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Given advanced enough telescopic technology, would it be possible to detect that a planet in the order of 50 light years away is inhabitable? (i.e. without having to actually go there)

I'm just looking for the basics here - presence of water, enough oxygen in the atmosphere, temperature, and sunlight (perhaps a bit dimmer than on Earth but still bearable) and assume the technology exists to "fine tune" i.e. remove unexpected toxic gasses from the atmosphere, turn the local algae into plant-growing soil, etc.

But here is the hard part, my scenario involves such a planet that people on Earth decided to colonise, but once they got there, they discovered that it wasn't inhabitable at all - lifeless and without atmosphere like the Moon, as well as unbearably cold and as dark as say, here on Earth in the moonlight.

So my question is, what could lead advanced astronomers to make such a mistake despite being really confident in their initial belief?

I'd prefer an answer where the astronomers got it wrong, rather than circumstances changing during the journey (e.g. a Solar flare destroying the atmosphere or something)

Edit after a bit of thinking time

I've been thinking that a huge undetected magnetic cloud made of handwavium particles between us and them could disrupt the light coming to us, messing with frequencies, etc. However, I think this would make the planet appear darker than it really is - I want it to appear brighter than it really is. Is there any kind of lensing effect something like this could have that would make either the local stars (it's a binary system) or the planet's own reflected light seem brighter? Maybe it could make the planet look closer to the star than it really is and they would deduce that the planet is brighter than it actually is?

It is OK if the astronomers involved are not 100% sure that they are right. External pressures will force them to launch the mission anyway. Also, there are no aliens any more advanced than maybe plant life.

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    $\begingroup$ Just a thought but how many astronomers? One organisation or many? Also how long would they be doing their scanning for? The astronomers making mistakes becomes easier to explain if there was only one group and they had to rush. $\endgroup$ – Rhubis Aug 19 '16 at 12:21
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    $\begingroup$ There has been a case where, for about a decade, there was a credible claim of detection of extrasolar planets that turned out to be nonsense. $\endgroup$ – John Dallman Aug 19 '16 at 13:07
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    $\begingroup$ Too short for a real answer; the telescopes were (accidentally) looking through a worm-hole at a different planet. $\endgroup$ – Marky Aug 19 '16 at 14:06
  • $\begingroup$ The real question seems to be why they did not send some robot there to check the environment. Is that impossible? $\endgroup$ – null Aug 19 '16 at 14:55
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    $\begingroup$ "Habitable" is a bit fuzzy. Read Larry Niven's Known Space series: the early colonies were certainly "habitable" from your definition, but a bit tricky to actually live on -- constant hurricane-force winds, or unexpectedly high gravity, or the only patch of solid ground is half the size of California, or... $\endgroup$ – Mark Aug 19 '16 at 18:45
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Getting any one factor wrong would be unlikely, but quite possible. But getting all of them (temperature, atmosphere, etc) wrong would defy belief.

Temperature would be the hardest to get wrong. Observing all of the planet's radiation gives a spectrum curve that translates into a surface temperature. If anything else was resulting in that radiation, it would have to move with the planet and match its expected output. Only a planet-sized object could do that, which would throw off all other observations.

Presence of water and oxygen will be a little more realistic to get wrong. In any case, you will be looking at some phenomenon that we've never observed before, making it so out-of-the-box that the astronomers don't consider the scenario.

It's all in the moons

During the planet's formation, it somehow lost all the light particles to nearby objects, leaving it without an atmosphere. Much later it captured these objects, giving it a ring of moons that are essentially blobs of dirty oxygen-rich water. Dark dust inside the moons absorbs a lot of sunlight, warming them to just above the melting point but not so much they evaporate. (This situation would normally not be stable over long periods of time)

Note: It needs to be liquid water because absorption lines are different for each phase of water, so astronomers would notice there is a lot of ice but no liquid water if the moons were frozen.

It's the one-in-ten-billion planet

This planet has an atmosphere (but no oxygen) and is covered with seas (but not of water). Instead, it has some amazing organic ooze. For some incredible reason, the ooze absorbs frequencies associated with hydrogen, oxygen, water, it even points to photosynthesis. Except they are all due to strange molecules that have nothing to do with water or anything. Truly a one-in-ten-billion coincidence.

It's a trap!

The astronomers are not to blame: Advanced aliens have put a cloud of nano-engineered particles around the planet that absorb exactly the frequencies that indicate the presence of those substances carbon-based lifeforms look for.

It may be a relic of a long-gone space war or pirate trap, but that no longer matters. Underneath the cloud, there's nothing.

It's... an anti-planet!

This is the most outlandish option: The entire solar system is made of anti-matter. It's exceptionally empty (all the floating anti-matter already having annihilated with normal matter from outside this area), so that the ship is fairly safe, but the lander sent out ahead explodes in a multi-megaton blast on first contact with the outer atmosphere.

The colonists can only stare at that beautiful living world, but they can never go there.

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  • $\begingroup$ Anti-matter seems an interesting idea. It may actually be a wonderful living world and you can observe it visually with any technology you like without suspecting anything. $\endgroup$ – eigenvalue Aug 19 '16 at 15:43
  • $\begingroup$ Too bad you can't have an undetected antimatter planet. There's a fine mist of hydrogen everywhere in space, one atom per cubic milimeter of interstellar medium, and this fine mist would start to glow as soon as it got in contact with the anti-planet. It might not be enough to destroy the planet but it would be enough to tell the scientists this ball emiting a large amount of gamma radiation might not be the best place for a visit. And yes, the lander would provide a really nice firework. $\endgroup$ – John Dvorak Aug 19 '16 at 18:56
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    $\begingroup$ The solar system would have to be far out in a very empty area of space, and anti-hydrogen + solar wind would hopefully have cleared out the hydrogen from the entire system. I have no good explanation for how, but then, that's how the astronomers in the story get fooled too. $\endgroup$ – Cyrus Aug 19 '16 at 21:14
  • $\begingroup$ The moons idea is clever but actual planned techniques will see that the atmosphere is not hugging the planet; and the ring will show up in light curves before doing the spectral measurements. $\endgroup$ – JDługosz Aug 19 '16 at 22:09
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Cold and without light is a huge stretch. Especially when there is no atmosphere. I mean you can have a thick atmosphere blocking the sunlight but without that, it all depends on the location of the planet. Only viable idea is that someone fakes the results.

However, if you are OK with a warm planet with no oxygen. It could happen. We detect elements from the light spectrum they reflect. Some molecule in the air could cause similar reflections to oxygen and can fool astronomers.

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  • $\begingroup$ Could something passing between Earth and the planet have an effect? (e.g. an undetected gas cloud) - I would imagine that this would have the opposite effect and make the planet or its star seem colder and darker than it really was, but would the opposite be possible? $\endgroup$ – colmde Aug 19 '16 at 12:07
  • $\begingroup$ I would say no, because we do detections at different times, giving us the ability to measure the distance. The heat of the planet depends on its location, thus will not be affected by this measurement. $\endgroup$ – Cem Kalyoncu Aug 19 '16 at 12:14
  • $\begingroup$ See this presentation. A simple cloud in the way won’t cause confusion, since the measurement along the limb is compared to background. You get details on layers, not just a simple yes/no. $\endgroup$ – JDługosz Aug 19 '16 at 22:06
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It was nice when the light left

In the last 100 years (50 for light to get to us + 50 to get there) things have gone downhill. Maybe something hit it, maybe locals did themselves in in grand style, maybe alien colonizers strip mined it first.

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It's possible that there was a planetary factor that, in the zeal of trying to escape the Earth or colonize, was forgotten. For example, the orbit could have been erratic, and a small group of scientists saw and understood it, but when they tried to go public the populace rejected them. This was like the discovery of the 'missing link' several years ago which, by chance or intention, was fabricated. So, there could have been a problem with the planet, but the people rejected it due to humanity's confirmation bias.

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Deciding a planet was habitable depends on looking for the presence of molecules that indicate life is present on the planet. The most likely candidates are water, oxygen, and chlorophyll. As discussed in this paper here

Since the planet in question is lifeless and devoid of atmosphere, the presence of water, oxygen and chlorophyll seems unlikely. But if they were present, this give a false positive. So in what circumstances would any of these substances be present to give a false positive?

Firstly, water is most detectable of the trio of life-signatures. What if aliens coated the planet with a layer of water to give a false reading? The coating wouldn't cover the entire planet. Just enough to make it look like an inhabitable planet. The water would have to be contained in thin sandwich of materials that would be transparent in the bandwidths used for detection of water.

If necessary, the aliens might plant oxygen and chlorophyll in similar layers to increase the probability of fooling non-alien astronomers. Deliberative fraud is a possible explanation for why astronomers were deceived into believing they had observed a habitable planet. As for reasons why the aliens have done this can be left an exercise for the reader.

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  • $\begingroup$ Would this still work if it was ice rather than water? $\endgroup$ – colmde Aug 19 '16 at 13:59
  • $\begingroup$ @colmde. Good question! I thought about that myself, but I really don't know the answer. Probably, it is because the molecular structure of H2O is the same in water and ice. They may be physically different forms f the same stuff, but their molecules are the same. $\endgroup$ – a4android Aug 20 '16 at 2:14

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