"you'll have to set up camp on the next nearest lifeless ball" - or just send a robot? Or a few disposable colonists?

Also a dead world can be dangerous: image a world with plenty of exposed asbestos deposits.

@DohnJoe doesn't even need to be an actual toxin... the moon and mars are covered in very fine highly abrasive dust that will play havoc with seals and mechanical equipment and cause no end of problems with static electricity and are so fine that they'll end up in your bloodstream if you breathe contaminated air. Possibly toxic, possibly carcinogenic, definitely bad news.

I think that the statistically relevant number of people could be used to drive the plot. There might not be anything wrong with sampling with humans from their perspective. You should not assert present ethical paradigms to space colonists.

@user3644640 space travel is unlikely to be cheap, outwith magical science-fantasy hyperdrives. Transporting hundreds of sacrificial humans to see if a world might be habitable over a 30+ year experiment seems like a pretty pointless and extremely wasteful gamble, compared to just visiting a sterile world with a hydrosphere.

@StarfishPrime If you have a fleet it can easily leave behind colonies of hundreds with means to communicate whether the place is liveable. Per the colony, the cost is marginal compared to operating a fleet. Traveltimes between worlds with hydrosphere might be quite long, so the fleet has time to reproduce the lost people. They might not be that picky if the planet does not happen to be sterile because they are already there and desperate for resources as they seldom find a new suitable planet. The protagonist could be part of such colonist team that gets one of these unsterile missions.

@user3644640 sure, if you handwave away the difficulties of interstellar travel, then interstellar travel is easy.

@StarfishPrime I'm guessing – extrapolating from a limited sample – that “active hydrosphere” makes the dust less jagged.

@AntonSherwood sure, but it was just an example. It doesn't have to rain cyanide and be filled with things that fart nitromethane in order to ruin everything ;-)

Slugs that use a neurotoxin as locomotive slime, alien algae that grow rather well in the jelly of the eyeball, remnants of an ancient civilisation’s unthinkably large power plant... All this and more can be found at your nearest lovable livable world!

“Congressman, can you explain to our TV audience why one generation ship is named The Intrepid Explorer and the other is Canon Fodder?” “Well, we are going to need statistically significant interaction testing...”

@JoeBloggs "We named this planet Golidlocks World not because it was somehow just right, but because of the presence of angry, hungry ursoids. HappiWerld Colonial Corporation would like to apologise for any confusion that may have arisen."

A productive observation for understanding this seemingly counterintuitive observation: consider what happens when Earth pathogens cross species boundaries. This includes most "plague" diseases, as well as ebola, among many others. The problem is that the pathogen usually doesn't actually want its host to die quickly: it usually propagates better if its host survives at least for a relatively long time.

(Cont.) But when pathogens cross species lines, the substances they produce wind up being much nastier to the new type of host than the old one. On an alien world this would probably be similar, except it is possible that their biochemistry would be so foreign as to barely interact with our biochemistry.

@StarfishPrime I am describing a moving group responsible of finding and colonizing many new worlds. I would assume that moving many people at once is cheaper per person than a few colonizing parties individually. With great enough number of people reproducing ~500 could reproduce 100 people of excess to leave per planet. With a civilization able of interstellar travel, the reason to colonize has something to do with restrictions of physics like not being able to create energy or material (or alternate freely between them). It is not a lifetime where the project must yield profit.

@user3644640 even if you can afford to bring along a load of sacrificial people, their presence does not guarantee success. They would be part of a 30+ year experiment, which might fail (rendering your whole colonisation operation a failure and possibly a doomed one) during which time you must set up camp somewhere other than the world with a biosphere anyway, and you'll have to stay there if the experiment tells you that biosphere is hazardous. This is a pointless gamble, especially if you have cheap interstellar travel.

@StarfishPrime I am talking about many hundreds years mission. Determining whether the planet has life before getting close enough could be impossible from the distance of tens of light years. It is because it is expensive that you would sacrifice 100 people and continue their journey; not stay there and wait. If that group fails, it is not that big of a deal because the fleet has a long distance to the next world and time to reproduce.

@user3644640 again, you assume that life and interstellar travel is cheap and easy, and everyone else on the mission is happy to gamble 30+ years of their life plus the risk of another interstellar flight (assuming that's even possible, which depends on your ship remaining spaceworthy and its drive system still being useful, none of which are at all guaranteed) when instead they could just go straight to a world that doesn't have apparent signs of life and live happily ever after there instead.

@StarfishPrime Oh, now I see that you mean the marginal cost of travelling not the total cost. But you also assume that they could know it beforehand whether the planet is liveable. I think that it comes to the: it depends. I assume that the level of technology to produce near the speed of light velocity is extremely advanced and the reason for travel is that the homeworld needs specific resources as they are not yet in the stage to freely play with the mass-energy equivalence. Fixing can be done on site. Minds are really flexible. If there is a need there is motivation to achieve it.

@user3644640 as I said before, if you handwave away the difficulties of interstellar travel, then interstellar travel is easy.

@StarfishPrime I think you are stuck in your own framework of interstellar travel with its difficulties and motivations you may consider trivial. As I said: "It depends". As always there is the whole world behind which makes sense. In my described world there is a clear motivation to have a reach of hundreds of light years, so it makes sense to stop by on the way to potential candidates. You have a handwaving way of detecting life from afar and know not to go there. I think we cannot know whether the information we get from that afar is enough to certainly know that.

@user3644640 there's a clear technological path towards larger telescopes with longer baselines. There's no path to magical easy interstellar travel, other than magic. But you do you.

@StarfishPrime What is your clear path to a difficult interstellar travel which is not advanced enough to have moveable means of fixing? Why are they even traveling? Are you certain that there are no limitations to your clear technological path. Are you certain that the information about the planet that travels to earth is enough to determine life 100%? If they could only produce 99% probability that there is no life, would they not go to that planet in the 1% of cases when there was life? Especially if their ship is broken? This has astray far from my point that future ethics are not known.

I just mean that I can think of a reason for people to go or end up into such a situation where they would sample with people and gave one example where it is not that big of a deal to put 100 people on an adventure on a planet full of life.