Gravitational microlensing to the rescue. We now have evidence for the existence of the rogue planets — interstellar wanderers moving through space unattached to any star system — that we talked about just the other day. It’s been assumed that such planets existed, because early solar systems are turbulent and unstable, with planetary migrations like those that lead to ‘hot Jupiters’ in the inner system. Moving gas giants into orbits closer to their star would cause serious gravitational consequences for other worlds in the system, ejecting some entirely.
But while we’ve been thinking in terms of detecting such worlds through auroral emissions like those produced by Jupiter, researchers at two microlensing projects have made a series of detections by using gravity’s effects upon spacetime. Specifically, a stellar system passing in front of a far more distant background star will warp the light of the background object. The resulting magnification and brightening flags the presence of the intermediate object, and surveys like Microlensing Observations in Astrophysics (MOA), based in New Zealand, have developed the necessary expertise to distinguish between intermediate stars and planets.
Both MOA, which scans the galactic center for these microlensing events, and the Optical Gravitational Lensing Experiment (OGLE), using a 1.3 meter telescope in Chile, have studied and built a case for the existence of up to 10 rogue planets of roughly Jupiter mass. Microlensing because of its nature picks up objects a long way from our stellar neighborhood — these average between 10,000 and 20,000 light years from Earth. Extrapolating from the lensing probabilities, the efficiency of their equipment and the rate of lensing, the researchers now conclude that there could be as many as 400 billion rogue planets in the Milky Way.
How Do Rogue Planets Form?
That’s a big number, but at this point we’re still shooting in the dark. After all, lower-mass planets should be ejected from young solar systems more frequently than the gas giants this work has detected, which is why planet hunter Debra Fischer (Yale University) told Nature News in a related story that lighter planets “…might be littering the galaxy.” What a scenario, particularly given the possibility that a hydrogen atmosphere could trap enough heat to allow the presence of liquid oceans. Unfortunately, the current survey was not sensitive enough to detect planets smaller than Saturn.
This work is getting huge play in the press, but I think it raises as many questions as it answers, for the scenario shifts depending on how these wandering worlds were formed. The current work draws on the idea that they were the result of ejection from solar systems. In fact, David Bennett (University of Notre Dame), a co-author of the study in Nature, assumes ejection as the primary mechanism:
“If free-floating planets formed like stars, then we would have expected to see only one or two of them in our survey instead of 10,” Bennett said. “Our results suggest that planetary systems often become unstable, with planets being kicked out from their places of birth.”
And if ejection is the driver here, then we should assume a huge population of low-mass planets moving through space without any star, just like these gas giants. But if there is another formation mechanism at work (Greg Laughlin speculates about this in the Nature News article I linked to above), then the low-mass wanderers are much less prevalent. Right now we just don’t know, because we would need a formation mechanism that would account for objects not much larger than Jupiter, “…something more similar to that of a tiny star than a giant planet,” Laughlin adds. Whether or not ejection is the mechanism thus becomes crucial for any hypothesis about rogue ‘Earths.’
Outer Orbits and Unseen Hosts
Also in play is the question of whether the ten detections could be of gas giants in planetary orbits around stars that were simply not detected. The study sees no host stars within 10 AU, a figure that remains relatively close to any potential host. We don’t have a firm answer, and I see that Alan Boss (Carnegie Institution) told the New York Times‘ Dennis Overbye that this scenario is the more likely one. If that’s the case, then we should look with even greater interest at data from the WISE (Wide-Field Infrared Survey Explorer) mission, which should have been able to spot any gas giant lurking in the distant regions of the Oort Cloud. Ten detections like this would imply such outer orbits may be common around stars.
Where we go from here seems obvious: We need to confirm there are no host stars. If we do, then the presence of twice as many rogue gas giants as there are stars in the galaxy is enough to take the breath away, whether or not they’re in the company of rogue ‘Earths.’ The planned Wide-Field Infrared Survey Telescope (WFIRST) might be able to make a detection of such Earth-mass rogue planets and give us some constraints on their numbers. We’ll also learn, as we continue the study of galactic wanderers, to tighten up our theories of planet formation and migration to account for the suddenly increased population of sub-stellar objects among the stars.
The paper is Sumi et al., “Unbound or distant planetary mass population detected by gravitational microlensing,” Nature 473 (19 May 2011), 349-352 (abstract).
Perhaps human interstellar expansion will turn out to be more like Polynesian island hopping than crossing the Atlantic.
Considering how many stars more massive than the sun have already died, I’d expect a large number of rogue planets in the galaxy simply because they move outwards as their parent stars lose mass.
Though when the star is in its final throes and ejects large amounts of material – one has to wonder whether the material drags the planet in (through friction) or if it helps push the planet out.
This does definitely raise a few suspicions, especially since if ejection is the mechanism then we ought to be seeing a large population of extremely massive gas giants orbiting relatively close-in to stars (i.e. the objects which ejected the rogue planets in the first place). Unfortunately observations suggest that such massive planets are quite rare.
I think Dr. Boss is right about these planets still orbiting stars, but at far away distances, say 30-50 AUs out. The WISE data results should be useful here.
If these gas giants are common, smaller planets should be even more common. The Kepler results suggests that Neptune sized planets are most common, with a roll off in the number of planets both smaller and larger than gas giants, with gas giants still being more common than Earth-sized worlds. If there are 400 billion gas giants, there are going to be 1600 billion Neptunes floating around out there. That’s a lot of planets.
I don’t know the sensitivity of the WISE instrumentation. But it seems to me that it should find one or more Neptunes within, say, 10000 AU of us if they do exist.
the researchers now conclude that there could be as many as 400 million rogue planets in the Milky Way.
Should that be 400 million or billion?
Right you are, ad, and thanks for catching that. I’ve corrected it in the text.
I can’t help thinking that 20 years ago, the only planets we knew were the ones in our solar system. Now, more than 500 exoplanets have been confirmed, and this new study suggests there might be more planets than stars in our Galaxy…
Another new discovery of mass in the universe. Recently I read an article postulating massive black holes without surrounding galaxies and another stating that there are likely many average black holes spread throughout galaxies. I’m starting to wonder just how much more normal mass is out there just waiting to be discovered. We may eventually not need any sort of dark matter or dark energy to explain missing mass. Just wondering.
kurt9,
WISE data will be of little use in determining whether these planets really are free-floating or are orbiting some star because of the resolution. Even at the shortest wavelength, the resolution is about 5 arcseconds which means that faint stars will not be cleanly separated in the images. At the distances of these planets, any associated star will be faint if its spectral class is similar to the Sun or later.
By the way, the idea of “rogue planets” is not new. Harlow Shapley proposed the idea in the late 195o’s in a popular level book.
There were some similar findings for a globular cluster a few years back. Several “delocalised planets” were found in the cluster. So it’s not that new a finding really.
The interesting thing is, are they primarily ejected planets, or are they primarily the low mass end of the IMF? If they are the latter, the WISE data should show there are hundreds of billions of cool brown dwarfs in the galaxy.
In any case, fascinating stuff because it looks like there’s more and more stuff out there to explore. It can’t add up to the missing dark matter mass though.
David,
If rogue planets are common, there should be one or two of them relatively close to our solar system (but not orbiting our sun), perhaps about 20,000-30,000 AU out from us. These planets, if big enough, should be detectable by WISE.
To me this finding makes sense. I view it as the evolution of planets and stars. Small rocky rogues passing through a gas cloud attract either more rocky material and become earthlike,and large ones could attract enough gas to become Jupiters. Then the Jupiters collect more gas and become stars. red dwarfs could attract more gas and become larger stars. it all depends on the makeup of the dust and gas clouds they encounter.
If there really are so many billions or trillions of rogue worlds just in our galaxy alone, floating about like cosmic pinballs let loose for eons, the fact that we have not been smacked by any so far as we know or even see one or two pass by shows just how voluminous space really is.
This may also add to the reason why we haven’t found ETI or they us: It’s a darn big galaxy, so big that you can’t even get hit by a giant alien Jupiter for love or money. Finding a few talking monkeys with car keys who are barely noticable past a few light years is going be even harder for an ETI than we imagined.
In order to lock down the case for or against Jupiter sized planets in the Oort cloud, we need another round of Wise Data. The probe was only operating one year which makes it difficult to sift out the motion of these putative distant worlds from the background of red stars with debris disks and foreground of closer objects that do not have calculated orbits. It looks like the mighty JWST will have to wait up to ten more years to launch (!) and besides, it has such a narrow field of view it is useless for system wide 40,000 degree squared surveys. similarly WFIRST is on economic hold and a closer look reveals that is is going to operate relative short wavelengths ( ~2 micron/ not a great way to look for gas giants.
By contrast the now “warm” WISE system still operates down to 5 micron and thus in the temperature range for gas giants and more distant brown dwarfs.
so for another 5 to 10 million we could turn Wise back on next year and get another read on object position. Things will have moved around quite a bit by then, and we could have three or more position data points so..we can see a pattern. From there ground scopes and nail down the particulars. I think the WISE team and the technology has more than proven itself worthy of receiving less than 1% of the JWST budget. (Hint to NASA_ who knows? the right discoveries would add a lot of interest in increased space research funding ) Imagine probe to a gas giant at 500 AU or even to the much more distant Oort cloud!
One can’t help but wonder how serious a hazard to navigation many billions of rogue planets (and not only big Jovian but smaller rocky worlds) would be.
That is if the ejection process is mainly responsible for these free wanderers as I think is likely.
If one can travel at any significant fraction of c the route had better be scoped out very throughly in advance. Even a hard to spot rogue dwarf planet would spoil the trip. Space is big but sooner or later……………
Not another possible solution to Fermi’s paradox !?!
Given the vast numbers of stars and planets and the long history of the galaxy before there even was a Sun, you can bet that just about every scenario we think is remotely feasible has in fact happened many many times. Ages ago.
So yes, there are many tidal powered rogues out there. And some with long long histories of bio-chemical evolution and extinctions. Remember that age after age after age has already happened out there. Long before any human ancestors came down from the trees, there was time for many many stories to play out . We are the young ones, children happily surprised at every turn.
Mike, I think the Bill B model is so much more envisionable than yours as to render the hazard that rogue worlds present an unlikely answer to the Fermi paradox.
If you follow the Polynesian expansion you find that repeated population crises must be the rate limiting drivers of each major new wave of expansion not the want of exploration. The most crucial factor then becomes their food resource stability is much lower than that of their material resources for boat building. It seems to me than on these new worlds the security of their food production would also be very low compared to the material available for spacecraft construction and fueling.
Thus if we ever began the process of colonising these worlds, yet found direct travel to the stars too difficult for any reason the Bill B mechanism might get us there, given that these worlds are numerous enough. That condition would have been met (and handsomely exceeded) if they were so common as to present a practical hazard to startravel.
Interesting news, but it opens up freightening scenarios too. If the galaxy is full of these rogue planets then nothing forbids one of them being in a route of collision with Earth and we don’t even know it’s coming, because we don’t know it exists even!
It’s quite a worst case scenario this one, tough… a much more plausible one is that a rogue gas giant comes through the Solar System totally disrupting all of the gravitational balances between its members, which were formed in billions of years. Couldn’t this cause the movement of Earth from the habitable zone to a more “hostile” orbit, like the one of Neptune or the one of Mercury?!?
As of now we don’t even know if we can change the orbit of an asteroid in collision route with Earth: changing the orbit of a rogue gas giant in route of collision with Earth is just science fiction.
I think as we know more and more about the universe, we learn that it is less and less empty than what we thought some years before and that it is really dangerous. Then the usual thought of the average man is “let’s stay safely at home”, meaning space colonization is a nonsense as we live on a paradise already: the Earth. Well that’s true, for now.
But it’s also true that our dear planet Earth stays in this really dangerous universe, it’s not like it’s in a separate dimension or something.
Actually it’s the most dangerous place in the universe for humanity: it gave birth to humanity, it hosts humanity and allow humanity to keep living and prospering on its surface. It gives humanity a sense of safety, making lots of people thinking space colonization is useless: this sense of safery may very well mean the doom of humanity someday. The first serious threat slamming on Earth it could mean humanity extinction at worst, returning to Middle Age at best!
We need to travel to other stars, to spread through the universe in order to avoid the end of our species only because we’re located in one single place.
Is there any possibility that highly eccentric (but not rogue) [super] Jupiter size planets might be the cause of periodic extinction events? The sedate planetary formation model now seems to be just one model, and I’m intrigued by the models of formation that imply gas giants to become close to their suns while throwing others out of,or into highly eccentric orbits, their systems.
I’m thinking a Jupiter with a perihelion in the Kuiper belt, and an aphelion well outside it, perhaps with high inclination. The hypothesis would suggest small, but periodic rises in cometary bombardment. Would this pattern be detectable on various moons – perhaps icy moons around Jupiter or Saturn with different aged surfaces?
Hello,
Another fine triumph of modern observational astronomy! Kudos to all of those involved.
From what I can tell, the teams involved did a rigorous statistical analysis, ruled out alternative interpretations, etc. However, I have an important question regarding this work. The observations were made during the MOA 2006-2007 season. Now, that was 4-5 years ago and as we know, MOA and OGLE have both been making high quality catalogs of microlensing events during the intervening years.
My question: Why then did the teams only include 2006-2007 observations? Afterall, 10 objects is a relatively small number. I’d be willing to bet that the 2008, 2009, and 2010 MOA/OGLE observing seasons have more of these events. Adding statistics from these additional years would significantly strengthen the claim about the galactic population of unbound and/or planets.
10 AU is quite close, about the orbit of Saturn. It appears we do not know if these are really “rogue”, or just Saturn- and Neptune-like. I think we got carried away by an exciting notion that is more fiction than fact. Again. Sigh…
“If Jupiter were kicked out of the Solar System, its surface temperature would drop by only about 15 kelvin, he says – although it would still be unsuitable for supporting life.”
Not with terraforming, I would contend – there would be a high thermal stockpile within a rogue jupiter, which could be exploited to warm the moons. Or perhaps, if there are no moons, we could build a halo around it… at any rate, there’d be abundant electricity…
“Should that be 400 million or billion?”
400 milliard, actually; 400,000 million, or 0.4 billion. I presume you mean 10e9?
“If these gas giants are common, smaller planets should be even more common. The Kepler results suggests that Neptune sized planets are most common, with a roll off in the number of planets both smaller and larger than gas giants, with gas giants still being more common than Earth-sized worlds. If there are 400 billion gas giants, there are going to be 1600 billion Neptunes floating around out there. That’s a lot of planets.”
I hope so; Neptunes, with their abundant Helium stockpiles, coupled with ~1g “surfaces” and high ice fractions, as well as thermal stockpiles which can be tapped, are ideal colonization targets (well, as ideal as you get in interstellar space). Humans would have to live in floating colonies, of course, unless there’s a suitable Triton equivilent they could terraform. How close could we expect one?
Although, isn’t Kepler biased in favour of the larger worlds?
@Drakend I think we have to bear in mind, if there are 400bn stars and 400bn free planets, the space density of free planets in our galactic locality is the same as that of the stars. Average separation is 7 light years.
People have computer-modelled rogue planets and stars interracting with the solar system. Turns out, the chances of a Jupiter or BD barging through changing planetary orbits are rather small. That is because it would have to come through within a very few degrees of the plane of the ecliptic. Otherwise the probability of it approaching another major solar system body closely enough to have any effect is tiny.
“Although, isn’t Kepler biased in favour of the larger worlds?”
The current results should a distribution with Neptune planets being the most common with a distinct roll-off of the availability of planets both smaller and larger than Neptune. It is possible that the roll-off for the smaller planets is an artifact of the instrument sensitivity. But, I don’t think this is the case as Kepler was designed to find Earth-sized planets and has actually found them. This suggests that the roll-off is real.
Yes, rogue Neptunes would be better than rogue Jupiters as the gravity would be lower. However, instead of floating colonies, orbital O’niell style habitats (with internal light source of course) would be used.
I’ve always thought of it that way.
It’s easy to forget how huge outer space is. Even if rogue planets are commonplace, a direct collision on an interstellar voyage would still be highly unlikely. With that said, we would do well to chart the way ahead carefully – a collision between a starship at relativistic speeds and a planet would be an unfortunate event for both parties.
Given our models of solar system formation, I expect that there are a number of wandering exo-earths and gas giants out there. We’re still in the early phases of finding exoplanets, so the fact that we haven’t spotted them right away is no surprise. If they’re common enough, they could make great waypoints and refueling stations for interstellar trips. An exo-earth with a thick atmosphere could even be habitable.
In any case, it’s always great to find more stuff out there.
As an aside, lol @ MOA being based in New Zealand. Moa is also the name for a huge, flightless bird native to New Zealand which is now extinct. I wonder if it was intentional?
“If they’re common enough, they could make great waypoints and refueling stations for interstellar trips.”
Space travel isn’t like air travel, and space is not an ocean. Stopping means killing your velocity and wasting a lot of fuel. Much easier just to keep straight on. Settling them, however, is different… perhaps, somekind of cycler network? :)
“An exo-earth with a thick atmosphere could even be habitable.”
If you call 1 kilobar habitable… ;)
“The current results should a distribution with Neptune planets being the most common with a distinct roll-off of the availability of planets both smaller and larger than Neptune. It is possible that the roll-off for the smaller planets is an artifact of the instrument sensitivity. But, I don’t think this is the case as Kepler was designed to find Earth-sized planets and has actually found them. This suggests that the roll-off is real.”
Ah, right. Oh well, I’ve always prefered Neptunes to Terra’s – larger surface area for the same gravity, and with habitable tempeatures at whatever distance from the stars (it’s at the 150 bar point, mind…). Perhaps, if more Neptunes form, several more formed in our solar system, of which only two remain in the “inner” system (Uranus and Neptune)? One has to wonder whether there are a few Neptunes hiding in the Oort cloud, having been slung out to wider orbits by interactions in the early days of the solar system…
“Yes, rogue Neptunes would be better than rogue Jupiters as the gravity would be lower. However, instead of floating colonies, orbital O’niell style habitats (with internal light source of course) would be used.”
But I don’t want to live in a glorified tin can… if we’ve mastered fusion or effective energy beaming (and, with manned interstellar craft, we quite probably would have), the gravity well shouldn’t be an issue. I’d prefer to live in an airship than an O’Neill colony – although a small ringworld wouldn’t go amiss. You might as well live where the energy is. A few people will, yes, have to live in orbit, but as for me, I’d prefer to have my feet on an aerostat. There’s far more “terraforming” potential on a planet – imagine a biomechanical structure, held up by hydrogen gas, covering the planet at the 1g point to provide a solid surface…
A good point. They would lend themselves to a cycler network. And it is important to remember that space is not an ocean. It’s too easy to think of it as one, since there is an analogy, and it is often treated as one in sci-fi (spaceships, the bridge, docking, etc.)
I haven’t done the math on how much atmospheric pressure would be required for a temperate rogue earth. What about an ice-age earth where we could survive with technology, where conditions were always wintry but not cryogenic?
As to the rest of your post, I agree that aerostat colonies must not be overlooked. They have great potential for creating a livable habitat and efficiently harvesting gasses. I’ve read that there’s a level in the Venusian atmosphere that is temperate and about 1 bar of pressure, and the fast winds would carry an aerostat around the planet to create a day/night cycle. Whether its Venus or a gas giant, thick atmospheres have mining potential.
http://www.gps.caltech.edu/uploads/File/People/djs/interstellar_planets.pdf
If we’re talking liquid oceans, look below the ice. Even a few kilometres below might be enough for temperate microclimates, and one can always dig out ice tubes to colonize; personally, I tend towards the vertical shaft city concept, with a “harbour” open to the ocean at the bottom for ships, or in the more likely case of their being no gas layer between the ocean and the ice, submersibles.
Aerostat floating cities or orbital habitats, any kind of habitat or space colony will necessarily be the space-based versions of Tokyo, Hong Kong, and Singapore. The future of humanity is in such city-state. If you want to live in space, you should live in one of these city-state in order to get used to what it would be like.
I realized this while reading “Schizmatrix” when I was on a Shinkansen in Japan in the early 90’s.
kurt9 said: ” The current results should a distribution with Neptune planets being the most common with a distinct roll-off of the availability of planets both smaller and larger than Neptune.”
But this what kepler scientist say about Earth-size planets:
“As for the existence of many of the even smaller planets, those Earth-size worlds that astronomers have long sought, the jury is still out. “We see a very rapid rise as we go to smaller objects, then a precipitous fall as we go to Earth-size and smaller planets,” Kepler principal investigator Bill Borucki of NASA Ames Research Center said at the meeting. “We don’t know what that represents.” It could be that the small dimming signals produced by such tiny worlds are simply hard to find in the noise of the data. Both Borucki and Marcy cautioned that it was too early to make any inferences on the frequency of Earth-size planets, which are expected to take more time to identify”
http://www.scientificamerican.com/article.cfm?id=kepler-planet-census
its too early to jump to conclusions than neptune-size planets are more common and that earth-size and small are less common.
“Aerostat floating cities or orbital habitats, any kind of habitat or space colony will necessarily be the space-based versions of Tokyo, Hong Kong, and Singapore. The future of humanity is in such city-state. If you want to live in space, you should live in one of these city-state in order to get used to what it would be like.”
I don’t get your reasoning. Why would people choose to live crowded together when they don’t have to? If you can build one O’Neill cylinder, you can build another… given the need for land for agricultural purposes (which we’re going to need, even with aeroponics, in some form), there’s no reason to bunch people together. On aerostat habitats, smaller, more numerous habitats are better, being more redundant. If you want to live in space, you should live in a medium sized town in order to get used to what it would be like.
Hello,
I also agree that it is too early to conclude that planets smaller than Neptune are more common than Earth-sized worlds. However, it is interesting from a what if standpoint to entertain the possibility that planets Earth-sized and smaller are actually rarer. What physical process could account for such a steep drop in planet occurrence below 2 Re? Is there any reason, based on existing theory, why that might be the case?
The core accretion theory of planet formation states that planets are built up from dust to sand, from sand to pebble, from pebble to boulder, from boulder to planetesimal, and finally, from planetesimal to planet. One of the predictions of this theory is that there should be more and more smaller objects. If the sub-Neptune drop off in planet occurrence is real, then core accretion may be in serious trouble unless there is some process that gets rid of these smaller planets after they form. I am no expert, but I will throw out a few ways this could happen:
1). Smaller planets are more easily scattered and shot out of their solar systems than are larger planets.
2). Smaller planets are more susceptible to spiraling into their central parent stars than are larger planets.
Or, may be planets simply just do not form by way of core accretion. This I find unlikely to be the case. But maybe gravitational collapse is the way most planets form and, below 2 Re, gravitational collapse is extremely inefficient at producing planets. I see this as an unlikely scenario because we already have evidence of several very dense rocky terrestrial planets, including all of the inner planets in our solar system.
There is a paper by Andrew Youdin of the Harvard-Smithsonian Center for Astrophysics (title is “The Exoplanet Census: A General Method, Applied to Kepler”) that specifically addresses the issue of small planet occurrence in light of the Kepler observations. It can be a found on the arXiv.org and may be worthy of a thread here on Centauri Dreams given that it concludes that the Kepler data imply essentially all Sun-like stars have planets. There is a graph in this paper that shows that there is a steep decline in detection efficiency below 1-2 Earth radii. What I am not sure about is how he is able, based on this fact alone, to conclude how many planets exist down to 0.5 Re.
“1). Smaller planets are more easily scattered and shot out of their solar systems than are larger planets.
2). Smaller planets are more susceptible to spiraling into their central parent stars than are larger planets.
Or, may be planets simply just do not form by way of core accretion.”
Or perhaps, above a certain size it’s hard to not gain enough material to become a Neptune. Maybe the Solar System got lucky, and didn’t have too much material in the inner solar system for the terrestrial planets to gather. Can you imagine if Terra had formed in the outer solar system, with plentiful (almost wrote planetful :) ) ices which could aggregate on to it? If the drop off is real, it’s certainly worth exploring as an idea (a possible PhD project for me in 2015?)…
I don’t get your reasoning. Why would people choose to live crowded together when they don’t have to? If you can build one O’Neill cylinder, you can build another… given the need for land for agricultural purposes (which we’re going to need, even with aeroponics, in some form), there’s no reason to bunch people together. On aerostat habitats, smaller, more numerous habitats are better, being more redundant.
I’m assuming that for any given amount of land area, that habitats will be inherently expensive to build. Perhaps with some kind of self-replicating robotics or nanotechnology, they can be built cheaply and, thus, have low population density in them.
Are we sure that all these so-called planets are natural? Maybe we got a few multigenerational colony ships amongst ’em.
“I’m assuming that for any given amount of land area, that habitats will be inherently expensive to build. Perhaps with some kind of self-replicating robotics or nanotechnology, they can be built cheaply and, thus, have low population density in them.”
Or, more likely, our bioengineering capabilities will be good enough to grow habitats.
Once you have the infrastructure to build one habitat, the next one will be cheaper anyway – it’s the initial mining facilities, fabricating stations etc that will cost the most. Also, expense is a tricky concept when dealing with space colonization – much better to use the term resources, since it doesn’t have the same monetary connotations. If someone manages to develop a design for cheap “microhabitats”, about the size of a small town or even a homestead, there’ll be a lot of takers. Say, two habitats linked by several, redundant, tethers, with elevators between them – such a thing might be quite simple to construct from a small asteroid. More complex designs can be imagined, the next step up being three linked together in a triangular arrangement, with extra cables linking in the middle for increased redundancy.
Cost is unlikely to have much to do with crowding in O’Neil habitats, save for the first couple of decades of the establishment phase. GDP per person has risen remorselessly and exponentially since the beginning of technological civilisation. The problem soon becomes why would all spacefaring citizens want to cram all their wealth into such a small space?
To me there are only the three reasons of real-estate: location, location, and location. People will trade-in large portions of the rest of their wealth to dwell in the right place. If high density living does turn out to be entrancingly desirable perhaps this will be driven by wanting to be wired in intimately with as many others of wealth or celebrity as possible.