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With our current technology, or technology available in the near future (up to 2025), how would we detect a planet exactly like our own, and how close would it have to be to be detectable?

Which methods thus far have proven the most effective for detecting Earth-like planets and what would they be able to reveal regarding our planet's atmosphere, orbital properties, and our species?

This question is interesting food for thought on both how easily an extraterrestrial civilization with comparable technology might find us, and for how easily we might find them.

For the purpose of the question we will assume that there is a planet with a civilisation with equivalent technological advancements that could exist anywhere.

Jordan
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    What I'm trying to get at is that if there was a planet/civilisation exactly like us, would we be able to detect that. – Jordan Nov 30 '13 at 06:30
  • see also http://astronomy.stackexchange.com/questions/8605/if-alpha-centauri-as-solar-system-exactly-mirrored-our-own-what-would-we-be-ab?lq=1 – ProfRob May 03 '15 at 14:21
  • If they were exactly like us and within 60LY we would be watching their "I Love Lucy" shows. – Jack R. Woods Sep 14 '15 at 20:03
  • Actually, one of the answers below talks about what would be needed to watch Lucy. There are obviously a lot of people who support SETI who think we can find intelligent life with technology we have right now. If you want "exactly like us", that probably doesn't exist in our galaxy given the tiny period of time we have been technological and how quickly technology changes. Assuming you are speaking hypothetically and talking about a technical civilization, it would depend on whether or not they want to be found. Otherwise, we're back to looking for Lucy (figuratively speaking). – Jack R. Woods Sep 14 '15 at 20:34
  • Afaik an Earth-like civilization would be detectable by their radio output from 100 ly away. – peterh Dec 14 '20 at 11:18

4 Answers4

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Using current technology (and by that I mean experiments and telescopes that are available now) we have not detected an "Earth-like" planet and we would probably be unable to detect life on Earth even if observed from a distance of a few light years. Therefore there is currently no prospect of detecting life on an "Earth doppelganger". I elaborate below:

  1. No planets 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. With current technology, it is just out of reach. Therefore any directed search for life on an Earth-like planet wouldn't actually know where 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). 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): "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): "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, 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, 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 - launch 2018), or even more collecting area than will be provided by the E-ELT in the next decade. For example Snellen (2013) 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). 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).

ProfRob
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  • Perhaps you should be asking how hard is it to find us. I know we aren't supposed to give opinion here, but I would suggest that any "nearby" civilization has known for a long time that there is life here (or if they are within 100LY, assuming super-luminal communication is impossible, know that we are technological). I think that it will be possible for us to send probes to other stars one day and that they probably already have. In other words, there may be "super-stealth" probes in our own solar system right now that we will never detect. (Thank-you Mr. Clarke.) – Jack R. Woods Sep 14 '15 at 21:25
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    Would you consider updating this excellent answer with any new results from the five years since you posted it? – N. Virgo Mar 18 '20 at 14:11
  • @jack-r-woods That's really wildly speculative (basically science fiction) and not the purpose of this SE. – StephenG - Help Ukraine Jun 05 '23 at 19:57
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Avi Loeb and Edwin Turner have written a paper about the possibility of detecting a civilization similar to our own on another world by looking for city lights.

Their proposed method suggests observing the dark sides of planets when they transit in front of their parent star. Though they said that this method will require future generations of telescopes.

Source:

called2voyage
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"probably not". Unless it were so close that television or radio signals like we've been inadvertently sending out can be detected above background radiation levels we can't know about the civilisation.
The signals are extremely weak, and not on any band scientifically interesting (deliberately of course, because we don't want our televisions to pick up natural signals that would interfere with our artificial signals) unless your scientific interest is specifically to snoop on communications sent out by potential aliens (SETI comes to mind, a nice hobby and something to do with radio telescopes for the times there's nothing more interesting to listen to).

As to planets the size of earth, we've never yet been able to find one, let alone a rocky one in the habitable zone of a star. The mechanisms used to detect exoplanets simply aren't sensitive enough.

Leaves one option: their equivalent of Voyager 1 enters our solar system and we actually spot and recognise it for what it is, and find a way to recover it. But If they're at our level of technology it will be thousands of years yet before that happens.

jwenting
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    The universe is pretty old, they might have sent it a long time a go in a galaxy far away... – Jordan Dec 06 '13 at 09:13
  • you want to detect something that's identical to us, assumption is they're identical to us right now... – jwenting Dec 06 '13 at 09:35
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    That's not the point of the question, assuming there is a planet identical to us, would we be able to detect it and realise it has intelligent life. – Jordan Dec 07 '13 at 06:09
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    @JordanBrown detect it has intelligent life NOW? No. Speed of light makes that impossible. Detect it might at some point have had intelligent life? Only if they sent out incredibly strong signals (way beyond what we're doing, and as our technology improves what we send out gets weaker as we're moving away from omnidirectional EM transmissions) and/or is very, very close. – jwenting Dec 09 '13 at 06:03
  • -1 Radio signals are not the only way to detect a civilization. – called2voyage Dec 09 '13 at 15:33
  • @called2voyage I'm nowhere saying they are, but the same principle holds for every other signal as well (unless you claim to have a way to send out a type of signal that does not degrade in strength over interstellar distances AND can be detected by us). – jwenting Dec 10 '13 at 06:01
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    "As to planets the size of earth, we've never yet been able to find one, let alone a rocky one in the habitable zone of a star." -- This is out of date: http://en.wikipedia.org/wiki/Exoplanet#Earth-size_planets – Keith Thompson Jan 19 '15 at 21:48
  • If SKA starts working before 2025 (that is roughly when it is scheduled) then it would be capable of detecting leaked radio/TV signals similar to those we emit and out to an interesting number of parsecs away. – ProfRob May 03 '15 at 14:20
  • @RobJeffries which changes nothing. You'd still detect a signal that was sent thousands to tens of thousands of years ago. The civilisation that sent it may well no longer exist. The planet may no longer exist. It may no longer be habitable. It may never have been earth like (it's feasible that life develops on planets that aren't earth like, though we'd likely not recognise it as such by looking at it) – jwenting May 03 '15 at 18:22
  • @jwenting "which changes nothing" ? Beg pardon - the detection of an unmistakeable signature of intelligent life elsewhere in the universe would be the single biggest discovery in the history of science. The paper by Loeb & Zaldarriaga I reference in my answer suggest detectability to 150 light years. i.e. The signals would be <150 years old, though the age of the signals is actually what changes nothing. They suggest exactly what you suggest in the first sentence of your answer is possible when SKA begins operation. – ProfRob May 03 '15 at 20:11
  • @RobJeffries it changes nothing in the fact that we can't ever detect if a planet is earthlike NOW. We can only ever detect if it were earthlike at some point in the (distant) past. – jwenting May 03 '15 at 20:46
  • @jwenting I think we have different definitions of distant: <150 years old does not sound so distant to me. – called2voyage Nov 11 '15 at 19:46
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If there were a planet with same civilazion "exactly like us", probability that it were near enough to be detectable are extremely remote. Not zero, but as low as those of trespassing a wall by means of quantum mechanics.

Envite
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    In recent years we have been discovering hundreds of exoplanets and your saying that if one of those was as advanced as us, we would have no way of telling? – Jordan Dec 06 '13 at 03:32
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    @JordanBrown any planet the size of earth would be extremely difficult to detect, yes. Any civilisation on the level of our own on such a planet would be impossible to detect with any sensors we possess. – jwenting Dec 06 '13 at 08:25
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    -1 Does not answer the question. Even if the probability were remote, there are definite techniques that could be used to detect such a planet should it happen to be close enough to detect. The question asks "how would we" not "could we" so this does not answer the question. – called2voyage Dec 09 '13 at 15:35
  • @called2voyage IF there were such a planet close enough to detect, it would be one of our Solar System, so it would have been detected long ago. We can not detect them nowadays (nor in any near future), as explained by jwenting – Envite Dec 09 '13 at 23:34