Newbie here wondering if there's some consensus answer to the following:

Given a planet similar to Earth (I'll call it Searth), exists in a star system S light years distant...

With our current technology, what's the value of S?

- How dependent would S be on the Searth's sun's magnitude or other characteristics?

- How would the value of S change in 10 years? 20 years? 100 years?

Another question:

How far distant could Earth be detected by an alien civilization with technology 50 years more advance than our current? (I'm assuming they had some sort of orbiting telescope with a clear view at a stable Lagrange point).

Huh? You said S was the number of lightyears distant (from us?) None of those factors you asked about have to do with S, then.

Since we don't know how advanced our technology will be in 50 years, we can't give a clear answer.

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Welcome to BAUT, mike!!!

There's a similar discussion going on over here

These are also related:
- astronomers-please-stop-shouting-out-universe
- our-radio-waves-going-out-into-space-will-they-last-forever

I think you mean distance at which Searth is detectable with current technology.

In that case yes, S depends a great deal on star's characteristics.

Detecting Searth around a main-sequence star is just about impossible at present. IF Searth's orbit is edge-on to us (i.e. from our viewpoint Searth traverses its sun's disk once every orbit) AND the star is a red dwarf, then there is a very small chance one of space telescopes currently in operation will detect the tiny dip in star's brightness. This chance will increase considerably -- and will include yellow stars similar to Sun, -- when Kepler space telescope is launched next February, as Kepler is designed specifically for this purpose. Star's magnitude limit for Kepler is 14, so S here is about 2500 light-years for sunlike stars, perhaps 50-200 light-years for red dwarf stars.

Searth orbiting a neutron star is far easier to detect, and in fact have been found. The detection method is virtually independent of distance, and works as far as a neutron star can be detected.

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AAAAH, now I get it

<i>I think you mean distance at which Searth is detectable with current technology.</i>

Yeah. That's what I meant. I guess posting at 2am local isn't the best.
Pardon my lack of clarity. Sheesh, what a way to start...

Regarding other comments: Yes, I know I'm asking people to speculate when it comes to detection capabilities in 50 years.

Ilya, thanks for the detailed answer.

So this is for detection a an earth-sized planet, right?

I wonder how far away we will be able to also do a basic analysis of its composition, atmosphere (if it has one), etc?

It is plausile that characteristic sonic modulation of the generation of the stellar wind or the characteristic spectrum from the parent star might affect the aurorae of the planets in detectable ways. This should take place in a part of the electromagnetic spectrum that is isolatable from the overwhelming energy of the parent star's radiation.

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You have to be able to detect/isolate actual photons from the planet itself in order to do spectroscopy. Extraordinarily difficult to do with such a small object close to its star at stellar distances. It's not really a matter of distance.

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