Close, but no cigar...

The initial mission will therefore last for two years, and cover almost the entire sky, looking at 200,000 stars in total for exoplanets.

Given what we’ve learned about exoplanets from previous searches (like with Kepler), TESS [the Transiting Exoplanet Survey Satellite] is expected to find anywhere from 4,500 to upwards of 20,000 such worlds. Mind you, the first exoplanet was discovered in 1992, and we’ve found roughly 4,000 more in the 26 years since then. TESS will likely double that number in just two years.

Phil Plait of Bad Astonomy fame. 

It sounds exciting, and it is. The first exoplanet wasn't discovered until 1992. Since then, astronomers have found thousands and it seems from their initial results that planets are incredibly common. This wasn't always what astronomers believed. I've got a very old guide to astronomy, The Stars In Their Courses by Sir James Jeans, first published in 1931.  

As I've blogged before, I like this book, particularly for the way it conveys something of the incomprehensible vastness of our solar system and the distances to the stars using simple language and scale comparisons with everyday objects and distances on earth.

But it is also a book of its time and it includes a number of theories which have now been superseded. Notably, Jeans supposes that the solar system came into being as the result of a close encounter between the sun and another star which passed close by in the distant past. The gravitational pull of the passing star, he thought, ripped a "long filament of hot filmy gas" shaped "rather like a cigar" from the surface of the sun. It was from this cigar of matter that he supposed the planets condensed, with tiny Mercury and Pluto (Pluto was still an official planet back then), forming at the thin ends of the cigar and Jupiter and Saturn condensing in the fat middle where there was more stuff:

 

If the solar system really had been the result of such a chance encounter, we might expect planets to be rare and for most stars to shine their lonely lights on planetless neighbourhoods. Our world - in fact, our whole solar system - would be a rare aberration.

But now astronomers believe that planets formed from clouds of gas and dust left over from star formation. These protoplanetary discs are thought to be a normal part of star formation, meaning that we should expect most stars to be accompanied by the planets which condense out of these discs.

Less than a century ago, many astronomers thought that most other stars were barren, companionless points of light. Less than three decades ago, they hadn't detected a single planet around another star. Now they've found thousands and expect to find as many more in the next couple of years. And that's just scratching the surface of the billions that are probably out there in our galaxy.

Contemporary readers of James Jeans' book would have been staggered, as I was, by the sheer inhuman immensity of the universe. What they didn't know about was the sheer number of worlds, scattered like dust across that vast emptiness.