# How to slow down human discovery of new planets?

I'm building a sci-fi world set a couple hundred years into the future. If humans were to suddenly discover FTL space travel (which are handwaved away as generic sci-fi warp speed or something) in a nearby year like 2020, then by something like the year 2400, surely humans would have colonized tons of planets. The problem is though, I don't have the time to write down ultra-detailed histories or map out entire continents for thousands of planets, so how can I force the humans to only discover a few planets (say, 40 or 50) and stop, or at least slow down the rate of discovery massively?

• More than 50 planets in 300y seems like a lot. Planets will take time to terraform and build basic infrastructure. Reproduction rates are also limiting (even if the government provides incentives),since all newborns must be fed,either from Earth or from (yet) barely developed colonies. Also,we need a sh.. a LOT of processed resources for building colony infrastructure and the FTL fuel. If you dont wanna leave Earth dry,some (most?) of those resources must come from colonies,hence they must wait for them to develop. I think 30-50 planets in 300 years is reasonable and you don't have to worry. – xDaizu Jan 5 '18 at 9:15
• Just because you have FTL doesn't mean it is a) cheap and b) fast. If you need billions to build a FTL ship you won't have many. If FTL means 10 times speed of light you still need several years to reach an other star system... – m.fuss Jan 5 '18 at 9:44
• War? Huh! What is it good for? – Mawg says reinstate Monica Jan 5 '18 at 11:50
• Cut space agencies budgets, for reasons. That will slow planet discovery down. – Renan Jan 5 '18 at 14:58
• then by something like the year 2400, surely humans would have colonized tons of planets. Why? Sure, it's possible, but why would people be rushing out to embark on a long, expensive, dangerous journey to build a colony from scratch when they could... do nothing and stay on Earth? If you look at the history of colonization in the real world, you didn't have masses of people clamoring to get to North America and start a new life form scratch... it was people who were driven out of Europe and/or had nothing to lose. I imagine the barrier to interstellar colonization would be even higher. – HopelessN00b Jan 5 '18 at 17:23

Wrong tool for the job.

The problem is though, I don't have the time to write down ultra-detailed histories or map out entire continents for thousands of planets, so how can I force the humans to only discover a few planets (say, 40 or 50) and stop, or at least slow down the rate of discovery massively?

You don't need to write down this amount of detail.

You only need to write down the detail that is needed for your story.

The rest is just sketch, outline.

Also this is wrong :

If humans were to suddenly discover FTL space travel in a nearby year like 2020, then by something like the year 2400, surely humans would have colonized tons of planets.

It's 2020 you have cutting edge tech that allows FTL.

This is a long, long, long way from having a transport infrastructure capable of moving thousands (or tens of thousands) of people to form a viable colony. And all the kit they'll need to survive (animals, plants, seeds ).

At least some of the people will need to be highly qualified (engineers, doctors, even police - stuff happens, the list is quite long). They have to be young (to produce children) yet experienced and knowledgeable enough to be expert in their fields as well as being psychologically suitable. These people won't necessarily be easy to find for what will be a one way trip.

And who pays for this and why ? Let's see world governments and major corporations thrash that one out ! Just agreeing who pays (and what they get for their money) would take a long time, maybe the odd war or ten.

But even if you can agree all this, traveling to a new planet is not at all the same as colonizing it. Is it safe ? Are there microbes or chemicals that will kill us - this is for the long haul, so those studies have to be comprehensive and will take decades - perhaps a century per planet.

Which bits of the planet ate stable ? Which are prone to flooding, earthquakes, wild fires ? What's the weather like ?

This won't be like Star Trek. No way are four guys beaming down (without environmental suits) with a couple of tricorders and half an hour later we're colonizing the place. It's a century of study and testing just to check whether it's possible at all and what the best options are for long term sustainable survival.

So even if you nail down FTL you're looking at maybe four planets with colonies after 400 years !

OK, we'll get better at that with practice and time, but it's not going to be hundreds of colonies, it's going to be ten, maybe.

Factor in a single disaster and everyone who doesn't want it done will create a political climate on Earth that makes it impossible to get any more colonies started for ten for even fifty years.

So politics, money and the requirements of science and engineering will slow this down naturally.

• Couldn’t you send teams to many planets at the same time to prepare to colonize them? If 10,000 teams are sent to promising planets, you’ll have those 10,000 (or maybe 8,000, taking into account factors we couldn’t see from Earth) planets ready in 100 years. – J F Jan 5 '18 at 14:04
• +1, infrastructure, not only that of transportation, and population growth will make this a very long process. You need people that actually want to leave mother earth, and while you will find many, compared to the numbers needed to create a self sustaining civilization on another planet, they will be few. – r41n Jan 5 '18 at 14:05
• @JF, that might work in a game, not reality. Even if those 10.000 teams were made of 10.000 humans each, that's 100 million, where do you get so many adventurous and capable people? Resource requirements to build those ships would be staggering, and a very lengthy process. After 100 years you would only have a population of a small town on each planet. Modern society couldn't be maintained there without help from earth. That would mean that earth needs to provide support to 10.000 outposts. – r41n Jan 5 '18 at 14:22
• @JF, if all you want is a few medium sized cities on that planet, essentially unable to do much at all, constantly in need of help from earth, then yes, you have done that. If you need a viable alternative to earth, that doesn't cut it. It took the "new world" (a small part of this planet) 250 years to be colonized and become independent, and even after 400 years there's plenty of free land. That happened on this planet, a joke compared to interstellar colonization. – r41n Jan 5 '18 at 15:49
• @r41n The problem is not simply finding people who want to go, but who are qualified to go as well. This isn't like populating that mythical thing "the old west", where any idiot (or criminal) who could afford a ticket or work their passage could go. This is like selecting astronauts - thousands of them at a time. The elite of the elite, and of those only the ones who never expect to see Earth and family again and are willing to risk death. And you can't have people going who want to e.g. set up a community based on their own ideas. You need pure selfless explorers. – StephenG Jan 5 '18 at 16:56

Expand quickly initially and then have expansion limited by lack of desire.

Most humans live near cities. They might like the idea of more space, but apparently they value nearness to civilization more. There are no cities on frontier planets. So most people who don't leave initially won't be that interested.

The initial colonization might happen quickly, as the government could encourage it as a solution to conflicts. Not happy in Syria? Ship them off to New Syria, New Levant, or New Kurdistan. No need for refugees in Europe.

Also, there are a certain number of people who find the concept of space colonization appealing. They might power the first few waves.

People who reject local laws might move. For example, polygamists might found the planet of Deseret.

As these groups establish planets though, they leave fewer and fewer people on Earth with similar beliefs. Or people can choose existing planets. For example, Muslims from the United States might go to New Levant, as there is already a Muslim community there. These are planets. They're huge. Not every community will need a whole planet. Some will be satisfied with a continent or even an island like New Zealand or Madagascar.

### Not enough population

It's also worth noting that in richer countries, fertility has dropped below replacements levels. It used to be common to expect a need for sharp limitations to the number of children per family. But the actual problem is increasingly that there are not enough children. If our population is shrinking, then why expand to other planets?

### Expensive FTL

You can also make your FTL drive arbitrarily expensive to use. Maybe we know how to go to new planets, but most people don't want to pay to do so. Or it requires a resource that is currently singular and no one wants to build a new one. E.g. to fuel an interstellar ship requires a gigantic solar array and takes a year; that's less than four hundred ships total. No one wants to devote the resources to building a second one.

### No inhabitable planets

Most planets may be uninhabitable. So living there might require massive terraforming, which can take centuries. Consider the Earth a mere three billion years ago, prior to the Great Oxygenation Event. There was life on Earth, but it was not anything compatible with us. Most planets might be more like Venus, where the atmosphere would be poisonous to us. Or like Mars, where there is no atmosphere.

### Alien conflict

Perhaps the planets are already inhabited. The aliens tell us that we can have so many planets and no more. We are already surrounded by other civilizations. Beyond our fifty planets, there are no more available.

• The concept of "expand quickly initially" sounds weird to me. At first you have less resources (only the ones on Earth) and your colonization technologies are on version 1.0 or beta. The more you colonize, the more access to resources you have and the more experience and better colonization technologies you have. Colonization would start sloooooowly and then snowball. And after a few centuries, the fad will probably fade, as you stated (lack of desire), or the politics and government would get weird (even war-ish) and the colonization will stop/slow down until they stabilize again (if) :) – xDaizu Jan 5 '18 at 9:21

# Few planets are habitable

There are a lot of planets out there. But given that we might have only a limited FTL capacity (like 2c or something), then there might not be a lot of planets immediately accessible. Lets say that only those stars within 100 ly are able to have been colonized for various reasons including speed of movement. Keep in mind, if you can only travel/communicate at 2c, then it takes 50 years to even get from Earth to a place that far away. That is a disincentive to colonization by itself.

### How many planets within 100 ly?

Solsystem has summaries of known stars within 100 ly for spectral classes A, B, F, G, K, and M. There are 80 type A and B stars, and those are probably not suitable for forming appropriate planets. F, G, and K have 303, 512, and 947. These are probably the most likely stars for a habitable planet as we know it. M are the red dwaves, of which there are 2000+. So all in all, there are in the range of 4000 stars within 100 ly. Right now the current estimates are in the range of 1 planet per star, so we can estimate 4000 planets withing 100 ly.

### Are any of these planets suitable for a colony?

Now, many of the planets will be gas giants. Many others will be too close to the star (Mercury), too far from the star(Pluto, if it were still a planet), without an atmosphere (Mars), with a hostile atmosphere (Venus), etc.

It would be very reasonable to say that no planet matches Earth's conditions well. Let say the most Earth-like planet has about 1g, about 1 bar of atmosphere, and an average surface temperature -40 C. Sounds pretty Earth-like! Its a much better colonization candidate than Mars! On the other hand, it also sounds exactly like Antarctica, and nobody lives there. Sure scientists are all over, and without treaties there might be some mining, but no one has 'colonized' and set up shop with their families. It is just too hostile. I think that even the best of your 4000 nearby planets will be too hostile to colonize.

### Are any of these planets even suitable for scientists and miners?

Now, lets walk back that similarity to Earth a bit. What are the changes that a planet has a near-Earth temperature, pressure and gravity? All those are necessary to even have a scientific station on the surface. For example, Venus has Earth-like gravity, but the crushing pressure and ridiculous temperatures means that it is very unlikely any humans would live on the surface until the far-future.

Mars, on the other hand, has a potentially dangerous to health low gravity and no atmosphere to speak of. Any colony of the surface would need support from Earth for decades to keep it working. While that might be appropriate for Mars, since it is so close, it is increasingly unlikely with distance that there is the money and political will to support such a colony. Now, for science and mining the cost of maintaining an outpost might be well worth it, so it is much more likely that you will see such outposts in other systems.

### Terraforming takes a long time

Just as a last note, your timeline isn't really long enough for terraforming either. Perhaps Mars, being right next door, could be partially terraformed within 400 years, but for planets years of travel away, the likelihood that enough resources were sent to give terraforming more than a start is low.

### Gravity wells are expensive

The last note is that going down into a gravity well is expensive. The hardest part of space travel from Earth is blasting things into space. Any planet with similar gravity to Earth would have a similar problem. In general, it is easier if you can just stay in space. If you really want to colonize, and all the potential planets are too hot, or without atmosphere or whatever, why not just put a space station in orbit and colonize space?

# Conclusion

Given the likely paucity of planets very similar to Earth, it is completely reasonable that no planets have been colonized by 2400. After all, no one has colonized Antarctica for ~200 years since its discovery.

A much more likely condition is that the most mineral rich or otherwise valuable ~50 of the nearby 4000 star systems have some sort of mining or research outpost in them. In cases where there is something interesting on the planet itself (like, life) there might be research stations on the planet's surface. But otherwise, most economic activity would be done in space, if possible, to avoid the cost of gravity wells.

Finally, if there were people who wanted to leave the sol system real bad for whatever reason, it seems likely that they would colonize in a space station with conditions amenable to human life, instead of on a hostile planet's surface.

OK, so you have FTL, but the question is, how much FTL? FTL doesn't mean "any speed you like". Maybe you can go double the speed of light. It still takes about 2 years to get to the next nearest star, and maybe 10s or 100s of years to get to the habitable planets.

Simply make the maximum speed be whatever you need it to be, to give the radius of exploration which will provide your desired number of planets!

Well you can take the easy way or the hard way.

The easy way is to put them in a very dire and resource draining conflict. It could be a war with aliens or with each other, or even something more devious like the discovery of some type of dangerous life form that can survive in space or seems to be on every new planet they find (think the Xenomorph).

The hard way is to design some type of obstacle to the travel itself. Maybe there is some type of cosmic background radiation that slows or even stops FTL travel and just so happens to be very weak in the areas your humans have colonized, but is much stronger everywhere else. Or maybe they travel by using wormholes which are rapidly becoming harder to find. I think this way is slightly more difficult because it requires you to figure out how FTL travel works in the story. Obviously these aren't your only options, but they might work for you.

• I was favoring the "wormhole" idea myself. Similar to how it's done in Bujold's Vorkosigan books, or the Starfire universe. Each system only leads you to one or two other systems, or sometimes you find you're in a dead end and must backtrack. Maybe combine this with the terraforming idea that others have mentioned? "Naturally Earth-like planets are far and few between. A hundred other planets about the same size as Earth are still in the middle of a centuries-long terraforming process before they will be suitable for colonization." – Lorendiac Jan 6 '18 at 1:33
• The game AI war fleet command also uses a wormhole system like this and it does significantly slow down travel, especially as the worm holes could be spaced out to the point that they are years or longer apart when traveling at conventional speeds. – Nick Jan 6 '18 at 4:55

You don't have to slow down planet finding at all. We've found lots of planets already, yes. But, none of them to our knowledge are habitable by humans.

Even spectroscopy can't verify that a planet is habitable. Sure, there may be water, but there may also be incredible gravity, or the planet is just that bit too close to the sun to be temperate, or it may be tidally locked to its sun, making the ring of dusk the only habitable area (assuming the water is there), etc.

The truth is that if we discovered FTL technologies in the next couple of years, it's entirely plausible that it could take a couple of hundred years to find one of the exoplanets that's ripe for colonisation. Finding 40 or 50 means you'd probably have to speed up exoplanet detection and FTL exploration & scouting because the percentage of habitable planets out of those we've found is going to be incredibly low.

There are a couple of ways you could handle this:

## Weaken FTL Travel

• Make FTL Travel Infrastructure Dependent: This can take a couple of forms. One is that FTL depends on wormholes, or other physical characteristics that only exist in certain places. Another is that it works more like a railroad. That you have to put things down ahead of a ship for it to travel at FTL speeds (One hypothetical version of the Albucierre drive does exactly this by using pre positioned masses to maintain a warp bubble). So while you can travel between already colonized areas at FTL speeds, the frontier advances at less than c.
• Mild FTL Travel: You can travel at faster than light speeds, but you can only do so at twice the speed of light. This is still very fast, but would limit the number of settled planets to a considerable degree.

## Make Large Parts of the Galaxy Uninhabitable

• Very Few Planets Worth Inhabiting or Colonizing: Pretty self explanatory, but inhabitable planets could be very rare. Thus fairly few have been found.

• Hazards Make Large Swathes of the Galaxy Dangerous: Very intense radiation from the Galaxy's core could make large regions of space impassable for transit and uninhabitable. Alternately, very strong extragalactic radiation or other hazards could make star systems on the edge of the galaxy uninhabitable.

I think what you want is to slow down discovery of potential habitable planets. This requires the ability to detect potential life signs on exoplanets. One way to stop it is to have a government that refuses to fund the required technology. What the government would specifically need to not fund is the development of planetary coronagraphs. This could possibly be due to collusion between the government and a corporation, or other entity, that would benefit from decreased colonization.

Coupled with a space telescope with sufficient resolution to see the exoplanets as points of light, the coronagraph blocks the light from the host star and incorporates a spectrometer that can detect life related gases (like oxygen, water vapor, methane, etc).

Possibly the first to be launched is a planetary coronagraph that incorporates the PISCES integral field spectrograph on the WFIRST telescope, scheduled to be launched in the 2020s.

You stop the flow of  and the research and development comes to a screeching halt.

https://en.wikipedia.org/wiki/Coronagraph

Planetary coronagraph with PISCES on WFIRST:
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150020906.pdf

## No economical incentives

When the New World was colonized, a group of pilgrims could finance the ship and equipment they needed to reach and build their new home.

You can make FTL travel as cheap or expensive as you like, but i'd imagine a space ship at least in the first few hundreds of years to be pretty expensive, plus all the equipment needed to create a colony on a planet that has a completely different ecosphere than earth.

This will need a financial backer willing to spend a few billion dollars, and financiers expect a return on their investment. A colony might pay of eventually, but it will take quite some time. Not a lot of companies will be willing to spend money with (maybe!) a break-even in 100 years.

I guess Elon Musk would gladly finance such an endeavour, and there will be a few others. Also, every major government will like to grab some land for themselves (but there will be political ramifications up to and including war about the choice pieces of real estate), but after the first wave of enthusiasts, economic reality will set in and the colonization efforts will slow down a lot.

It's often useful to compare a proposed reality to the real world.

At the moment we have the knowledge of how to make transports that could travel reasonably rapidly to Mars, even more rapidly to the Moon. We can also reach all the other bodies in our solar system.

But I notice a significant lack of colonization on Mars, the moon, or anything else in our own solar system.

Why not? Because it's devastatingly inhospitable, and we lack all the other necessary science advancements, like the ability to create a living biosphere. Our best effort at this was Biosphere 2, and look what a complete comedy of errors that debacle was.

So even if we develop really awesome FTL travel, so that we can visit any planet on any star in the galaxy and come back within an hour... we'd do that. We'd come back. Astronomers and astronauts and planetary scientists would go out, study these planets for an 8-hour work day, then commute back home and put their feet up under earth gravity, with the windows open to the earth air, and the earth birds singing, eating earth food.

So... colonization? Why? Sure, there'll be a minority of people that want to set up a colony just because it sounds romantic, or better than their existing life, or something. And some will even be willing to fund that dream.

But given that there are almost certainly approximately zero other planets in the galaxy with breathable atmospheres, what's out there for them, that wouldn't also exist in an underground apartment right here on earth?

Really, there are very few draws I can see.

• Installing, maintaining and operating terraforming or mining equipment.
• Isolation of dangerous people.
• Secret bases for research, training, retreat in event of war, etc.

Sports/Recreation/Vacation:

• Low-gravity sports and recreation.
• High-gravity sports and military training.

Science:

• Planetary science research.
• Biological study of extraterrestrial life, if any found.
• Sociological/archaeological study of extraterrestrial societies, if any.
• Other science: humans/biology/physics/etc in different gravities, etc.

With so few draws, creating permanent outposts on even a dozen other planets within 20 years seems very improbable given the costs involved in just getting them habitable.

Actually colonizing them, which I would interpret as having self-supporting environments in which people can live out a whole life-cycle, including pregnancy, birth and childhood... not very likely within the next 20 years for even one of them.

So it seems to me that given the amount of hand-waving you'd need to do to get even a single colony established, even close by like on Mars or the Moon, the problem is definitely not in restricting the number and size of colonies, but rather in how you could convincingly get a colony to grow large enough to be interesting.

# Terraforming takes a long time. A REALLY long time.

First of all, did you know that oxygen likes to bond with stuff? If you leave metal outside, it will rust - that's oxygen bonding with the metal. One simplistic way to describe most fires is oxygen bonding with materials rather vigorously. So why do we still have oxygen in our atmosphere? Simple - there are a lot of things on Earth (plants and phytoplankton) producing oxygen. Without them, we wouldn't expect to see much more than trace amounts of free oxygen in the atmosphere. So if there isn't oxygen-producing life on a planet, it will have to be terraformed before it will have a breathable atmosphere.

Let's look at what it would take to terraform the atmosphere of a planet. On Earth the atmosphere is about 21% oxygen, though we could acclimatize to closer to 15%. Earth's atmosphere weighs about 5*10^18 kg, so on a planet of about Earth's size (which you'll want in order to have reasonable gravity) you need to have about 7.5*10^17 kg of oxygen in the atmosphere before it becomes reasonably breathable. A quick search suggests that a tree produces about 100 kg of oxygen per yer. This means that you need 7.5*10^15 tree-years in order to produce enough oxygen to make a breathable atmosphere. In order to do this in 250 years, you need 3*10^13 trees - around 10 times as many as there are on Earth.

There's more to it than that - I haven't factored in other sources of oxygen, I've ignored what we'd need to do to reduce CO2 to a breathable level, and I'm sure I've neglected other things we'd need to worry about. However, this should give you an idea of the scale of the problem - it's really hard to change the atmosphere of a planet significantly in a short time frame.

With this, it's easy to explain why only a few planets have been colonized. If you're lucky enough to find a planet that has oxygen-producing life, that planet could be fully colonized within a reasonably short timeframe. If there isn't already life on the otherwise-habitable planet, the colony will be restricted to controlled-atmosphere dwellings and will require a lot of outside help before it would have enough infrastructure built up to be self-sustaining, and even then it would not have a large volume of surplus resources to use for growth.

The TL;DR version: for however many planets you want to have colonized by 2400, say that a handful of habitable planets without life are being colonized and are in the early stages of being terraformed, with the rest of the planets being ones where simple oxygen-producing life already existed. If you want the planets to be more spread out, simply say that life is rarer and we had to go farther to find it.

I was writing a similar setting in a sci-fi story. In my story, I had humans slow their expansion by diverting resources towards terra-formation of the worlds they discovered. I kept Earth-like worlds extremely numbered.

1. The vast majority of planets aren't inhabitable by humans.
2. Of the few planets potentially inhabitable by humans, some have significant show stoppers (such as plants that use chlorine in their metabolism, resulting in an atmosphere that will kill an unprotected human).
3. Of the few planets potentially inhabitable that can be terraformed in a reasonable amount of time, that still requires a lot of effort and a long time.
4. Unless you're writing an encyclopedia, why would you even bother writing detailed information on so many planets when it's unnecessary? Even in writing set on Earth in the present day, the readers don't get or expect a detailed history and map of Indonesia just because one character happens to come from there.
• Chlorine atmosphere are not reality. – Loren Pechtel Jan 6 '18 at 3:55
• Not a chlorine atmosphere, an atmosphere that has a dangerous level of chlorine for humans. There's a difference. – Keith Morrison Jan 7 '18 at 21:46
• Except the chlorine would react with the environment and not stay in the atmosphere. – Loren Pechtel Jan 7 '18 at 22:04
• The same is true for oxygen, yet you're breathing it right now. – Keith Morrison Jan 8 '18 at 2:54
• We have free oxygen because we have organisms that unbind it and release it into the atmosphere at quite a rate. Even then we wouldn't have free oxygen if the natural binding of the planet itself wasn't satisfied. (Rocks are mostly some form of silicon oxide.) – Loren Pechtel Jan 8 '18 at 22:34

Make it really fucking expensive and unlikely to pay off as a business investment. For example handwavery-engines cost billions to operate so naturally companies and governments try to offset this cost with asteroid mining or something. Long story short, they succeed. Every knows the materials came from an asteroid though and earth bound mining companies don't like that so they lie about the superiority of earthern minerals and stuff. One thing leads to another which leads to several more things and for whatever reason the Company is only able to get 80% of the billions it cost to run handwavery-engines out of the asteroid that should have been worth at least 3 trillion. Suddenly no one wants to fund space exploration because it's too damn costly. Offset this with like one person who accidently discovers a cure to cancer on venus or something. If space travel is high risk high reward with an extreme barrier to entry, then exploration will slow down because very few people will be able to do it.

tl;dr if you make it expensive high risk high reward that hasn't gone so well for a lot of people, then you only a few really rich people will even be able to explore space.

Another factor involved: NASA and the CDC are going to put some pretty major restrictions on dealings with alien planets that might be of colonization interest.

NASA because they want to make sure there's no indigenous life before they allow you to land stuff that hasn't been highly sterilized, the CDC because they want to be sure there's no potentially dangerous local life before they allow you to return anything that hasn't been highly sterilized.

Now, it's likely that alien bugs are harmless but the CDC won't go along with risking the human race until they're sure. And while it's unlikely that alien diseases will get us I would be much more worried about alien pests. Imagine a planet with a more evolved ecology than ours--the local plant-munchers need nothing but calories and trace minerals and can munch most anything organic to get it. They're at least mildly toxic and they have a considerable degree of cold tolerance (go passive but don't die.)

Something like that without it's local enemies would redefine invasive species--and if not caught quickly enough it could become an extinction event.

Light-speed travel merely means that I can travel somewhere in what seems a short time for me. However, due to Special Relativity (assuming that you're not bent on violating it), when I accelerate I "rotate" myself in spacetime such that I experience time (and a dimension of space) differently from those in the reference frame I left behind.

If I travel to Proxima Centauri b and back at light speed, it will seem roughly instantaneous to me and will take over 8 years from an Earth perspective. This is without a "ramp-up" time for safe acceleration -- much Sci-Fi ignores the limits on accelerating humans or presumes tech which handles that.

If many, many habitable planets have been discovered, people may be quite choosy about where to go, for many practical, social, and idealistic reasons. (You can think of interesting ones. For example, some may prefer a spot where time flows more slowly relative to other populated regions, so that they can develop military or mercantile applications more rapidly than others. Some may want to get the hell away from areas where wars have broken out or disasters have occurred. Some may seek to live near certain features -- like a nebula with a particular form -- because of religions beliefs. Many may have simply "left the map".)

If people are choosy about destinations and can travel very far in an instant (from their perspective), it may take centuries for them to arrive, from the Earth's perspective. Pioneers may even like the fact that Earth time leaves them behind. They may anticipate greater progress to have occurred within Humanity once they've reached their new home, or they may have traveled to one planet, found that Humanity isn't to their taste, and then chosen to relocate in order to toss the dice again and alight once again a century or so later (per Earth time).

If this came to pass, there might in two hundred years be a sizable portion of Humanity now compressed (relative to Earth) into flatness along their direction of travel, frozen in time (relative to Earth) until they reach their destinations years later.

Now, if this were all Faster-Than-Light, it depends on your Physics. The people might again simply remain "frozen" until arrival, but the craft would reach the destination in less than the time it would take light. Another approach might have the passengers regress in time during the trip -- essentially, a new past, causally consistent with the outcome at arrival, would be created, so you might want to think of a creative means of assuring that they still arrived with the ship. Either way, unless communications were FTL, all the diaspora would still be confronted with the problem of long-distance communication delays.

Truly FTL might allow ships to arrive in the past, which would presumably send them into an alternate branch of reality -- the one in which they lived on a distant world a few centuries ago. Consequently, we don't hear anything from those pioneers, unless maybe we just now heard from their ancestors, who turned us on to that FTL technology in the first place.

Another possibility (I'll just toss this in) is that the FTL method employs spacetime warping, which works great for the traveling craft but stretches out space in its wake, making it less traversible (including for electromagnetism, interfering with observation of that part of space). This sort of spacetime pollution could become a controversial topic. It's a bit like the guy before you in a public restroom urinating all over the toilet seat. He'll never need it again and he's long gone, and you get stuck with the mess. The difference in the analogy here is that the messy toilet seat also happens to have created stretch marks in the fabric of space and time which altered reality behind him, even if you bothered to track him down.