<p>I had put off answering this question because it seems too broad without specifying what sort of detection methods are proposed. But if you answer it directly from the perspective of - if we were to take the solar system and put it at some distance from us, would we able to detect signs of life on planet Earth - then the answer is probably not.</p>

<p>Using <em>current technology</em> (and by that I mean experiments and telescopes that are available now) we would probably be unable to detect life on Earth even if observed from a distance of a few light years. Therefore there are no stars within this sphere (other than the Sun).</p>

<ol>
<li><p>No planets quite like the Earth have yet been detected around another star. That is to say, none that have a similar mass, radius and orbit at 1 au (or close to it) from a solar-type star [EDIT: There is of course now a close contender in Kepler-452b, though it is 60% bigger than Earth; <a href="http://arxiv.org/abs/1507.06723" rel="noreferrer">Jenkins et al. 2015</a>.]. With current technology, it is just about in reach. Therefore any directed search for life on Earth has a limited number of places in which to start. If you can't detect the planet <em>at all</em> then there is absolutely no chance of looking at its atmospheric composition to look for biomarkers (e.g. oxygen along with a reducing gas like methane, or chlorofluorocarbons from an industrial civilisation - <a href="http://adsabs.harvard.edu/abs/2014ApJ...792L...7L" rel="noreferrer">Lin et al. 2014</a>). The only exoplanets for which atmospheric compositions have been (crudely and tentatively) measured are "hot Jupiters". - giant exoplanets orbiting very close to their parent stars. </p></li>
<li><p>A "blind" search could look for radio signatures and of course this is what SETI has been doing. If we are talking about detecting "Earth", then we must assume that we are not talking about deliberate beamed attempts at communication, and so must rely on detecting random radio "chatter" and accidental signals generated by our civilisation.
The SETI Phoenix project was the most advanced search for radio signals from other intelligent life. Quoting from <a href="http://adsabs.harvard.edu/full/2000ASPC..213..451C" rel="noreferrer">Cullers et al. (2000)</a>: "<em>Typical signals, as opposed to our strongest signals, fall below the detection threshold of most surveys, even if the signal were to originate from the nearest star</em>". Quoting from <a href="http://adsabs.harvard.edu/abs/2001ARA%26A..39..511T" rel="noreferrer">Tarter (2001)</a>: "<em>At current levels of sensitivity, targeted microwave searches could detect the equivalent power of strong TV transmitters at a distance of 1 light year (within which there are no other stars)...</em>". The equivocation in these statements is due to the fact that we <em>do</em> emit stronger beamed signals in certain well-defined directions, for example to conduct metrology in the solar system using radar. Such signals have been calculated to be observable over a thousand light years or more. But these signals are brief, beamed into an extremely narrow angle and unlikely to be repeated. You would have to be very lucky to be observing in the right direction at the right time if you were performing targeted searches. </p></li>
</ol>

<p>Hence my assertion that with current methods and telescopes there is not much chance of success. But of course technology advances and in the next 10-20 years there may be better opportunities.</p>

<p>The first step in a directed search would be to find planets like Earth. The first major opportunity will be with the <a href="http://tess.gsfc.nasa.gov/" rel="noreferrer">TESS spacecraft</a>, launching in 2017, capable of detecting earth-sized planets around the brightest 500,000 stars. However, it's 2-year mission would limit the ability to detect an Earth-analogue. The best bet for finding other Earths will come later (2024 perhaps) with the launch of <a href="http://www.esa.int/Our_Activities/Space_Science/ESA_selects_planet-hunting_PLATO_mission" rel="noreferrer">Plato</a>, a six-year mission that again, studies the brightest stars. However, there is then a big leap forward required to perform studies of the atmospheres of these planets. Direct imaging and spectroscopy would probably require space-borne nulling interferometers; indirect observations of phase-effects and transmission spectroscopy through an exoplanet atmosphere does not require great angular resolution, just massive precision and collecting area. Spectroscopy of something the size of Earth around a normal star will probably require a bigger <em>successor</em> to the James Webb Space Telescope (<a href="http://www.jwst.nasa.gov/" rel="noreferrer">JWST</a> - launch  2018), or even more collecting area than will be provided by the E-ELT in the next decade. For example <a href="http://www.epj-conferences.org/articles/epjconf/abs/2013/08/epjconf_hpcs2012_11001/epjconf_hpcs2012_11001.html" rel="noreferrer">Snellen (2013)</a> argues it would take 80-400 transits-worth of exposure time (i.e. 80-400 years!) to detect the biomarker signal of an Earth-analogue with the E-ELT!</p>

<p>It has been suggested that new radio telescope projects and technology like the Square Kilometre Array may be capable of serendipitously detecting radio "chatter" out to distances of 50 pc ($\sim 150$ light years) - see <a href="http://adsabs.harvard.edu/abs/2007JCAP...01..020L" rel="noreferrer">Loeb &amp; Zaldarriaga (2007)</a>. This array, due to begin full operation some time after 2025 could also monitor a multitude of directions at once for beamed signals. A good overview of what might be possible in the near future is given by <a href="http://adsabs.harvard.edu/abs/2009astro2010S.294T" rel="noreferrer">Tarter et al. (2009)</a>.</p>

CC BY-SA 4.0
2020-01-07T18:07:54.300Z
Jan 2019 - [TESS has officially detected its first Earth-size planet in its star’s habitable zone](https://www.nasa.gov/feature/goddard/2020/nasa-planet-hunter-finds-its-1st-earth-size-habitable-zone-world), orbiting [a red dwarf ~101.4 light years away](https://en.wikipedia.org/wiki/TOI-700)
brichins
CC BY-SA 3.0
2015-01-24T15:33:19.923Z
2017-01-31T12:21:54.590Z
2017-01-31T12:21:54.590Z
ProfRob
<p>I'm finding this very tough to answer, the method of detection is critical in how far we can detect. There are two probable methods I can think of, one superior to the other. The first method involves the speed of light and our production of waves. The second involves how we have tailored our atmosphere.</p>

<p>Our production of waves (radio) started late 19th century, if we use a point of reference, say 1900; we have been broadcasting for 115 years, at light speed a species no further than 115 light years away could detect us. Hence the idea of the SETI program as Rahul has suggested, with the intention of broadcasting ourselves.</p>

<p>The best method, and the one I can see working for humans in their search for others, is atmospheric poisoning. There are specific hydrocarbons in our atmosphere which are thought to only be produced by man, if we think like this, then it's plausible we may also detect atmospheric poisoning around an exoplanet. Detecting oxygen simply is not enough, as it's not indicative that life exists, oxygen can be produced naturally in limited quantities as found elsewhere in the solar system however to sustain carbon based lifeforms such as ourselves there would have to be a major abundance. Detecting pollutants is the more logical way to conceive detection. If we are able to produce elements not naturally found, it is a clear indication that a species put it there. This also relies on the speed of light, however man made pollutants have existed pre-wave era, and have had longer to transmit light than our production of waves. The downside is detection method of the pollutants, currently as humans we rely on either using a star with a transiting planet to determine composition, or less precisely spectrum data (which doesn't indicate atmospheric material).</p>

<p>Another viewpoint is looking at the <a href="https://en.wikipedia.org/wiki/Kardashev_scale" rel="nofollow">Kardashev scale</a>, one could put forth that we have the technology to determine that answer based on energy consumption. If we could detect a massive gravitational field and no apparent source of energy, the energy may well be harvested by another species; such as a Dyson sphere. Such a detection I believe would be too easy to overlook as it's not something our species is actively searching for. While this holds truer for more of a theoretical detection, another species may be able to detect energy consumption on our planet, through lighting of our planet and atmosphere along with increasing surface temperatures.</p>

<p>I believe at best, as for human interference, we may be looking in the range of 100-150 light years. As for detection of life in general, I can't imagine pre-modern era if there was a simple way to determine that life existed if viewed from elsewhere apart from the fact we had a stable system containing liquid water and atmospheric oxygen.</p>

<p>We may be too reliant on providing the argument from our point of view being carbon based lifeforms, if another species advanced or more than us wasn't carbon based, it could very well be they are looking for other indications more localized to their own species, in the same way we look for indications which we imagine detecting ourselves with.</p>

<p><strong>EDIT:</strong> As requested by Rob Jeffries; NO, using transit photometry using today's current technology is not yet possible. At <code>1ly</code> Earth would appear as <code>2.776*10^-4″</code>-><code>3600*(180/π)*(12734/9.460*10^12)</code> or <code>2.776mas</code>, which is possible by <a href="http://www.eso.org/public/teles-instr/vlt/" rel="nofollow">ESO's Very Large Telescope</a> which has an angular resolution able to image in milliarcseconds. At <code>10ly</code> Earth would appear as <code>2.776*10^-5″</code>-><code>3600*(180/π)*(12734/9.460*10^13)</code> or <code>277.6μas</code>, possible after completion of the <a href="http://arxiv.org/abs/1205.5743" rel="nofollow">Cherenkov Telescope Array</a> which has an angular resolution able to image in microarcseconds. Whilst the Cherenkov Telescope Array, is limited to <code>100μas</code> at <code>400nm</code> and not able to image <code>1μas</code>, at this next level it's we are imaging at <code>100ly</code>. The <a href="http://en.wikipedia.org/wiki/Gaia_%28spacecraft%29" rel="nofollow">Gaia spacecraft</a> can resolve up to <code>20μas</code> however is not able to image at this level. The NASA Ames Research Center is demonstrating resolution abilities down to <code>5μas</code> at the attempt to resolve down to <code>1μas</code>, however again that is not imaging resolution. For radio waves, truly enough I had not mentioned the inverse square law and wave degradation. For us as humans, yes a few light-years may be possible with a realm of possibility opening up with the <a href="http://en.wikipedia.org/wiki/Square_Kilometre_Array" rel="nofollow">Square Kilometer Array</a>.</p>

<p>If you want me to retract my guesstimate from the first time, pollution and transit photometry are in fact possible using today's existing technology within <code>1ly</code>, on par with existing radio receivers within <code>1yr</code>. If you deter from the fact that the new instruments are however not built yet you can vastly increase this up to <code>100ly</code>, just because something isn't built does not make the technology non-existent (Is the SKA technology feasible? Yes, we have the technology to make it happen right now, we just haven't done so. That does <strong>not</strong> make it technology that does not exist).</p>

<p><a href="http://www.seti.org/seti-institute/kepler-186f-first-earth-sized-planet-orbiting-in-habitable-zone-of-another-star" rel="nofollow">Seti Home</a> has published finding of the the first earth sized planet detected from transiting. Further publication by <a href="http://arxiv.org/abs/1404.5667" rel="nofollow">Cornell University Library</a> claims that the planet is within the habitable zone and implies it's within possibility of having an atmosphere and liquid H20 on it's surface. The <a href="http://en.wikipedia.org/wiki/Kepler_%28spacecraft%29" rel="nofollow">Kepler</a> spacecraft detected this finding, in case you are unaware, Kepler maps light curves as a body transits across the face of another body, this is called <a href="http://en.wikipedia.org/wiki/Transit_%28astronomy%29" rel="nofollow">Transit</a>. To even suggest that this technology doesn't already exist is preposterous, if you want a true analogue to Earth as is, with technology already existing; <code>1ly</code>, if you want to use technology possible however not built; <code>100ly</code>.</p>

CC BY-SA 3.0
2015-01-24T10:20:45.717Z
The question asks how far away could we detect life, not the other way around. In principle we could detect radio signals from as far away as you like if the signals were powerful enough (or directed). I really can't see where you have come up with the arbitrary figure of 100-150 light years.
ProfRob
CC BY-SA 3.0
2015-01-24T13:18:05.160Z
@RobJeffries, as stated if we were to use radio signals emanating from earth and a point of reference from 1900, that gives a minimum 115 light years. (as light travels 1 light year per year). If we use a figure such as atmospheric pollution, I am unsure when unnatural pollutants began however if you base it from the industrial era that's as early as 1760, given enough time that smog would become abundantly obvious to another species, it could be a later reference point. That expands the range up to 255 light years. If you actually read what I wrote, it's in perspective of another species.
Ashley James
CC BY-SA 3.0
2015-01-24T13:36:35.140Z
@RobJeffries, I have re-read the information from the initial post and I apologize for turning the question around. It does however exactly answer the question posted, **how far away could we detect that earth has life?** I am sorry if the first question on the page contradicts the rest of the content.
Ashley James
CC BY-SA 3.0
2015-01-24T13:55:31.363Z
Well, no it doesn't answer this unless you explain how we could establish that the Earth has life, using current technology, from a distance of 100-150 light years. I don't think that is currently possible at all.
ProfRob
CC BY-SA 3.0
2015-01-24T14:01:26.607Z
@RobJeffries, [Transit](http://en.wikipedia.org/wiki/Transit_%28astronomy%29) explains how we would detect pollution, it's theorized this may work against another planet as a detection method. As for waves, we have been transmitting for over 100 years. I don't need to explain if we have been transmitting for 100 years, we have had technology to receive it for 100 years. I can additionally cite [Radio wave](http://en.wikipedia.org/wiki/Radio_wave) and [Pollution](http://en.wikipedia.org/wiki/Pollution#Urban_Pollution) if you don't understand.
Ashley James
CC BY-SA 3.0
2015-01-24T14:51:20.453Z
We have yet detected NO "earth-like" planets via the transit technique (that is Earth-sized objects in earth-sized orbits around sun-like stars). Currently, the "pollution" idea has been floated as something that might work for close-in planets to a white dwarf WHEN AND IF JWST is working. Yes you DO need to explain how radio/TV signals intended for terrestrial receivers can be detected 100 light years away by current technology on earth - they can't. What might be detected is occasional beamed radar transmissions used for solar system metrology.
ProfRob
CC BY-SA 3.0
2015-01-25T02:15:00.430Z
Transit measurements are not limited by angular resolution, but by photometric precision and the need to fly satellites with a big enough telescope for long enough to detect several transits. That's why Earth-sized, *not* Earth-like planets have been found. Giving them spectrographs capable of transmission spectroscopy for CFCs is a futher step in complexity. JWST may do this, but is incapable of identifying the targets. Angular resolution is required for *direct* imaging, but equally important is contrast. It requires space-based nulling interferometers to do this for an Earth-like planet.
ProfRob
CC BY-SA 3.0
2015-01-25T02:39:48.730Z
I didn't want to extend the discussion however to set it right, we are both wrong factually. Transit techniques are used to map orbital parameters and characteristics, photometry is used to image, or spectrophotometry. Imaging an atmosphere is related to imaging, not orbital parameters, and requires angular resolution to discern the atmosphere from other flux sources. This is typically substituted for a point spread function in cluttered regions. We can find planets smaller, less massive and same distance as Earth, because we haven't found one doesn't mean we don't have technology to do so.
Ashley James
CC BY-SA 3.0
2015-01-25T02:57:50.590Z
Your comments make no sense. The transit techniques *are* photometric they do not require angular resolution and the angular resolution of space and ground-based transit experiments is *poor*. Transmission spectroscopy is done with big telescopes and HST not because it requires angular resolution but because it requires massive *precision*. The question was not about *finding* earths, though that is a prerequisite. I agree, if you have 500 million USD it could be done soon. Finding CFCs in earth-like planet atmospheres cannot.
ProfRob
CC BY-SA 3.0
2015-01-25T03:13:53.837Z
To characterise SKA as current technology is rather demeaning to those who are spending their entire careers *developing* the technologies and computing power required for SKA to be fully operational *at the earliest by 2025*.
ProfRob
CC BY-SA 3.0
2015-01-25T04:19:23.300Z
I will only continue this in chat and downgrade further responses, as I will not make and cannot make a 100 page essay to you in comments. You are attempting to discredit information provided from professional resources. **PUT IT THIS WAY** If you want to image something, you need the resolution to image it. A planet transiting a star passes line of sight, at the edge of transit the atmosphere passes through starlight, this is how we can can detect an atmosphere. A CCD is a photometer to detect and capture light, you can then filter that light into a spectrum.
Ashley James
CC BY-SA 3.0
2015-01-25T04:24:43.870Z
If you want to further ridicule, consider the fact of the questions I answered. My discussion was never talking about detecting atmospheres for an exoplanet, I am talking about Earth from a distance. We have that technology, and I provided examples to do that along with how we could do that. If you want to write a thesis on on transmission spectrography this is not where you do it. If you want to discuss mathematics, I will begin doing my research and will ridicule you every step of the way. Take it elsewhere.
Ashley James
CC BY-SA 3.0
2015-01-25T11:38:13.123Z
It is not ridicule to point out that someone is incorrect; and you continue to be incorrect in your statements about observational techniques to study exoplanets (or the Earth at a distance) and what is currently possible. Please stick to the science and maintain a civilised discussion.
ProfRob
CC BY-SA 3.0
2015-01-24T05:22:35.980Z
2015-01-25T01:00:09.773Z
2015-01-25T01:00:09.773Z
Ashley James
<p>Depends on what you mean by detecting life. As is explained in <a href="https://what-if.xkcd.com/47/" rel="noreferrer">this what-if post by Randall Munroe</a>, the algae on earth will tell the aliens about us before we can tell them about us.</p>

<p>If you consider the presence of liquid water or the presence of $O_2$ as detecting life, then such detection can be made by studying the spectrum of extra-solar planets, measurements that we can currently make. The furthest extra-solar planet discovered so far is <a href="http://exoplanets.co/extrasolar-planets/farthest-exoplanet-from-earth.html" rel="noreferrer">at a distance of 27,700 light years</a>. So, a partial answer to your questions would be to study the spectrum of every extra-solar planet found within the circumstellar habitable zone to look for signatures of tell-tale signs of life. We currently do have the technology to measure the optical reflection spectrum of an extra-solar planet, for example <a href="http://www.eso.org/public/news/eso1002/" rel="noreferrer">ESO's VLT</a>, the <a href="http://spie.org/x109304.xml" rel="noreferrer">Gemini Observatory</a> and the <a href="http://astrobites.org/2012/11/11/detecting-exoplanet-atmospheres-from-the-ground/" rel="noreferrer">OSIRIS instrument on GTC</a> but I don't know if SETI has that capability. You could further look up the work of <a href="http://seagerexoplanets.mit.edu/research.htm" rel="noreferrer">Dr. Sara Seager</a>.</p>

CC BY-SA 3.0
2015-01-17T21:45:06.110Z
Could you elaborate on what current technology we have that enables the detection of oxygen in an exoplanetary spectrum? There have been claims of water (steam) in some hot Jupiters, but that's not oxygen and there can be no liquid water on a hot Jupiter.
ProfRob
CC BY-SA 3.0
2016-09-16T01:39:26.527Z
@RobJeffries: Why claims? The data is out there. Hubble WFC3 and Spitzer in it's warm mission were successful in giving us the first ~20 transmission spectra of Hot Jupiters during their transits. And in those planets that are not dominated by Rayleigh-scattering, usually water is seen. There are even survey papers published. If you're interested in them I can check my notes.
AtmosphericPrisonEscape
CC BY-SA 3.0
2016-09-16T06:17:40.600Z
@AtmosphericPrisonEscape I work in a department with exoplanet experts. They say "claims" - implying that they believe the evidence is less than conclusive. But perhaps things have moved on since my comment 20 months ago.
ProfRob
CC BY-SA 3.0
2016-09-16T21:06:59.357Z
@RobJeffries: Hmm I think this would best be resolved by looking at the data. The spectra reviewed in Sing+2015, doi:10.1038/nature16068  looks conclusive to me, that there is water. But I'm not a spectroscopist, so I cannot say how degenerate those features are with other possible molecules. Maybe you have some time/motivation to skim over the article.
AtmosphericPrisonEscape
CC BY-SA 3.0
2016-09-16T23:45:04.900Z
@AtmosphericPrisonEscape It hardly changes my point, which is that the only chance of detecting something at present is *steam* in the atmospheres of transiting hot Jupiters, which is not liquid water on an Earth analogue. JWST will improve matters a lot but it isn't here yet.
ProfRob
CC BY-SA 3.0
2016-09-17T00:38:22.213Z
@RobJeffries: True, I shall retract my critique and vow to read the answer in question more carefully next time.
AtmosphericPrisonEscape
CC BY-SA 3.0
2014-12-05T19:53:28.853Z
2014-12-05T19:53:28.853Z
Poruri Sai Rahul
<p>My guess is that life-bearing planets are too far apart to be detected. I think we can only find the ones within a sphere around our planet that is hundreds of light years in diameter but I suspect life-bearing planets may be much farther away than that.</p>
<p>I would like to estimate the diameter of the sphere within which we could detect life on another planet and then estimate the probability that there is life within that sphere.</p>
<p>For example, given our current technology, what is the furthest distance would be able to detect life on Earth?
How many stars like our Sun are in that sphere?
How long would it take for <a href="http://www.seti.org/" rel="nofollow noreferrer">SETI</a> to rule out each of those stars?</p>

CC BY-SA 3.0
2014-12-04T23:21:31.300Z
There are a lot of ways we could figure out that there's life somewhere, such as radio transmissions. Is there a specific method you have in mind, or is this more of a general overview?
HDE 226868
CC BY-SA 3.0
2014-12-04T23:33:35.847Z
I'm not sure which is the best detection technology but we should pick the one that allows us to have the biggest detection sphere. If that detection sphere contains too many stars to search, we could select a smaller sphere based on our estimates that there should be a least one life bearing planet within the smaller sphere.
Software Framework
CC BY-SA 3.0
2014-12-05T15:36:15.470Z
Related question: http://space.stackexchange.com/questions/1766/what-properties-are-used-to-quantify-the-odds-of-a-star-harboring-earth-like-lif
Jerard Puckett
<exoplanet><life>
How far away could we detect that Earth has life?
Software Framework
# How far away could we detect that Earth has life?

(Asked by: username_0 on 2014-12-04)

My guess is that life-bearing planets are too far apart to be detected. I think we can only find the ones within a sphere around our planet that is hundreds of light years in diameter but I suspect life-bearing planets may be much farther away than that.

I would like to estimate the diameter of the sphere within which we could detect life on another planet and then estimate the probability that there is life within that sphere.

For example, given our current technology, what is the furthest distance would be able to detect life on Earth?
How many stars like our Sun are in that sphere?
How long would it take for [SETI](http://www.seti.org/) to rule out each of those stars?

**Tags:** exoplanet, life

> **Comments:**
> **username_1**: There are a lot of ways we could figure out that there's life somewhere, such as radio transmissions. Is there a specific method you have in mind, or is this more of a general overview?  
> **username_0**: I'm not sure which is the best detection technology but we should pick the one that allows us to have the biggest detection sphere. If that detection sphere contains too many stars to search, we could select a smaller sphere based on our estimates that there should be a least one life bearing planet within the smaller sphere.  
> **username_2**: Related question: http://space.stackexchange.com/questions/1766/what-properties-are-used-to-quantify-the-odds-of-a-star-harboring-earth-like-lif  

---

## 3 answers

### Answer by username_6 on 2014-12-05. Score: 8 | Accepted answer

Depends on what you mean by detecting life. As is explained in [this what-if post by Randall Munroe](https://what-if.xkcd.com/47/), the algae on earth will tell the aliens about us before we can tell them about us.

If you consider the presence of liquid water or the presence of $O\_2$ as detecting life, then such detection can be made by studying the spectrum of extra-solar planets, measurements that we can currently make. The furthest extra-solar planet discovered so far is [at a distance of 27,700 light years](http://exoplanets.co/extrasolar-planets/farthest-exoplanet-from-earth.html). So, a partial answer to your questions would be to study the spectrum of every extra-solar planet found within the circumstellar habitable zone to look for signatures of tell-tale signs of life. We currently do have the technology to measure the optical reflection spectrum of an extra-solar planet, for example [ESO's VLT](http://www.eso.org/public/news/eso1002/), the [Gemini Observatory](http://spie.org/x109304.xml) and the [OSIRIS instrument on GTC](http://astrobites.org/2012/11/11/detecting-exoplanet-atmospheres-from-the-ground/) but I don't know if SETI has that capability. You could further look up the work of [Dr. Sara Seager](http://seagerexoplanets.mit.edu/research.htm).

> **Comments:**
> **username_3**: Could you elaborate on what current technology we have that enables the detection of oxygen in an exoplanetary spectrum? There have been claims of water (steam) in some hot Jupiters, but that's not oxygen and there can be no liquid water on a hot Jupiter.  
> **username_7**: @RobJeffries: Why claims? The data is out there. Hubble WFC3 and Spitzer in it's warm mission were successful in giving us the first ~20 transmission spectra of Hot Jupiters during their transits. And in those planets that are not dominated by Rayleigh-scattering, usually water is seen. There are even survey papers published. If you're interested in them I can check my notes.  
> **username_3**: @username_7 I work in a department with exoplanet experts. They say "claims" - implying that they believe the evidence is less than conclusive. But perhaps things have moved on since my comment 20 months ago.  
> **username_7**: @RobJeffries: Hmm I think this would best be resolved by looking at the data. The spectra reviewed in Sing+2015, doi:10.1038/nature16068  looks conclusive to me, that there is water. But I'm not a spectroscopist, so I cannot say how degenerate those features are with other possible molecules. Maybe you have some time/motivation to skim over the article.  
> **username_3**: @username_7 It hardly changes my point, which is that the only chance of detecting something at present is *steam* in the atmospheres of transiting hot Jupiters, which is not liquid water on an Earth analogue. JWST will improve matters a lot but it isn't here yet.  
> **username_7**: @RobJeffries: True, I shall retract my critique and vow to read the answer in question more carefully next time.  

---

### Answer by username_3 on 2015-01-24. Score: 14

I had put off answering this question because it seems too broad without specifying what sort of detection methods are proposed. But if you answer it directly from the perspective of - if we were to take the solar system and put it at some distance from us, would we able to detect signs of life on planet Earth - then the answer is probably not.

Using *current technology* (and by that I mean experiments and telescopes that are available now) we would probably be unable to detect life on Earth even if observed from a distance of a few light years. Therefore there are no stars within this sphere (other than the Sun).

1. No planets quite like the Earth have yet been detected around another star. That is to say, none that have a similar mass, radius and orbit at 1 au (or close to it) from a solar-type star [EDIT: There is of course now a close contender in Kepler-452b, though it is 60% bigger than Earth; [Jenkins et al. 2015](http://arxiv.org/abs/1507.06723).]. With current technology, it is just about in reach. Therefore any directed search for life on Earth has a limited number of places in which to start. If you can't detect the planet *at all* then there is absolutely no chance of looking at its atmospheric composition to look for biomarkers (e.g. oxygen along with a reducing gas like methane, or chlorofluorocarbons from an industrial civilisation - [Lin et al. 2014](http://adsabs.harvard.edu/abs/2014ApJ...792L...7L)). The only exoplanets for which atmospheric compositions have been (crudely and tentatively) measured are "hot Jupiters". - giant exoplanets orbiting very close to their parent stars.
2. A "blind" search could look for radio signatures and of course this is what SETI has been doing. If we are talking about detecting "Earth", then we must assume that we are not talking about deliberate beamed attempts at communication, and so must rely on detecting random radio "chatter" and accidental signals generated by our civilisation.
   The SETI Phoenix project was the most advanced search for radio signals from other intelligent life. Quoting from [Cullers et al. (2000)](http://adsabs.harvard.edu/full/2000ASPC..213..451C): "*Typical signals, as opposed to our strongest signals, fall below the detection threshold of most surveys, even if the signal were to originate from the nearest star*". Quoting from [Tarter (2001)](http://adsabs.harvard.edu/abs/2001ARA%26A..39..511T): "*At current levels of sensitivity, targeted microwave searches could detect the equivalent power of strong TV transmitters at a distance of 1 light year (within which there are no other stars)...*". The equivocation in these statements is due to the fact that we *do* emit stronger beamed signals in certain well-defined directions, for example to conduct metrology in the solar system using radar. Such signals have been calculated to be observable over a thousand light years or more. But these signals are brief, beamed into an extremely narrow angle and unlikely to be repeated. You would have to be very lucky to be observing in the right direction at the right time if you were performing targeted searches.

Hence my assertion that with current methods and telescopes there is not much chance of success. But of course technology advances and in the next 10-20 years there may be better opportunities.

The first step in a directed search would be to find planets like Earth. The first major opportunity will be with the [TESS spacecraft](http://tess.gsfc.nasa.gov/), launching in 2017, capable of detecting earth-sized planets around the brightest 500,000 stars. However, it's 2-year mission would limit the ability to detect an Earth-analogue. The best bet for finding other Earths will come later (2024 perhaps) with the launch of [Plato](http://www.esa.int/Our_Activities/Space_Science/ESA_selects_planet-hunting_PLATO_mission), a six-year mission that again, studies the brightest stars. However, there is then a big leap forward required to perform studies of the atmospheres of these planets. Direct imaging and spectroscopy would probably require space-borne nulling interferometers; indirect observations of phase-effects and transmission spectroscopy through an exoplanet atmosphere does not require great angular resolution, just massive precision and collecting area. Spectroscopy of something the size of Earth around a normal star will probably require a bigger *successor* to the James Webb Space Telescope ([JWST](http://www.jwst.nasa.gov/) - launch 2018), or even more collecting area than will be provided by the E-ELT in the next decade. For example [Snellen (2013)](http://www.epj-conferences.org/articles/epjconf/abs/2013/08/epjconf_hpcs2012_11001/epjconf_hpcs2012_11001.html) argues it would take 80-400 transits-worth of exposure time (i.e. 80-400 years!) to detect the biomarker signal of an Earth-analogue with the E-ELT!

It has been suggested that new radio telescope projects and technology like the Square Kilometre Array may be capable of serendipitously detecting radio "chatter" out to distances of 50 pc ($\sim 150$ light years) - see [Loeb & Zaldarriaga (2007)](http://adsabs.harvard.edu/abs/2007JCAP...01..020L). This array, due to begin full operation some time after 2025 could also monitor a multitude of directions at once for beamed signals. A good overview of what might be possible in the near future is given by [Tarter et al. (2009)](http://adsabs.harvard.edu/abs/2009astro2010S.294T).

> **Comments:**
> **username_4**: Jan 2019 - [TESS has officially detected its first Earth-size planet in its star’s habitable zone](https://www.nasa.gov/feature/goddard/2020/nasa-planet-hunter-finds-its-1st-earth-size-habitable-zone-world), orbiting [a red dwarf ~101.4 light years away](https://en.wikipedia.org/wiki/TOI-700)  

---

### Answer by username_5 on 2015-01-24. Score: 2

I'm finding this very tough to answer, the method of detection is critical in how far we can detect. There are two probable methods I can think of, one superior to the other. The first method involves the speed of light and our production of waves. The second involves how we have tailored our atmosphere.

Our production of waves (radio) started late 19th century, if we use a point of reference, say 1900; we have been broadcasting for 115 years, at light speed a species no further than 115 light years away could detect us. Hence the idea of the SETI program as Rahul has suggested, with the intention of broadcasting ourselves.

The best method, and the one I can see working for humans in their search for others, is atmospheric poisoning. There are specific hydrocarbons in our atmosphere which are thought to only be produced by man, if we think like this, then it's plausible we may also detect atmospheric poisoning around an exoplanet. Detecting oxygen simply is not enough, as it's not indicative that life exists, oxygen can be produced naturally in limited quantities as found elsewhere in the solar system however to sustain carbon based lifeforms such as ourselves there would have to be a major abundance. Detecting pollutants is the more logical way to conceive detection. If we are able to produce elements not naturally found, it is a clear indication that a species put it there. This also relies on the speed of light, however man made pollutants have existed pre-wave era, and have had longer to transmit light than our production of waves. The downside is detection method of the pollutants, currently as humans we rely on either using a star with a transiting planet to determine composition, or less precisely spectrum data (which doesn't indicate atmospheric material).

Another viewpoint is looking at the [Kardashev scale](https://en.wikipedia.org/wiki/Kardashev_scale), one could put forth that we have the technology to determine that answer based on energy consumption. If we could detect a massive gravitational field and no apparent source of energy, the energy may well be harvested by another species; such as a Dyson sphere. Such a detection I believe would be too easy to overlook as it's not something our species is actively searching for. While this holds truer for more of a theoretical detection, another species may be able to detect energy consumption on our planet, through lighting of our planet and atmosphere along with increasing surface temperatures.

I believe at best, as for human interference, we may be looking in the range of 100-150 light years. As for detection of life in general, I can't imagine pre-modern era if there was a simple way to determine that life existed if viewed from elsewhere apart from the fact we had a stable system containing liquid water and atmospheric oxygen.

We may be too reliant on providing the argument from our point of view being carbon based lifeforms, if another species advanced or more than us wasn't carbon based, it could very well be they are looking for other indications more localized to their own species, in the same way we look for indications which we imagine detecting ourselves with.

**EDIT:** As requested by Rob Jeffries; NO, using transit photometry using today's current technology is not yet possible. At `1ly` Earth would appear as `2.776*10^-4″`->`3600*(180/π)*(12734/9.460*10^12)` or `2.776mas`, which is possible by [ESO's Very Large Telescope](http://www.eso.org/public/teles-instr/vlt/) which has an angular resolution able to image in milliarcseconds. At `10ly` Earth would appear as `2.776*10^-5″`->`3600*(180/π)*(12734/9.460*10^13)` or `277.6μas`, possible after completion of the [Cherenkov Telescope Array](http://arxiv.org/abs/1205.5743) which has an angular resolution able to image in microarcseconds. Whilst the Cherenkov Telescope Array, is limited to `100μas` at `400nm` and not able to image `1μas`, at this next level it's we are imaging at `100ly`. The [Gaia spacecraft](http://en.wikipedia.org/wiki/Gaia_%28spacecraft%29) can resolve up to `20μas` however is not able to image at this level. The NASA Ames Research Center is demonstrating resolution abilities down to `5μas` at the attempt to resolve down to `1μas`, however again that is not imaging resolution. For radio waves, truly enough I had not mentioned the inverse square law and wave degradation. For us as humans, yes a few light-years may be possible with a realm of possibility opening up with the [Square Kilometer Array](http://en.wikipedia.org/wiki/Square_Kilometre_Array).

If you want me to retract my guesstimate from the first time, pollution and transit photometry are in fact possible using today's existing technology within `1ly`, on par with existing radio receivers within `1yr`. If you deter from the fact that the new instruments are however not built yet you can vastly increase this up to `100ly`, just because something isn't built does not make the technology non-existent (Is the SKA technology feasible? Yes, we have the technology to make it happen right now, we just haven't done so. That does **not** make it technology that does not exist).

[Seti Home](http://www.seti.org/seti-institute/kepler-186f-first-earth-sized-planet-orbiting-in-habitable-zone-of-another-star) has published finding of the the first earth sized planet detected from transiting. Further publication by [Cornell University Library](http://arxiv.org/abs/1404.5667) claims that the planet is within the habitable zone and implies it's within possibility of having an atmosphere and liquid H20 on it's surface. The [Kepler](http://en.wikipedia.org/wiki/Kepler_%28spacecraft%29) spacecraft detected this finding, in case you are unaware, Kepler maps light curves as a body transits across the face of another body, this is called [Transit](http://en.wikipedia.org/wiki/Transit_%28astronomy%29). To even suggest that this technology doesn't already exist is preposterous, if you want a true analogue to Earth as is, with technology already existing; `1ly`, if you want to use technology possible however not built; `100ly`.

> **Comments:**
> **username_3**: The question asks how far away could we detect life, not the other way around. In principle we could detect radio signals from as far away as you like if the signals were powerful enough (or directed). I really can't see where you have come up with the arbitrary figure of 100-150 light years.  
> **username_5**: @RobJeffries, as stated if we were to use radio signals emanating from earth and a point of reference from 1900, that gives a minimum 115 light years. (as light travels 1 light year per year). If we use a figure such as atmospheric pollution, I am unsure when unnatural pollutants began however if you base it from the industrial era that's as early as 1760, given enough time that smog would become abundantly obvious to another species, it could be a later reference point. That expands the range up to 255 light years. If you actually read what I wrote, it's in perspective of another species.  
> **username_5**: @RobJeffries, I have re-read the information from the initial post and I apologize for turning the question around. It does however exactly answer the question posted, **how far away could we detect that earth has life?** I am sorry if the first question on the page contradicts the rest of the content.  
> **username_3**: Well, no it doesn't answer this unless you explain how we could establish that the Earth has life, using current technology, from a distance of 100-150 light years. I don't think that is currently possible at all.  
> **username_5**: @RobJeffries, [Transit](http://en.wikipedia.org/wiki/Transit_%28astronomy%29) explains how we would detect pollution, it's theorized this may work against another planet as a detection method. As for waves, we have been transmitting for over 100 years. I don't need to explain if we have been transmitting for 100 years, we have had technology to receive it for 100 years. I can additionally cite [Radio wave](http://en.wikipedia.org/wiki/Radio_wave) and [Pollution](http://en.wikipedia.org/wiki/Pollution#Urban_Pollution) if you don't understand.  
> **username_3**: We have yet detected NO "earth-like" planets via the transit technique (that is Earth-sized objects in earth-sized orbits around sun-like stars). Currently, the "pollution" idea has been floated as something that might work for close-in planets to a white dwarf WHEN AND IF JWST is working. Yes you DO need to explain how radio/TV signals intended for terrestrial receivers can be detected 100 light years away by current technology on earth - they can't. What might be detected is occasional beamed radar transmissions used for solar system metrology.  
> **username_3**: Transit measurements are not limited by angular resolution, but by photometric precision and the need to fly satellites with a big enough telescope for long enough to detect several transits. That's why Earth-sized, *not* Earth-like planets have been found. Giving them spectrographs capable of transmission spectroscopy for CFCs is a futher step in complexity. JWST may do this, but is incapable of identifying the targets. Angular resolution is required for *direct* imaging, but equally important is contrast. It requires space-based nulling interferometers to do this for an Earth-like planet.  
> **username_5**: I didn't want to extend the discussion however to set it right, we are both wrong factually. Transit techniques are used to map orbital parameters and characteristics, photometry is used to image, or spectrophotometry. Imaging an atmosphere is related to imaging, not orbital parameters, and requires angular resolution to discern the atmosphere from other flux sources. This is typically substituted for a point spread function in cluttered regions. We can find planets smaller, less massive and same distance as Earth, because we haven't found one doesn't mean we don't have technology to do so.  
> **username_3**: Your comments make no sense. The transit techniques *are* photometric they do not require angular resolution and the angular resolution of space and ground-based transit experiments is *poor*. Transmission spectroscopy is done with big telescopes and HST not because it requires angular resolution but because it requires massive *precision*. The question was not about *finding* earths, though that is a prerequisite. I agree, if you have 500 million USD it could be done soon. Finding CFCs in earth-like planet atmospheres cannot.  
> **username_3**: To characterise SKA as current technology is rather demeaning to those who are spending their entire careers *developing* the technologies and computing power required for SKA to be fully operational *at the earliest by 2025*.  
> **username_5**: I will only continue this in chat and downgrade further responses, as I will not make and cannot make a 100 page essay to you in comments. You are attempting to discredit information provided from professional resources. **PUT IT THIS WAY** If you want to image something, you need the resolution to image it. A planet transiting a star passes line of sight, at the edge of transit the atmosphere passes through starlight, this is how we can can detect an atmosphere. A CCD is a photometer to detect and capture light, you can then filter that light into a spectrum.  
> **username_5**: If you want to further ridicule, consider the fact of the questions I answered. My discussion was never talking about detecting atmospheres for an exoplanet, I am talking about Earth from a distance. We have that technology, and I provided examples to do that along with how we could do that. If you want to write a thesis on on transmission spectrography this is not where you do it. If you want to discuss mathematics, I will begin doing my research and will ridicule you every step of the way. Take it elsewhere.  
> **username_3**: It is not ridicule to point out that someone is incorrect; and you continue to be incorrect in your statements about observational techniques to study exoplanets (or the Earth at a distance) and what is currently possible. Please stick to the science and maintain a civilised discussion.  

---

