5 ms·

This might be a dump question but if we were to look at our planet from far away what exactly would we see?

elihu 9y ago
From lightyears away, probably nothing except maybe a slight drop in the sun's output if the Earth happens to pass in front of it and a periodic wobble of the sun's location as the sun and Earth orbit the center of gravity of the sun-Earth system. The Earth is pretty small and far from the sun compared to the kinds of exoplanets we've been able to detect, so the wobble (which might be detectable via doppler shift of light from certain elements in the sun) would be slight and probably hard to detect without some pretty sophisticated instruments. It might be possible to detect something the size of the Earth from several light years away with a good enough telescope, but I'm not sure what the limit of that is.
- wallace_f 9y ago
  Question, given the sun's wobble would result from rotating around the solar system's center of gravity, how feasible is it to actually know with certainty what the solar system is made up of?
  - saagarjha 9y ago
    Composition analysis is usually done using spectroscopy, which looks for the characteristic wavelengths of compounds that show up when analyzing spectra.
    - wallace_f 9y ago
      This comment seems out of place.
      - saagarjha 9y ago
        I was answering your question in a roundabout way: usually "wobble" gives very little information about the chemical composition of an exoplanet (other than maybe that it's a large gas giant).
        wallace_f 9y ago
        I see. I actually thought your comment might have been said in bad faith as an insult, but I now think it was focused on the context of the post, while mines was focused on the context of what I was replying to. I was replying to the aforementioned method of detecting planets via dopplar shifts in a star's light... I've always wondered about that--with how much precision can you actually know what is causing the shifts. How many gas giants, smaller planets, etc? For example, I suppose two planets of equal size, orbital plane, and 180 degrees from one another would mute their gravitational effect on their host star.
        saagarjha 9y ago
        > For example, I suppose two planets of equal size, orbital plane, and 180 degrees from one another would mute their gravitational effect on their host star. Such orbits are extremely rare. Any such orbit would be highly unstable, as any perturbation would cause the planet to deviate from its location. (Strictly speaking, there are five points where objects can have an orbit that matches that of another planet. They're called the Lagrangian points, and the only ones that are stable are L4 and L5, 60˚ ahead and behind the planet in its orbit.)
  - elihu 9y ago
    From the wobble you can determine the approximate mass of the planet and how long it takes to orbit it's sun. I suppose you might be able to deduce some things about the shape of its orbit as well. Knowing what a planet is made of would require some other technique. I think the best you could do from looking at the wobble would be to say things like "planets that orbit stars like this and having a mass of such and such with an orbital period of so many days are likely to be made of the same things that other planets with those characteristics are made of". On the other hand, we can know pretty easily quite a lot about what it's host star is made of by examining the light it emits.
    - neom 9y ago
      If we can detect wobble, can we detect composition? (If someone observing us was more advanced, if they can detect wobble they can probably deduce composition?)
      - saagarjha 9y ago
        Probably, because we’ve been doing it for other exoplanets already. Wobble and composition are detected in different ways, though.
    - wallace_f 9y ago
      I suppose two Jupiters on the same orbital plane, 180 degrees from each other, would cancel out? Is it possible for observers to really know we have 4 rocky planets, 4 gas giants, and what their mass is, etc? Or, with what degree of certainty could this be known, and from what distance?
      - animal531 9y ago
        I don't believe you can have 2 planets in the same orbit, and once they're in different orbits they'll take a different amount of time to orbit; so you should be able to get individual measurements. Edit: Well, as per the simulated stable thread, apparently you can have Trojan etc pairs of planets in the same orbit. Interesting.
- mturmon 9y ago
  Everything you say is true, but we are farther along than your comment indicates. With a 6.5 meter space telescope and a star shade, we pretty much have the capability within just a few years to blot out the direct light from the host star and perform spectral characterization of exo-Earths using the light from their host star that is bouncing off of them. In particular, this is a direct exoplanet observation, not indirect like the host-star radial velocity method. An example being studied right now is HabEx (https://en.m.wikipedia.org/wiki/Habitable_Exoplanet_Imaging_Mission https://en.m.wikipedia.org/wiki/Habitable_Exoplanet_Imaging_...). From such spectra, we can use chemical and general circulation models to constrain the atmospheric composition, height, and temperature. We can investigate whether the atmosphere is in chemical equilibrium, or if there is some non-chemical process active. The paper referenced in the article actually does some of this (although this is a larger planet).
  - fullstackchris 9y ago
    Would a chemical equilibrium or non-chemical process indicate the possibility of life? Or are neither exact signals?
    - avian 9y ago
      They are a "hm, that's interesting" kind of a signal. They simply mean that a simplistic chemical view of the atmosphere does not explain the observation. It could mean the presence of life, or it could mean some other unknown, inorganic process. See for example the Viking experiments: they detected signs of life on Mars, only to be disproved later by better understanding of soil chemistry. https://en.wikipedia.org/wiki/Viking_lander_biological_experiments https://en.wikipedia.org/wiki/Viking_lander_biological_exper...
- samstave 9y ago
  If we are so small as compared to our sun, and these exoplanets may be so much larger - and we have the idea of a “habitable zone” for planets: is there a “habitable gravity” level for planet size that we should consider for harboring life?
  - acqq 9y ago
    There are actualy the limits: https://www.forbes.com/sites/startswithabang/2017/02/16/super-earths-are-common-but-incapable-of-supporting-life-like-ours/ https://www.forbes.com/sites/startswithabang/2017/02/16/supe...
  - Beltiras 9y ago
    Maybe you would see silicate life happening at higher atmospheric pressures and gravities.
garmaine 9y ago
Not a dumb question at all NASA pointed the Galileo Satellite at earth during one of its flyby to see if they could detect life via the techniques they would like to apply to extrasolar planets: https://www.astrobio.net/alien-life/seeing-earth-exoplanet-signs-life-visible/ https://www.astrobio.net/alien-life/seeing-earth-exoplanet-s...
lloeki 9y ago
This[0]. And if you're wondering about the techniques and possibility of detecting life, IIRC the eponymous book has a whole chapter about exactly that. [0]: https://en.wikipedia.org/wiki/Pale_Blue_Dot https://en.wikipedia.org/wiki/Pale_Blue_Dot

From lightyears away, probably nothing except maybe a slight drop in the sun's output if the Earth happens to pass in front of it and a periodic wobble of the sun's location as the sun and Earth orbit the center of gravity of the sun-Earth system. The Earth is pretty small and far from the sun compared to the kinds of exoplanets we've been able to detect, so the wobble (which might be detectable via doppler shift of light from certain elements in the sun) would be slight and probably hard to detect without some pretty sophisticated instruments. It might be possible to detect something the size of the Earth from several light years away with a good enough telescope, but I'm not sure what the limit of that is.

wallace_f 9y ago
Question, given the sun's wobble would result from rotating around the solar system's center of gravity, how feasible is it to actually know with certainty what the solar system is made up of?
- saagarjha 9y ago
  Composition analysis is usually done using spectroscopy, which looks for the characteristic wavelengths of compounds that show up when analyzing spectra.
  - wallace_f 9y ago
    This comment seems out of place.
    - saagarjha 9y ago
      I was answering your question in a roundabout way: usually "wobble" gives very little information about the chemical composition of an exoplanet (other than maybe that it's a large gas giant).
      - wallace_f 9y ago
        I see. I actually thought your comment might have been said in bad faith as an insult, but I now think it was focused on the context of the post, while mines was focused on the context of what I was replying to. I was replying to the aforementioned method of detecting planets via dopplar shifts in a star's light... I've always wondered about that--with how much precision can you actually know what is causing the shifts. How many gas giants, smaller planets, etc? For example, I suppose two planets of equal size, orbital plane, and 180 degrees from one another would mute their gravitational effect on their host star.
        saagarjha 9y ago
        > For example, I suppose two planets of equal size, orbital plane, and 180 degrees from one another would mute their gravitational effect on their host star. Such orbits are extremely rare. Any such orbit would be highly unstable, as any perturbation would cause the planet to deviate from its location. (Strictly speaking, there are five points where objects can have an orbit that matches that of another planet. They're called the Lagrangian points, and the only ones that are stable are L4 and L5, 60˚ ahead and behind the planet in its orbit.)
- elihu 9y ago
  From the wobble you can determine the approximate mass of the planet and how long it takes to orbit it's sun. I suppose you might be able to deduce some things about the shape of its orbit as well. Knowing what a planet is made of would require some other technique. I think the best you could do from looking at the wobble would be to say things like "planets that orbit stars like this and having a mass of such and such with an orbital period of so many days are likely to be made of the same things that other planets with those characteristics are made of". On the other hand, we can know pretty easily quite a lot about what it's host star is made of by examining the light it emits.
  - neom 9y ago
    If we can detect wobble, can we detect composition? (If someone observing us was more advanced, if they can detect wobble they can probably deduce composition?)
    - saagarjha 9y ago
      Probably, because we’ve been doing it for other exoplanets already. Wobble and composition are detected in different ways, though.
  - wallace_f 9y ago
    I suppose two Jupiters on the same orbital plane, 180 degrees from each other, would cancel out? Is it possible for observers to really know we have 4 rocky planets, 4 gas giants, and what their mass is, etc? Or, with what degree of certainty could this be known, and from what distance?
    - animal531 9y ago
      I don't believe you can have 2 planets in the same orbit, and once they're in different orbits they'll take a different amount of time to orbit; so you should be able to get individual measurements. Edit: Well, as per the simulated stable thread, apparently you can have Trojan etc pairs of planets in the same orbit. Interesting.
mturmon 9y ago
Everything you say is true, but we are farther along than your comment indicates. With a 6.5 meter space telescope and a star shade, we pretty much have the capability within just a few years to blot out the direct light from the host star and perform spectral characterization of exo-Earths using the light from their host star that is bouncing off of them. In particular, this is a direct exoplanet observation, not indirect like the host-star radial velocity method. An example being studied right now is HabEx (https://en.m.wikipedia.org/wiki/Habitable_Exoplanet_Imaging_Mission https://en.m.wikipedia.org/wiki/Habitable_Exoplanet_Imaging_...). From such spectra, we can use chemical and general circulation models to constrain the atmospheric composition, height, and temperature. We can investigate whether the atmosphere is in chemical equilibrium, or if there is some non-chemical process active. The paper referenced in the article actually does some of this (although this is a larger planet).
- fullstackchris 9y ago
  Would a chemical equilibrium or non-chemical process indicate the possibility of life? Or are neither exact signals?
  - avian 9y ago
    They are a "hm, that's interesting" kind of a signal. They simply mean that a simplistic chemical view of the atmosphere does not explain the observation. It could mean the presence of life, or it could mean some other unknown, inorganic process. See for example the Viking experiments: they detected signs of life on Mars, only to be disproved later by better understanding of soil chemistry. https://en.wikipedia.org/wiki/Viking_lander_biological_experiments https://en.wikipedia.org/wiki/Viking_lander_biological_exper...
samstave 9y ago
If we are so small as compared to our sun, and these exoplanets may be so much larger - and we have the idea of a “habitable zone” for planets: is there a “habitable gravity” level for planet size that we should consider for harboring life?
- acqq 9y ago
  There are actualy the limits: https://www.forbes.com/sites/startswithabang/2017/02/16/super-earths-are-common-but-incapable-of-supporting-life-like-ours/ https://www.forbes.com/sites/startswithabang/2017/02/16/supe...
- Beltiras 9y ago
  Maybe you would see silicate life happening at higher atmospheric pressures and gravities.

garmaine 9y ago

Not a dumb question at all NASA pointed the Galileo Satellite at earth during one of its flyby to see if they could detect life via the techniques they would like to apply to extrasolar planets: https://www.astrobio.net/alien-life/seeing-earth-exoplanet-signs-life-visible/ https://www.astrobio.net/alien-life/seeing-earth-exoplanet-s...

lloeki 9y ago

This[0]. And if you're wondering about the techniques and possibility of detecting life, IIRC the eponymous book has a whole chapter about exactly that. [0]: https://en.wikipedia.org/wiki/Pale_Blue_Dot https://en.wikipedia.org/wiki/Pale_Blue_Dot