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If I were to attempt the colonization of a planet with preexisting life, how would I manage to pinpoint a candidate planet from the comforts of Earth's orbit? I am interested in the stars somewhere within 40 light years from Earth, and with relatively realistic technology (no FTL drives or time machines).

I am familiar with the technology behind spectrography, but it seems rather unreliable in terms of launching a multi trillion dollar mission. This suggests` that quite a few gasses that are correlated with life on Earth might not indicate life on exoplanets- so, how can we be certain enough of life to send thousands of colonists to a distant star?

  • Really big telescopes?
  • Spectography?

Different from this question, in that I'm asking about a much larger scale, and not trying to comprehend a trope.

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  • $\begingroup$ If you're looking for real world examples or even theories I don't think you are going find much. Even in science fiction I can't recall any situations where life could be detected across light years. I think you're better off handwaving some miracle device that can do this, but not exactly explain how it works. $\endgroup$ – Cradle2theGabe Oct 18 '16 at 18:10
  • $\begingroup$ Note: your real reason for sending colonists to a distant star is not "we found some interesting gasses." It is typically something extreme enough to warrant multi-trillion dollar expeditions in the first place, like "our planet is dying." $\endgroup$ – Cort Ammon Oct 18 '16 at 19:48
  • $\begingroup$ Related: How far away would an alien civilization need to be for us to not notice them? You can take the answer to that and work your way backwards for one aspect to answering your question. $\endgroup$ – a CVn Oct 18 '16 at 19:54
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    $\begingroup$ The short answer is that there is no remote way to definitively detect life -- the spectroscopy techniques we already have are the best thing we have come up with. Of course, if the life we're trying to observe is advanced enough, they could make their detection much easier by building something artificial and detectable. $\endgroup$ – Sigma Ori Oct 18 '16 at 21:02
  • $\begingroup$ What kind of life do you want? Are you OK with cyanobacteria, or should it be plants and fish etc.? The latter would need a probe, detecting unknown lifeforms from atmospheric trace gases is a lot of guesswork. $\endgroup$ – Karl Oct 19 '16 at 5:05
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You can detect what kind of gases the athmosphere consists of, if you use the so called transit method to research them. That method is up to now the most successful in finding exoplanets by far. And that is how it works:

You simply measure how bright a star is. If a planet orbits a star, it sometimes might happens to be exactly between the star and earth. In that case it darkens the star by a tiny margin, but enought to detect the planet.

A small part of the star is not darkened by the planet itself but by its atmosphere. If you have a very sophisticated telescope you can detect that. You have simply to use spectroscopy and do some math to calculate what you might find there.

While there are a lot of gases that can occur by geologic activity, there are some that are known to be produced only by living organisms. If you find even the slightest hint of one of those, you know there is live.

Remark: Up to now (October 2016) this hasn't been done successfully with exoplanets but some telescopes are being build to do exactly this. It's quite possible that between 2020 and 2030 we get the first evidence of life.

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  • $\begingroup$ So you know what gasses are present, but how is that helping you in detecting live? CO2, methane, and other gasses typically associated with life, could occur naturally as well, so there is no definitive answer. $\endgroup$ – ventsyv Oct 18 '16 at 19:21
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    $\begingroup$ That’s the real answer. See Sniffing Alien Atmospheres: Exoplanet spectrophotometry and Exoplanets: Under a Microscope, and Through a Wide-field Lens where she predicts when such a measurement will first be published. $\endgroup$ – JDługosz Oct 18 '16 at 19:38
  • $\begingroup$ @lurch welcome to worldbuilding! You might want to review my references and edit specific details into your post. Basically, that’s the answer I would have written.☺ $\endgroup$ – JDługosz Oct 18 '16 at 19:43
  • $\begingroup$ Actually, just because a planet orbits a star, that doesn't guarantee that the planet will ever be in inferior conjunction of the star as viewed from Earth, because the ecliptic of the distant solar system may be seen at an angle. For the simplest visualization, think of the galaxy as a cake, with each star system flat in a single layer. If someone in one star system then looks at another star system, conjunctions will only occur within the same layer. Now add to this that not everything is flat, nor neatly layered. $\endgroup$ – a CVn Oct 18 '16 at 20:01
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    $\begingroup$ @ventsyv I suppose it to some degree depends on how absolute the OP needs the answer to be. We can't even quite define what constitutes life here on Earth, so there's a long way to go before we can set stringent criteria for how to detect life on other planets. However, we can work with what we do have and get perhaps good enough approximations of an answer to the question of whether a planet harbors life, and spectroscopy can be a large part of that. $\endgroup$ – a CVn Oct 18 '16 at 20:06
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You send a probe.

Either an ultra-fast fly-by probe that just sends sensor data and images back to Earth or you send a slower but more intelligent Orbiter and Lander combo.

See Mars for more info.

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    $\begingroup$ I was also going to suggest a probe but it sounds like you were looking for something in the realm of remote sensing. Though if something is worth a multi-trillion dollar journey I'm sure they money can be spared to send a probe, which could then employ the techniques discussed in the question you referenced. $\endgroup$ – Cradle2theGabe Oct 18 '16 at 18:13
  • $\begingroup$ @Cradle2theGabe Ah, I assumed that as long as the observer remained at a distance any equipment could be placed in proximity. $\endgroup$ – Scott Downey Oct 18 '16 at 18:26
  • $\begingroup$ Sorry I meant to direct the comment to the question author. I like your idea and find it very practical. $\endgroup$ – Cradle2theGabe Oct 18 '16 at 18:29
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If temperature, radiation levels and water are OK, and you find a high oxygen level, then it is safe to assume some sort of life.

Oxygen can be present without life, but not to >10%, and not on a planet that has an average of 25°C, water, and is not radiation hell due to an unstable mother star.

Don't be to excited: Could be the only thing you finde are rocks coated with a thin, slimy cover of alien cyanobacteria. ;-)

There is a readymade plan for a space-based coronagraph that could easily take the necessary spectra. NASA didn't (yet) like the plan, because it requires an extra satellite which is flown at a distance of kilometres from the telescope in the direction of the target. So it eats a lot of fuel and doesn't last very long. ;-)

Bit more sci-fi? Google for the "exo earth imager".

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You need several probes.

Spectrography gives only few information about elements in the atmosphere, but to be survivable, other measures are important:

  • radiation from the star (do the planet have magnetic field),
  • the stability of climate (maybe we can survive, but we need to be ready for that),
  • stability of the planet system (I mean: if the planet system is new or unstable, it is probable to have frequent meteorites that could damage climate and colony.
  • resources on the crust of planet (to have the right tools to substain the colony).
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  • $\begingroup$ I dont see those issues: The magnetic field is irrelevant if the atmosphere is sufficiently thick. Mars -> problem, earth -> fine, no mass excinction has ever occured during the thousands of years without magnetic field during the many reversals of poles in earths pre-history. Resources? With real bad luck, the planet could be covered in one deep ocean. Otherwise Aluminium and Iron are surely abundant, and the rest of the PSE will also be there. Stability? If we can see one planet, we can also detect the whole system. If it's there and doesn't look very crazy, it is stable. $\endgroup$ – Karl Oct 19 '16 at 5:16

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