Any regular reader of these columns will know that I am fascinated by the question of the origin of life on Earth. If it began here, how did it come into existence? If it was imported from somewhere else, as Svante Arrhenius proposed almost a hundred years ago, where did it come from?

In Arrhenius's time, the answer was obvious: if life did not arise on Earth, then it did so on some other planet in our galaxy. This does not so much answer the original question as postpone addressing it, but those seemed the only options. Clearly, life could not come into existence in empty space.

However, there are places in our solar system and galaxy other than stars, planets, and moons. Two well-known scientists, Fred Hoyle and his one-time student, Chandra Wickramasinghe, were led to consider those other places by an oddly roundabout route. They were addressing the question of the composition of interstellar dust, which is present in space in sufficient amounts to interfere with observations of distant stars and galaxies. Early suggestions, that the dust might be minute particles of iron or water-ice, each about a micrometer across, were ruled out because the dust motes did not scatter or absorb light at the right frequencies.

Wickramasinghe began to explore other alternatives. One that seemed promising was graphite particles. Graphite is just a form of carbon, and stars during their lifetimes expel a good deal of carbon. The way that graphite scattered light in lab experiments was consistent with telescope observations, so it seemed for a while that the problem was solved. Unfortunately, later observations showed that interstellar dust also strongly absorbed infrared light at wavelengths of 3.4 and 10 micrometers. Graphite does not absorb at those wavelengths, so it had to be crossed off the list of candidates.

However, one of the unique properties of carbon is that it can form an enormous number of compounds. The field of chemistry that deals with carbon compounds is usually called "organic" chemistry, because for a long time it was believed that such compounds could only be created by living organisms. That is now known to be wrong, but we seem to be stuck with the name and we will use it here.

Hoyle and Wickramasinghe now had a new idea, one that led to a new problem. In addition to carbon, interstellar space also contains hydrogen, oxygen, and nitrogen. Possibly the dust was composed not of simple carbon, but of one of its compounds. But which one? There are hundreds of thousands or millions of possible candidates, ranging from simple molecules (such as methane or carbon dioxide) to extremely complex ones. How could all those possibilities be tested in a reasonable time?

Hoyle and Wickramasinghe, on the basis of the possible existence of complex organic molecules in space, were already tilting toward the notion that life itself might exist there. However, it seemed at first like a wild improbability, which could never be tested. Then Wickramasinghe had a bright idea. He had been worrying over the difficulty of testing the innumerable kinds of organic molecules. Doing them one at a time would take more than a scientist's lifetime. But there was a possible shortcut. Bacteria, as part of their normal life processes, produce a vast variety of different organic molecules. Many bacteria also are about the same size as grains of interstellar dust, something which at the time seemed coincidental.

Wickramasinghe and his students took the dead husks of bacteria and shone light through them. The general absorption pattern was very close to what had been observed for interstellar dust. More than that, light that had passed through the bacteria displayed another characteristic absorption profile between two and four micrometers. Here was a prediction, something that could be tested experimentally by examining the light from some suitable distant source. And Hoyle knew of just such a source, an object known as GC-IRS7 that is located toward the center of our galaxy and is a powerful emitter of infrared radiation.

Observations were made, in bootleg fashion, by Wickramasinghe's brother who happened to have a little extra observing time on an Australian telescope. The results agreed exactly with those predicted by the laboratory experiments. The dust of interstellar space, improbable as it seemed, looked like nothing more than dead bacteria.

Hoyle and Wickramasinghe's announcement of their results was, to put it mildly, not well received. Other astronomers, ranging in their reactions from frankly skeptical to scoffing, pointed out that the total mass of interstellar dust in our galaxy was about ten million times that of a star. And bacteria are tiny. Where could such a huge number of them come from?

That problem is not quite as it may seem, because given a suitable supply of nutrients, bacteria can reproduce at a phenomenal rate. With each bacterium dividing in two in an hour or less, it's easy to calculate that a week or two is more than long enough to generate the number necessary to fill the galaxy. The real question is, where could the supply of nutrients come from?

There is only one plausible answer. As I said at the beginning, space contains objects that are not stars, planets, or moons. There are also comets out there. In our own solar system, the Oort Cloud, far from the Sun, is believed to be made up of hundreds of billions of them. Comets are known to contain complex organic molecules and ice, and earlier in the history of the solar system they probably had interiors kept liquid by the energy of radioactive decay. They would be a suitable environment for the development of life.

One problem remains. Suppose that bacterial life did arise within a comet, somewhere in the galaxy. There is more than enough comet mass to account for the interstellar dust, but how would bacteria escape from a comet, and how would they spread?

That turns out to be rather easily answered. All the time the paths of comets are disturbed by gravitational forces. Some fall in closer to the parent star. Heat boils off cometary materials, including any bacteria, and radiation pressure is then more than enough to carry them out to the depths of interstellar space. There, they ultimately reach new homes in comets or planets orbiting other stars.

Including, eventually, the planet that we call the Earth.

Does all this sound a bit improbable? It does. But ask yourself: is what I have described more improbable than life itself?

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