Can We See If A Planet Is Inhabited 1 Million Lightyears away?

  1. Can we see if a planet is inhabited 1 million lightyears away (presently)?

    No, correct? We we actually be seeing the planent inhabited 1 million years ago, right?
  2. jcsd
  3. cepheid

    cepheid 5,189
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    Yes, you would be seeing a 1 million year old image of the planet. You cannot see the planet as it is presently.

    This is setting aside the impossibility of even resolving an image of that planet at that distance (its angular size would be too small)..

    ...and of seeing it in the first place (it would be too faint)

    ...and of it being there (1 million light years is in the void between our galaxy and others in the Local Group, where you likely wouldn't find any stars or planets).
  4. Chronos

    Chronos 10,278
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    Assuming we could see anything a million light years away, it would appear as it was a million years ago. We have difficulty seeing anything smaller than a football field on the moon, which is only a couple light seconds from earth. With modern equipment and techniques we can barely resolve the disc of nearby stars.
  5. Why is this? Is it because technology is limiting this? Will this be improved in the future?

    Regardless of how far we advance, we will NEVER be able to see a planet 1 million lightyears away presently?

    I know light is fixed (finite), but can't we change (increase) the speed of cameras? ...or "instruments" I should say to view them?
  6. russ_watters

    Staff: Mentor

    We have enough trouble seeing evidence of life on the moon. Telescope resolving power is a function of size and even the largest telescopes we have can barely make out the Apollo landing sites.
    Improved, sure, but we will likely never be able to see anything smaller than a continent on planets orbiting other stars. Evidence of life will come from studying the chemical makeup of the atmosphere.
    You mean how it is today? No. The light takes time to arrive. No amount of technology can change that.

    By the way -- a million light years is a little far. The galaxy is only on the order of a hundred thousand light years across and most of the stars we see in the sky are less than 100.
  7. Drakkith

    Staff: Mentor

    It has to do with the way light is focused down to a spot in an optical system such as a telescope. The basics, as Russ said, is that bigger telescopes have better resolution. This is inherent in the way light works and cannot be worked around.

    The speed of what? A camera, whether digital or film, simply opens a shutter and starts to collect light. When we feel the exposure is done we close the shutter and look at the light gathered. The rate at which light is gathered is not controllable at all, it simply depends on the brightness of the object you are viewing. Brighter objects allow for faster exposures, and very faint objects may requires the equivalent of days or weeks of exposure time in the form of many sub-exposures that are averaged together. (This is how the Hubble Space Telescope works)
  8. I personally read any sentence which says physics won't be able to do something in the future, with the prefix "With physics as we understand today, *".

    *"With physics as we understand today, you cannot see the planet as it is presently."

    * "With physics as we understand today, we will likely never be able to see anything smaller than a continent on planets orbiting other stars."

    *"With physics as we understand today, no amount of technology can change that."

    *"With physics as we understand today, this is inherent in the way light works and cannot be worked around.

    Keeps the hopes up for me. :)
  9. cepheid

    cepheid 5,189
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    I worked out that if you wanted to be able to resolve features on the planet surface at the 1000 km scale (some fraction of a continent size), at a distance of 1 million light years, and you want to do this at a wavelength of around 10 microns, which is roughly where the blackbody spectrum for something with an effective temperature of 300 K peaks, then you would need a telescope with a circular aperture of diameter approximately 115 MILLION kilometres. That's almost the distance between Earth and the sun. So you'd need telescope equal in size to half the Earth's orbit. And that STILL doesn't guarantee that you'd be able to see any emission from that planet (because it might be too faint, or it might be swamped by foreground sources or detector noise). If I worked things out right (which I may not have), such a planet would have a bolometric flux of ~2e-28 W/m2, leaving about 10 microwatts of power incident on the telescope. I'll leave you guys to convert that into a magnitude or into janskys or whatever it is you use.
  10. mfb

    Staff: Mentor

    I think this is a bit pessimistic. If you look at stars nearby (say, 10 light years), the required telescope size shrinks to ~100-1000km. You do not need a solid telescope of that size, several smaller mirrors in the right positions would allow to get the same resolution as well. It is beyond our current technology, but it is "just" an engineering issue. It might become possible within the 21st century.
  11. Correct, we can only ever see things in space as they were and not as they are. Even our own moon is seen as it was about 1.5 seconds ago or the Sun 8 minuets ago. In a very real sense the heavens are a recorded HD brodcast! If light were not limited by its velocity the night sky would be ablaze with light. But back to your inhabited planet hypotheses. no telescope on earth could resolve it. But if the inhabitants developed radio communications then it would be likely that at some point we would receive their signals, which is the purpose of the SETI program. There is one other possibility however, if the inhabitants were heavily industrialised it may be possible to detect traces of industrial output in their atmosphere as the planet passed in front of their sun. You would not for instance expect to see traces of plutonium-239 and MOX occurring in a naturale state, that would be a big indicator of intelligent life. It may only be a matter of time. In fact when it comes to observing the universe all is a matter of time.
  12. Vanadium 50

    Vanadium 50 18,262
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    For resolution, yes. For light collection, I'm not so sure. You're probably talking about a planet around magnitude 26. The optics are going to be challenging, since you'll have maybe a 3rd magnitude star a microradian away.
  13. mfb

    Staff: Mentor

    If you can get the resolution, light collection should not be an issue. Hubble can observe objects down to a magnitude of ~30, and that super-telescope would certainly have more light collecting area than Hubble.
  14. When you consider that Hubble in perfect seeing conditions cannot resolve the nearest and biggest stars into a disc it is highly improbable that we could ever build a scope with such a capability. Resolving power is however not as critical as the scopes ability to accurately receive spectral, occulation and luminosity data. It is this critical information that gives us the real big picture.
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