Turn Right At Orion by Mitchell Begelman

Author:Mitchell Begelman
Language: eng
Format: mobi, epub
Publisher: Basic Books
Published: 2012-02-29T23:00:00+00:00

18

The Shepherd

Serendipity—finding important things by chance—has always been one of the astronomer’s best friends. By definition it is unpredictable, and its impact is often most poignant when conditions look least hopeful. It is safe to say that the most significant discoveries made by my generation of astrophysicists were serendipitous; the jets of SS 433 offer a good example. Astronomers looking for one thing found something entirely different, and in many cases they weren’t even looking. The discovery figuratively descended from the sky and bopped them on the head. My brush with serendipity was not so dramatic. Faced with two unpalatable choices in my search for planets in formation, I unexpectedly found a third way.

Across a gap of what couldn’t have been more than 2 or 3 light-years, and partially embedded in the wall of molecular gas behind the luminous façade of the Orion Nebula, I spied another disk that had the earmarks of dust but also exhibited some interesting features that were missing from my present venue. This disk was banded with dark and narrow concentric rings, where dust seemed to be absent. Like the rings of Saturn, I thought. During my childhood days as an amateur astronomer, it had been considered an easy test of visual acuity to spot the “division” in the rings that had first been noticed by Cassini in the seventeenth century. This could be done with a small telescope. If you had a bigger scope and a steady eye, you could find hints of the many other narrow gaps that observers had discovered over the years and that stood out prominently when the first close-up pictures came back from Pioneer and Voyager. It later turned out that Uranus, Neptune, and even Jupiter had rings, although these were far beyond the detection capabilities of amateurs with small telescopes. The latter systems were like negative images of Saturn’s rings. Instead of dark gaps between bright annuli, the divisions consisted of narrow, bright rings of reflective particles separating broad, empty spaces.

A theory had been developed to explain both the gaps and the narrow rings. Neither had been expected, because even the small random motions of the particles (which would necessarily arise from the same kinds of gravitational effects that had generated them in the disk I had just visited) would cause narrow rings to spread and merge and gaps to fill in quickly. It was hypothesized that the rings were held in trim, and the gaps kept open and sharp, by small moons orbiting the planet. To clear out a gap, a single moon would have to orbit within it. And to channel particles into narrow rings, there would have to be a pair of moons, locked in synchronized orbits on either side. These moons didn’t have to be very big; in fact, their existence had been predicted simply because their presence would explain the gaps and rings, long before they were seen. When they finally were discovered, it caused a sensation in the world of planetary studies.

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