An example of analogy used in popular science writing:
"To make the Solar System easier to picture, I like to think of it as a city, with the different parts of it as neighbourhoods. The planets, comets and asteroids are all part of the overall city and its suburbs in some way, but the places where they are found represent very different areas of it, from the busy and lively downtown communities (i.e. the planets of the inner Solar System) to the quiet, calm and more sparsely populated suburbs (i.e. the comets.)"
"The size of the Solar System, and the problems in visiting its farthest corners, mean that scientists are still drawing a detailed picture of what the real estate surrounding our average star contains, and trying to understand how it formed."
"Before the planets formed, the Solar System was nothing but a swirling cloud of gas and dust, which was itself travelling through space. If we want to draw on the Solar-System-as-a-city metaphor here, we can think of this as the peaceful and luscious green countryside existing before our metropolis was built. This swirling cloud eventually became the construction site for everything we see [sic] today, including the planets, asteroids and comets, from the largest gas giant, Jupiter to the smallest specks of dust that travel around in the space between the planets.
The outer edge of this cloud eventually became the immensely cold and far fringes of today's Solar System – places that are hard to see from Earth with even the most powerful telescopes, where barely any of the Sun's energy can reach. This represents the true outskirts of our city, the boroughs that barely feel like they are part of the conurbation at all. Many comets formed at the very edge of this cloud and for well over 4 billion years they have lingered in this remote neighbourhood. These comets are so far away that they could be represented by the rural communities that often surround large cities. In the same way that daily life in these farming regions is little affected by the hustle and bustle of the capital, these comets have been able to remain perfectly in deep freeze, preserving the early captured planet-building ingredients that they scooped up billions of year ago."
"We can think of asteroids as the as the leftover Solar System building rubble that wasn't incorporated into one of the major city skyscrapers, or planets in the case of the Solar System."
"One of the problems of sampling and measuring comets and asteroids is that they reside very, very far away from Earth. In fact, much like the city dweller without access to a car who never ventures out as far as the greenbelt, no spacecraft has ever travelled as far as the outermost comet neighbourhood. Luckily, comets and asteroids occasionally exit their cold, dark home and pay a visit to the inner Solar System. We can, perhaps, think of these objects as the out-of-towners travelling into the city for some weekend sightseeing. These people zoom through the outskirts of the town on their high-speed train line, just like the comets and asteroids that whizz past the planets on their orbits around the Sun. When these usually far-flung space objects make such journeys, it gives scientists a fighting chance of getting near enough to them , within a reasonable timeframe to study them up close."
"Despite the fact that we may have to thank comets and asteroids for our very existence here on Earth, there are some serious downsides to them entering the city centre of the Solar System."
"One of the cometary neighbourhoods, the so-called Oort Cloud, is so far out that it is barely within the gravitational grasp of our star. This is the region of the Solar System that we likened to the rural communities existing in the greenbelt surrounding a city, each one located a great distance form the others, but all looking similar in nature. The closer comet neighbourhood is the Kuiper Belt, sometimes known as the Edgeworth-Kuiper Belt, which is still 10 times further from the Sun than the asteroid belt. The Kuiper Belt acts much like the outer suburbs of the city, not quite the very edge, but a journey into the city would be a long and arduous one."
"Some estimate suggest that the outer edges of the Oort Cloud may even be closer to the next star system than to our own Sun. These icy objects really are [sic] those more remote rural farms that are as close to our Solar System city as they are to the neighbouring city, yet still a great distance from both. It is the gravitational influence of passing stars, and even the Milky Way itself, that can perturb and tug at the outer Oort Cloud comets, loosely bound to the Sun because of its weak gravitational influence at this distance, disturbing them from their orbit and sending them careering into the inner Solar Systm. Perhaps if we stretch our Solar System-city metaphor a bit further we can suggest that these comets are like the farm workers who seldom get a night off from their hard labouring, but when they do they travel a long way to the city for a wild night out."
"Even if objects were formed in the far outer reaches of the Solar System, they haven't necessarily ended up staying there permanently, and vice versa. Much like the inhabitants of our Solar System-city having moved around throughout their lifetimes, perhaps starting their young life in the suburbs, migrating for work to the bright lights of the city and eventually back out again as they grow older, the Solar System objects have, in some cases, experienced a similar life.""The fact that the comet-forming region in the outer Solar System is vast, and sparse, means that most comets are separated by at least several hundreds of kilometres. Remember the analogy to those remote farms situated on the outskirts of the city?"
"The results show that you can't judge one comet by another, even those that grew up in the same neighbourhood."
"This applied, in particular, to the long-period comets that can appear in the inner Solar System rather unexpectedly, on a random orbit, after departing their home in the Oort Cloud."
Natalie Starkey
Starkey, N. (2018). Catching Stardust. Comets, asteroids and the birth of the solar system. Bloomsbury Sigma.
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