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  • #26
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Unless you know a priori that you don't have to check M's, you have to check them. Even if 99.99% of them can't support life, it leaves about 50 in the range. There are also thousands of K's And you need to be able to go down to Europa-sized. So maybe I was wrong with 35 and it's only 34.
 
  • #27
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100 years ago we didn't even know about Pluto, at magnitude ~15. Today we are about to launch a telescope that can find magnitude 34 objects, and LSST will monitor over half of the sky for objects down to magnitude 24-28. If surveys don't reach magnitude 35 or better in the next 100 years something went seriously wrong.
 
  • #28
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The problem with extrapolating is that if you draw a line between Orville Wright and Neil Armstrong, you don't get where were are today. You get Buck Rogers.

I don't want to say this is impossible, just that there are a number of hurdles one needs to jump over, and need to be considered. I am also going to take the OP's "all" (in italics) literally. To ensure that you get them all, you need to be substantially more sensitive that what you need to get most of them.

Step 1: get all 500,000 stars. We're not there yet. If you plot stellar density vs. distance, it starts falling over at 10 or 15 parsecs. We're missing stars that close. So we need at least a 100 times better threshold, and probably more like 500.

Step 2: see the planets. You only need to go out maybe 100 uradians from each star, so you only need to look at maybe 1/1000 of the sky. That's good. And bad - that's still a lot of sky. About 25,000 Hubble deep fields. (About 1500 years equivalent for the Hubble Deep Field).

Step 3: See the planets move. If you don't want fakes, you need at least 5 shots of each system, and more for systems with many planets. So we're at 7500 Hubble-years. If there's a century between now and the story, and we launch one Hubble per year, we'll just about get it.

Step 4: Pull spectra off all these planets. This is very photon-hungry and you'll have millions of targets. Maybe you could get colors with photometry and eliminate ones where you don't need to do a spectrum, but it's still a lot.

So you have maybe four steps to take and 2 or 3 decades for each step. Possible? Sure. Guaranteed? I don't think so.
 
  • #29
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I didn't take "all" literally. I guess 99.9% would be sufficient.
Step 1: get all 500,000 stars. We're not there yet. If you plot stellar density vs. distance, it starts falling over at 10 or 15 parsecs. We're missing stars that close. So we need at least a 100 times better threshold, and probably more like 500.
Get all stars with a chance to have habitable planets. Proxima Centauri is somewhere at the threshold, some people see it below. Absolute magnitude 15.6, easier to see in the infrared, but let's be really pessimistic and say were are looking for magnitude 18 stars. At 330 light years they have an apparent magnitude of 23. LSST will easily see them. Gaia will get distance estimates for all objects down to magnitude 20.

ELT will have 250 times the light collection of Hubble. A single ELT equivalent in space, operating for 20 years, would give you observation time equivalent to 7500 Hubble years. It wouldn't have the angular resolution needed for most planets, so you probably want a larger interferometer with the same mirror area (or more), but that's the point I mentioned earlier: The resolution is the limit, not the light collection.

Spectra of the planets will be much more difficult. You can limit it to planets in the habitable zone, that excludes various stars.

I don't say it is guaranteed, but it's something we can do with projects we could reasonably work on today. An ELT-like telescope in space isn't that outrageously beyond current technology.
 
  • #30
stefan r
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500 ly * 500 nm / (100 m) = 23 million km. Just at the edge of what you need to resolve e.g. something like Earth as separate from the star (as you need much more than 1 standard deviation to see a 1 in a billion contrast), but orders of magnitude too small to see features on the planet directly. Phase curves can still give some information.
With a thriving economy and rapid exploitation of the asteroid belt astronomers should be able to have arrays of telescope lenses and starshades built there. The diameter of the interferometer lens should be around 6 or 7 astronomical units 1012m.
 
  • #31
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The previous comment asked about an OWL-like telescope.
 
  • #32
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May I mention http://www.recons.org/

"The purpose of RECONS (REsearch Consortium On Nearby Stars) is to understand the nature of the Sun's nearest stellar neighbors, both individually and as a population. Our primary goals are to discover "missing" members of the stellar sample within 10 parsecs (32.6 light years), and to characterize all stars and their environments within that distance limit." etc etc...

And, by extrapolation, improve estimates of population beyond this 'thoroughly known space'...

Their next Solar Neighborhood (TSN) Series report (#46) should appear soon...

FWIW, whatever the source, I'm hoping for better data on tau Ceti 'f', a sub-neptunian-ish / super-earth in habitable zone. At present, orbital plane error-bars put an uncomfortably wide range on mass and probable composition...
 
  • #33
chasrob
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May I mention http://www.recons.org/

"The purpose of RECONS (REsearch Consortium On Nearby Stars) is to understand the nature of the Sun's nearest stellar neighbors, both individually and as a population. Our primary goals are to discover "missing" members of the stellar sample within 10 parsecs (32.6 light years), and to characterize all stars and their environments within that distance limit." etc etc...

And, by extrapolation, improve estimates of population beyond this 'thoroughly known space'...

Their next Solar Neighborhood (TSN) Series report (#46) should appear soon...

FWIW, whatever the source, I'm hoping for better data on tau Ceti 'f', a sub-neptunian-ish / super-earth in habitable zone. At present, orbital plane error-bars put an uncomfortably wide range on mass and probable composition...
I've been waiting for them to update their 100 nearest stars list...
 
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  • #34
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Me, too, but the field is currently moving so fast, with so many possibles, probables and confirmed, that even the http://exoplanet.eu/catalog/ is struggling to keep up. Plus there are a zillion follow-up observations to be done. Akin to spectral surveys before those 'fibre-optic strand placed by robot' systems automated the field...
 

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