My mental map of the universe has evolved over my life, partly due to new scientific discoveries and partly due to my own education. As if often the case, as you learn more, things get more complicated. The simplistic picture I had as a child was that the universe consisted of many galaxies in which there are many stars and around which there are planets, likely something similar to our own solar system. I have had to modify this model dozens of times, and perhaps I need to make another little tweak. This has to do with where planets exist.

First, galaxies are not randomly distributed throughout the universe. They are bound together in local groups, those groups are gravitationally bound into galaxy clusters, which in turn are part of superclusters which are finally organized into giant filaments, the largest gravitationally bound structures in the universe. So our universal address is – the Sol system within the Milky Galaxy, part of the Local Group within the Virgo Cluster which is part of the Laniakea Supercluster.

At some point I also learned that not all stars (and therefore, not all planets) exist within galaxies. Estimates of the number of stars within and between galaxies just overlap, so they may be equal, but the average estimates indicate that likely 1-10% of all stars are not in galaxies. They are wandering between galaxies, mostly within galaxy clusters. The first intergalactic star was discovered in 1997. It is likely that most such stars were formed within galaxies (you need clouds of gas and gravitational disturbances for stars to form) but then were flung out because of gravitational interactions with other objects, such as a black hole. Two galaxies colliding can also spray their stars throughout the cluster. It is also very likely that such stars would retain their planets.

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In Isaac Asimov’s The Caves of Steel he imagines a future in which most of humanity lives in gigantic cities, extending many levels underground. This leaves the majority of the Earth’s surface for industrial farming, necessary to feed all those densely populated cities. If you take a similar strategy, however, and keep the population at sustainable numbers, this could maximize land for natural ecosystems and also minimize the environmental footprint of the average person. So if we are going to plan our civilization around environmental sustainability we would not necessarily need Asimovian megacities, but we could concentrate the population in cities and in densely developed areas around cities, and leave large stretches of land in between undeveloped. This is far better than endless suburban sprawl.

But of course, we are not starting from scratch, and our current layout was not planned by some central committee but evolved organically. Pragmatically, the question we need to ask is – where do we go from here. Cities are growing dynamic things, so we can use city development to move in a certain desirable direction, even if we can’t tear it all down and start anew. This means it is important to study what the best city planning and development would be going forward.

When most people think of city planning to reduce the carbon emissions of transportation, the first thing that comes to mind is planning city centers so that they are walkable/bikeable and to provide public transportation, in order to minimize reliance on fossil-fuel burning cars. This also has the advantage of reducing city traffic, which can be a nightmare. However, a recent study suggests that, while important, this may be of secondary concern with respect to impact on CO2 emissions. For many cities, especially those with a single concentrated hub (as opposed to multicentric cities, like LA), the most impactful strategy might be the densification of a ring surrounding the city center. The range of this ring depends on the city, but is something like 10-20 km for a typical large city, but can extend to 40 km. Increased density can be accomplished with infill development, as many such zones are only moderately developed leaving lots of room for densification.

The idea is that the workplaces will be concentrated in the city center, and the workers will live in the ring around the city center, minimizing their commute distance. This could have a significant impact on the average commute distance that people have and therefore their transportation carbon footprint. This approach would work better for some cities than others, depending on geography. This plan could also maximize the impact of public transport, like buses and trains, dedicated to bringing people back and forth from this densified ring to the city center.

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