific community.”

    Thank you for your attention,
  6. Jan 9, 2013 #5
    Let's look at Drake's equation again, however much it might seem like an oversimplification.

    N = R**fp*ne*fl*fi*fc*L

    N = communicative civilizations in our Galaxy
    R* = rate of star formation
    fp = fraction of stars with planets
    ne = number of Earthlike planets when planets form
    fl = fraction of Earthlike planets that have life
    fi = fraction of planetary biospheres where intelligent species evolve
    fc = fraction of intelligent species that develop interstellar-communication technology and an interest in interstellar communication
    L = how long a communicative civilization lasts

    When Frank Drake proposed it in 1960, only R* was known with any reliability. But the large numbers of exoplanets discovered is putting 2 more within our reach: fp and ne. This recent finding is one step forward.
  7. Jan 9, 2013 #6
    There is a joker in the deck: water supply. Too little water, and a planet is a giant desert. Too much water, and a planet is covered with a thick ocean. The Earth seems conveniently in between.

    Depth of the Ocean
    Where is Earth's water? USGS Water-Science School

    If all the Earth's water was spread evenly over the Earth's surface, it would have a depth of about 2.4 km.

    From the USGS site:

    All crustal water -- oceans: 96.5%, saline groundwater: 0.93%, saline lakes: 0.07%, freshwater: 2.5%
    Freshwater -- glaciers and ice caps: 68.6%, groundwater 30.1%, surface and aerial water: 1.3%
    Surface and atmospheric water -- ice and snow: 73.1%, lakes: 20.1%, soil moisture: 3.52%, swamps and marshes: 2.53%, rivers: 0.46%, biological water: 0.22%, atmospheric water: 0.22%

    Freshwater would be about 60 m average depth.

    So it looks like we can have much less water than we do. Or can we?

    Let's take a closer look at water in the Earth's crust.
    Looking in Chemical Properties of Soil and Ground Waters, I find:

    Free, chemically bound, physically bound, total:
    0.2, 0.11, 0.42, 0.73 * 10^(24) g
    0.4, 0.2, 0.8, 1.4 km

    That's not much less than the water in the oceans, so much less total water would sink into the rocks and leave the surface bone-dry.
  8. Jan 9, 2013 #7
    Let's go the in the opposite direction and find out how deep the oceans can be before they cover any possible land.

    Their depth is about 4 km, and how does that compare with the highest possible land?

    If a mountain has height h, then the pressure underneath its peak is
    (density) * g * h

    g = surface gravity of planet

    So if a mountain has a height greater than a certain value, it will slump until its height reaches that value.

    Relative to its surroundings, the highest mountains on Earth are Mauna Kea and Mauna Loa in Hawaii, at about 10.2 km.

    On Mars, Olympus Mons is about 22 km high, and a mountain on Earth with the same base pressure would have a height of 8 km. Almost but not quite the height of those Hawaiian mountains.

    So if the Earth had 3 to 4 times as much water as it does, then it will be hard for a mountain to reach the water's surface.

    The other limit is 1/3 the of the amount.

    So an Earthlike planet with both continents and oceans ought to have 1/3 to 3 times the amount of water that the Earth does.
  9. Jan 9, 2013 #8


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    I don't understand your argument.
    Earth's crust has a height difference of nearly 20km between its lowest point (Mariana trench, -10.911 km) and its heighest point (Mount Everest, +8.848km). Sure, an ocean would erode the Himalaya summits significantly, reducing this value a bit. The average ocean depth is ~4km and covers 3/4 of its surface, and the average height of dry land is something below 500m I think. To increase the water level with our current height profile by 8km, you would need additional water corresponding to about 4/3*7.5/4=3.3 times the current ocean water. To remove any surface water, you would have to remove the whole ocean water - and a significant part of the water in the rock as well (probably everything above ~5km below the current ocean level, and more close to the Mariana trench).
  10. Jan 9, 2013 #9
    Actually, there's much less erosion below the ocean surface than above it. That's evident in the numerous submerged seamounts. The oldest mountain in the Hawaii-Emperor chain is the Meiji seamount, and that's about 82 million years old. It's next to a subduction zone that has likely destroyed older seamounts. By comparison, the oldest of the above-water Hawaiian Islands is about 5 million years old.

    My argument is that for there to be above-water land, there has to be some land areas that are more elevated than others, and I was estimating what the maximum possible elevation difference might be.

    Mt. Everest is about 8.8 km above sea level, and the Mariana Trench is about 10.9 km below sea level, adding to 19.7 km. However, Mt. Everest is about 4 km taller than its surroundings and the Mariana Trench is about 5.5 km deeper than its surroundings.
  11. Jan 9, 2013 #10


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    Thanks for the interesting post number five regarding Drake’s equation. You are exactly correct: now two of its variables, the fraction of those stars that have planets and the average number of planets that can potentially support life per star that has planets are now closer to being defined...truly a step forward.

    The equation continues to stimulate imaginations and reinforces the “we are not alone” emotion. Unfortunately, the Drake equation still remains outside the realm of science.

    “The problem, of course, is that none of the terms can be known, and most cannot even be estimated. The only way to work the equation is to fill in with guesses. [...] As a result, the Drake equation can have any value from "billions and billions" to zero. An expression that can mean anything means nothing. Speaking precisely, the Drake equation is literally meaningless...”

    One reply to such criticism is that even though the Drake equation currently involves speculation about unmeasured parameters, it was not meant to be science, but intended as a way to stimulate dialogue on these topics. Then the focus becomes how to proceed experimentally. Indeed, Drake originally formulated the equation merely as an agenda for discussion at the Green Bank conference.


  12. Jan 9, 2013 #11


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    Thank you for your interesting posts about Earth’s water in post number six, planetary geology in post number seven, and more earth geology in post number nine.

    Would these not more appropriately appear in a different part of our Forums such as “Other Sciences, Earth” subtitled “Geology, Meteorology, Oceanography...”?

    Is it possible you have some specific reason for posting these here in the “Astronomy” section that is not apparent?

  13. Jan 9, 2013 #12


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    It appears that the closest planet (as discovered to date) outside of our solar system that may be suitable for life is over 100 trillion miles or some 20 light years away. If our radio signals emitted over 40 years ago have reached that planet and been detected, we haven't heard back from 'them' yet. Is there life on other planets? This video is worth another look, then you decide......
    Last edited by a moderator: Sep 25, 2014
  14. Jan 10, 2013 #13
    Bobbywhy, my reason is to show what's necessary for a planet to have Earthlike continents and oceans.

    Here is some work on what compositions exoplanets may have.
    [1004.0971] The Compositional Diversity of Extrasolar Terrestrial Planets: I. In-Situ Simulations -- no migration
    [1209.5125] The Compositional Diversity of Extrasolar Terrestrial Planets: II. Migration Simulations -- a Jupiter-mass planet spiraling in to 1 AU

    The first one was for little or no mixing. None of the planets get oceans, and they don't get much nitrogen either. Their atmospheres may thus be mainly carbon dioxide and sulfur dioxide.

    The second one gives planets oceans, and often 100 to 1000 times more ocean than what the Earth has. The authors concede that they may be an overestimate because of effects like comet-style evaporation of water from planetesimals close to the star.

    Turning to observations, most of them are either of masses (radial velocity) or of radii (transit). In a few cases, it's been possible to get both, giving overall average densities. For GJ 1214 b, it is about 1.27 +- 0.4 g/cm^3, lower than what one would expect for rocky materials. So that planet likely has an ocean a few thousand km deep.
  15. Jan 10, 2013 #14


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    lpetrich, Thank you for posting those two references with simulations. Now I can understand why the posts are here.

  16. Jan 10, 2013 #15
    One more factor, but I'm not sure how express it clearly in English.

    Continental crust density - 2.7 g/cm^3
    Water density - 1 g/cm^3

    Continental plate floats. Its still nicely seen in Scandinavia which still rebound after being pressed during ice age.

    If you drop high enough amount of water, then actually it would increase somewhat buoyancy affecting continental shelf. If you drop let's say as much water to increase sea level by 10 km, then when the continents rebound you should get 10 - (10*1/2.7) relative sea level increase, roughly 6,29 km increase, with quite a few contemporary mountain ranges above sea level.

    Yes, but if you mean such discovered planets with mass density relation meaning hundreds kilometre deep ocean, then I have to agree.
    Last edited: Jan 10, 2013
  17. Feb 7, 2013 #16
    What does our earth have? The right ammount of water, I think just 1% more and we would be a waterworld, a solid magnetic field, habitable zone, the right gasses. I think if we could check for all those things the number would drop a lot.
  18. Feb 8, 2013 #17


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    The number of planets which look like earth is lower, of course - and it depends on the definition of "look like earth". But what is so special about earth, apart from the fact that we live here?
  19. Feb 8, 2013 #18
    The things I mentioned. Earth has just the right amount of water, enough to sustain life, but not so much there isnt a significant ammount of landmass. A liquid metal core that generates a good magnetic field, goldilocks zone, the size/gravity a fairly stable axis due to the moon/other planets.

    But yeah, even a tiny fraction where all those criteria are met of those 17 billions would ammount to hundreds of earths.
  20. Feb 8, 2013 #19


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    Earth had life long before it lived on land. A water world wouldn't preclude abiogenesis.
  21. Feb 8, 2013 #20


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    In addition, a magnetic field is probably irrelevant for life deep in the oceans, and liquid water (the main argument for the habitable zone) can exist elsewhere, too - Europa is a good candidate, Ganymede could have oceans, and even for Pluto this cannot be ruled out.

    See Ryan's signature - the same is true on a smaller scale. Earth is not fine-tuned to allow life. Life on earth is fine-tuned to survive on earth.
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