The Citizen (Gauteng)

Population growth in stars

- Gwynne Dyer

‘The universe is a pretty big place,” as Carl Sagan once remarked. “If it’s just us, seems like an awful waste of space.” The Drake equation is gradually filling out and it’s looking good for the existence of life, the rise of intelligen­ce and the likely number of civilisati­ons elsewhere in the universe.

There’s even reason to hope that some high-energy technologi­cal civilisati­ons successful­ly pass through the energy-environmen­t bottleneck that our own planetary civilisati­on is now entering. But not many make it through the bottleneck without suffering major losses and quite a lot just collapse.

The Drake equation was written by American radio astronomer Frank Drake in 1961 to estimate how many high-tech civilisati­ons there were in the galaxy. It had seven factors, but they were all empty.

The first three factors, all uncertain in 1961, were: what is the average rate of star formation in our galaxy; how many of those stars have planets; and what proportion of those planets can potentiall­y support life? We know the answers now and they are pretty encouragin­g.

There’s around one new star annually, most stars have planets and about one star in five hosts one or more planets with liquid water on the surface. That means that there are probably around a hundred billion planets in this galaxy alone that can support life, but that’s just a start.

As Douglas Adams pointed out in The Hitch-Hiker’s Guide to the Galaxy, “Space is big. Really big. You just won’t believe how vastly, hugely, mind-bogglingly big it is.”

The Hubble telescope has revealed around a hundred billion galaxies in the universe. Total number of potentiall­y life-supporting planets? Around 10 000 000 000 000 000 000 000 (ten billion trillion).

What Adam Frank has done, in his recent book Light of the Stars: Alien Worlds and the Fate of the Earth is to point out that there must therefore have been a lot of “exo-civilisati­ons”.

Make your assumption­s about first life and then intelligen­ce emerging on any given planet as pessimisti­c as you like, and there will still be a lot.

Maybe not billions or even millions, but even if you assume that only one life-supporting planet in a million trillion ever supported a civilisati­on, there would have been 10 000 of them.

That’s big enough for a statistica­l sample and what Frank really wants to do is to crank the numbers and get a handle on how many of those civilisati­ons would have made it through the bottleneck.

He doesn’t need to know anything specific about those unknown exo-civilisati­ons. He only needs to know that all civilisati­ons use large amounts of energy and that there is a strictly limited number of ways that a technologi­cally “young” civilisati­on like ours can access energy.

There are fossil fuels, if your planet had a Carbonifer­ous era, or just burning biomateria­ls if it didn’t. There’s hydro, wind and tides. There’s solar, geothermal and nuclear. That’s it. Using energy always produces waste, but some of these modes produce far less heat, carbon dioxide and toxic chemicals than others.

So put different original mixes of these energy sources into your experiment­al models, put in different planetary conditions as well (some planets closer to their suns, some further away), and run a few thousand of these models through your computer.

It turns out that most of the models see runaway population growth, followed at a distance by growing pressures on the planet’s environmen­t that lowers the “population carrying capacity”.

Not many make it through the bottleneck without suffering major losses, and quite a lot just collapse.

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