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How big of a space telescope would we need to see this planet in any actual detail? One of my sci-fi fantasies is to take a photo of an extrasolar planet and s

by api 10y ago

How big of a space telescope would we need to see this planet in any actual detail? One of my sci-fi fantasies is to take a photo of an extrasolar planet and see someone else's city lights. :) Of course if we could see that we could also probably detect their radio emissions, but seeing someone else's lights would somehow be cooler.

kyriakos 10y ago
I'm no expert but I'd say you need an orbiting telescope to do that.
themartorana 10y ago
At ~500 light years away, even if we did see lights, who would we be communicating with _now_? And would it be disappointing if we saw a technologically-similar civilization that hadn't contacted us yet, being 500 years ahead of us? Sometimes it's romantic to look up at stars that probably died a long time ago. Sometimes I hate that we can't see what's happening now.
- karmelapple 10y ago
  Where does the 500 light years number come from? Alpha Centauri is only 4.4 light years away from Earth.
  - ogrisel 10y ago
    This discovery is about Proxima Centauri which is slightly closer: 4.25 ly.
- bengarvey 10y ago
  It's only 4.25 light years away https://en.wikipedia.org/wiki/Proxima_Centauri https://en.wikipedia.org/wiki/Proxima_Centauri
- verandaguy 10y ago
  Why are people in this thread constantly citing 500 light-years? Proxima Centauri's 4 ly away, which, granted, isn't a walk in the park, but it's remarkably near to us.
- civilian 10y ago
  Why would a civilization be exactly 500 years ahead of us? While evolution is a powerful trend, I think there's so much randomness about... when their planet cooled off enough (and had enough alkaline vents) to support abiogenesis, when bacteria form eukaryotes, when eukaryotes become multi-cellular, when life moves out of the oceans, when plant-like organisms start producing fruit leading to animals that take advantage of the high-energy food source and are able to develop smart brains--- all of these events could happen +/- a million years from eachother. And that's assuming that their evolution of life followed a similar path! So worrying about a race having 500 years on us is just... a worry based on so many wrong assumptions.
evilduck 10y ago
That question pops up on /r/askscience fairly often. Here's one that does the math for the previously closest known Earthlike exoplanet (36ly) and Alpha Centauri (4.5ly). https://www.reddit.com/r/askscience/comments/137klp/will_there_be_space_telescopes_capable_of_taking/ https://www.reddit.com/r/askscience/comments/137klp/will_the...?
- Gravityloss 10y ago
  For starters, it doesn't seem impossible to build it, on earth or in space. You don't need a continuous mirror of that size. I wonder if any of the LIGO technology can be applied to optical telescopes? https://en.wikipedia.org/wiki/Aperture_synthesis https://en.wikipedia.org/wiki/Aperture_synthesis
throwanem 10y ago
> How big of a space telescope would we need to see this planet in any actual detail? Impossibly big. I'm sorry.
- TeMPOraL 10y ago
  From the link 'evilduck posted, it seems that's big indeed, but possibly big. 240km is a big thing, but within the realm of possibility; also I'm personally hoping someone will invent some magic mathematical trick around phased arrays or whatever, and cut that size by an order of magnitude or two :).
  - throwanem 10y ago
    240km is also what's required to resolve a feature no smaller than 100km across. I'm no mathematician and I am not equipped at the moment to pretend to be one, but to resolve a feature an order of magnitude smaller, I suspect you'd need a much larger instrument.
    - ianai 10y ago
      If I did the math correctly, it turns out to be ~2400km for 10km across (changing 100 to 10 in the eq). This could very easily be wrong as I only glanced through the comment. Think of it as Kirkian mathematics.
  - ianai 10y ago
    I would file 240km under very do-able. It's nothing like, say, a solar system sized particle accelerator.
    - throwanem 10y ago
      That simple? Well jeez, I don't know what I was even worried about. :D
darkerside 10y ago
Way cooler, because it would imply they share our visible light spectrum!
- wcoenen 10y ago
  It's not a coincidence that we can see in the 390 to 700 nm part of the spectrum. These are the wavelengths that pass through water most easily[1]. Any alien lifeforms which started their evolution in (illuminated) water could be expected to be sensitive to the same slice of the spectrum. [1] https://en.wikipedia.org/wiki/Electromagnetic_absorption_by_water#/media/File:Absorption_spectrum_of_liquid_water.png https://en.wikipedia.org/wiki/Electromagnetic_absorption_by_...
Thorondor 10y ago
The planet is about 0.05 AU from Proxima Centauri, meaning we need an angular resolution of about 1.9e-7 radians to even distinguish it from its host star. Is that realistic? In theory, an orbiting space telescope has a diffraction-limited resolution of approximately 1.22λ/D (λ = wavelength, D = aperture size). Modern image processing techniques can improve on this somewhat, but it makes a good order-of-magnitude estimate. Anyway, this formula tells us a 4-meter telescope has a maximum angular resolution of about 1.8e-7 radians at a typical visible light wavelength of 600 nm. That would be just good enough... except that we don't actually have a 4 meter orbiting space telescope. Resolving even large features on the planet would require a much larger telescope, probably kilometers or more. For ground based telescopes, the situation is even worse because of atmospheric effects. Despite being 10 meters in aperture, the Keck telescopes in Hawaii are limited to an angular resolution of about 2e-7 radians because of the atmosphere. However, there is reason to hope that the even larger European Extremely Large Telescope will have enough resolution (about 5e-8 radians? hard to tell from their official publications) to image Proxima b directly. https://www.eso.org/sci/meetings/2011/VLTI2011/presentations/Kissler-Patig.pdf https://www.eso.org/sci/meetings/2011/VLTI2011/presentations... Again, this is still not enough to resolve surface features. So, long story short the answer is unfortunately no at present. Maybe space-based manufacturing will let us build a big enough telescope someday?
- verandaguy 10y ago
  To put things in perspective, what was the angular resolution of Hubble's Ultra Deep Field measurements/photos? (keeping in mind that angular resolution isn't the only challenge here)
  - Thorondor 10y ago
    Hubble's angular resolution is about 0.05 arcsec = 2.4e-7 radians. And you're absolutely right that angular resolution isn't the only challenge here - some way of filtering out the extremely bright (relatively) light from the host star is needed as well, for starters.
- throwaway_yy2Di 10y ago
semaphoreP 10y ago
The most promising technique at this point is to use optical interferometry to resolve the surface of the planet. In this case, you will need two telescopes (likely in space) separated by a baseline distance, "d". The two telescopes will require extremely precise synchronization in both spatial position and timing so that the light they collect will interfere at precisely the right phase, but if this can be achieved, the angular resolution of such a telescope in radians is wavelength/d. An earth sized planet has a diameter of ~13,000 km. To "resolve the planet", we would need to be able to distinguish one half of the planet from the other, meaning we need to see at a resolution of ~6000 km on the surface. Proxima Centauri is ~4 light years away, meaning that this requires an angular resolution of 1.5e-10 radians or 30 microarcseconds. That's over 30,000 times smaller than the angular size of a human hair held at arms length!) To achieve 30 microarcsecond resolution with our optical interferometer operating at 600 nm (visible light), we need the two telescopes to be 4 km apart, which practically doesn't sound unfeasible. Alternatively, you could construct a 4 km telescope, but that's far bigger than any optical telescope we have now or in the near future (the biggest telescope in the next 20 years will be 40 meter in diameter).
- kilroy123 10y ago
  Interesting, I would love to see a feasibility study done on this.
  - semaphoreP 10y ago
    NASA actually did do a feasibility study on the Terrestrial Planet Finder (https://en.wikipedia.org/wiki/Terrestrial_Planet_Finder https://en.wikipedia.org/wiki/Terrestrial_Planet_Finder) as a space-based telescope to find terrestial planets orbiting other stars, and one of the versions is an infrared interferometer flying in formation in space. Unfortunately, they never followed through with it (I'm guessing due to technical challenges).

by api 10y ago

kyriakos 10y ago

I'm no expert but I'd say you need an orbiting telescope to do that.

themartorana 10y ago

At ~500 light years away, even if we did see lights, who would we be communicating with _now_? And would it be disappointing if we saw a technologically-similar civilization that hadn't contacted us yet, being 500 years ahead of us? Sometimes it's romantic to look up at stars that probably died a long time ago. Sometimes I hate that we can't see what's happening now.

karmelapple 10y ago
Where does the 500 light years number come from? Alpha Centauri is only 4.4 light years away from Earth.
- ogrisel 10y ago
  This discovery is about Proxima Centauri which is slightly closer: 4.25 ly.
bengarvey 10y ago
It's only 4.25 light years away https://en.wikipedia.org/wiki/Proxima_Centauri https://en.wikipedia.org/wiki/Proxima_Centauri
verandaguy 10y ago
Why are people in this thread constantly citing 500 light-years? Proxima Centauri's 4 ly away, which, granted, isn't a walk in the park, but it's remarkably near to us.
civilian 10y ago
Why would a civilization be exactly 500 years ahead of us? While evolution is a powerful trend, I think there's so much randomness about... when their planet cooled off enough (and had enough alkaline vents) to support abiogenesis, when bacteria form eukaryotes, when eukaryotes become multi-cellular, when life moves out of the oceans, when plant-like organisms start producing fruit leading to animals that take advantage of the high-energy food source and are able to develop smart brains--- all of these events could happen +/- a million years from eachother. And that's assuming that their evolution of life followed a similar path! So worrying about a race having 500 years on us is just... a worry based on so many wrong assumptions.

evilduck 10y ago

That question pops up on /r/askscience fairly often. Here's one that does the math for the previously closest known Earthlike exoplanet (36ly) and Alpha Centauri (4.5ly). https://www.reddit.com/r/askscience/comments/137klp/will_there_be_space_telescopes_capable_of_taking/ https://www.reddit.com/r/askscience/comments/137klp/will_the...?

Gravityloss 10y ago
For starters, it doesn't seem impossible to build it, on earth or in space. You don't need a continuous mirror of that size. I wonder if any of the LIGO technology can be applied to optical telescopes? https://en.wikipedia.org/wiki/Aperture_synthesis https://en.wikipedia.org/wiki/Aperture_synthesis

throwanem 10y ago

> How big of a space telescope would we need to see this planet in any actual detail? Impossibly big. I'm sorry.

TeMPOraL 10y ago
From the link 'evilduck posted, it seems that's big indeed, but possibly big. 240km is a big thing, but within the realm of possibility; also I'm personally hoping someone will invent some magic mathematical trick around phased arrays or whatever, and cut that size by an order of magnitude or two :).
- throwanem 10y ago
  240km is also what's required to resolve a feature no smaller than 100km across. I'm no mathematician and I am not equipped at the moment to pretend to be one, but to resolve a feature an order of magnitude smaller, I suspect you'd need a much larger instrument.
  - ianai 10y ago
    If I did the math correctly, it turns out to be ~2400km for 10km across (changing 100 to 10 in the eq). This could very easily be wrong as I only glanced through the comment. Think of it as Kirkian mathematics.
- ianai 10y ago
  I would file 240km under very do-able. It's nothing like, say, a solar system sized particle accelerator.
  - throwanem 10y ago
    That simple? Well jeez, I don't know what I was even worried about. :D

darkerside 10y ago

Way cooler, because it would imply they share our visible light spectrum!

wcoenen 10y ago
It's not a coincidence that we can see in the 390 to 700 nm part of the spectrum. These are the wavelengths that pass through water most easily[1]. Any alien lifeforms which started their evolution in (illuminated) water could be expected to be sensitive to the same slice of the spectrum. [1] https://en.wikipedia.org/wiki/Electromagnetic_absorption_by_water#/media/File:Absorption_spectrum_of_liquid_water.png https://en.wikipedia.org/wiki/Electromagnetic_absorption_by_...

Thorondor 10y ago

The planet is about 0.05 AU from Proxima Centauri, meaning we need an angular resolution of about 1.9e-7 radians to even distinguish it from its host star. Is that realistic? In theory, an orbiting space telescope has a diffraction-limited resolution of approximately 1.22λ/D (λ = wavelength, D = aperture size). Modern image processing techniques can improve on this somewhat, but it makes a good order-of-magnitude estimate. Anyway, this formula tells us a 4-meter telescope has a maximum angular resolution of about 1.8e-7 radians at a typical visible light wavelength of 600 nm. That would be just good enough... except that we don't actually have a 4 meter orbiting space telescope. Resolving even large features on the planet would require a much larger telescope, probably kilometers or more. For ground based telescopes, the situation is even worse because of atmospheric effects. Despite being 10 meters in aperture, the Keck telescopes in Hawaii are limited to an angular resolution of about 2e-7 radians because of the atmosphere. However, there is reason to hope that the even larger European Extremely Large Telescope will have enough resolution (about 5e-8 radians? hard to tell from their official publications) to image Proxima b directly. https://www.eso.org/sci/meetings/2011/VLTI2011/presentations/Kissler-Patig.pdf https://www.eso.org/sci/meetings/2011/VLTI2011/presentations... Again, this is still not enough to resolve surface features. So, long story short the answer is unfortunately no at present. Maybe space-based manufacturing will let us build a big enough telescope someday?

verandaguy 10y ago
To put things in perspective, what was the angular resolution of Hubble's Ultra Deep Field measurements/photos? (keeping in mind that angular resolution isn't the only challenge here)
- Thorondor 10y ago
  Hubble's angular resolution is about 0.05 arcsec = 2.4e-7 radians. And you're absolutely right that angular resolution isn't the only challenge here - some way of filtering out the extremely bright (relatively) light from the host star is needed as well, for starters.
throwaway_yy2Di 10y ago

semaphoreP 10y ago

The most promising technique at this point is to use optical interferometry to resolve the surface of the planet. In this case, you will need two telescopes (likely in space) separated by a baseline distance, "d". The two telescopes will require extremely precise synchronization in both spatial position and timing so that the light they collect will interfere at precisely the right phase, but if this can be achieved, the angular resolution of such a telescope in radians is wavelength/d. An earth sized planet has a diameter of ~13,000 km. To "resolve the planet", we would need to be able to distinguish one half of the planet from the other, meaning we need to see at a resolution of ~6000 km on the surface. Proxima Centauri is ~4 light years away, meaning that this requires an angular resolution of 1.5e-10 radians or 30 microarcseconds. That's over 30,000 times smaller than the angular size of a human hair held at arms length!) To achieve 30 microarcsecond resolution with our optical interferometer operating at 600 nm (visible light), we need the two telescopes to be 4 km apart, which practically doesn't sound unfeasible. Alternatively, you could construct a 4 km telescope, but that's far bigger than any optical telescope we have now or in the near future (the biggest telescope in the next 20 years will be 40 meter in diameter).

kilroy123 10y ago
Interesting, I would love to see a feasibility study done on this.
- semaphoreP 10y ago
  NASA actually did do a feasibility study on the Terrestrial Planet Finder (https://en.wikipedia.org/wiki/Terrestrial_Planet_Finder https://en.wikipedia.org/wiki/Terrestrial_Planet_Finder) as a space-based telescope to find terrestial planets orbiting other stars, and one of the versions is an infrared interferometer flying in formation in space. Unfortunately, they never followed through with it (I'm guessing due to technical challenges).