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10 october 2018

While I was reading Donald Goldsmith's Exoplanets, news broke that astronomers had for the first time detected the strong possibility that an exomoon exists. Exoplanets and exomoons ("exo-" in this case meaning outside our solar system) have been virtual certainties for a long time, providing the settings for most science fiction. But no individual exoplanet was identified till about 25 years ago, and no exomoon till just now. The process of discovering these vitally fascinating objects is a crystalline detective story, told with great clarity by Goldsmith in this new book.

Exoplanets are often described in news reports as if some astronomer had just snapped a picture of one. This impression is usually misleading. Most exoplanets are inferred from ingenious chains of reasoning backed by hoards of data. Goldsmith is especially good at sorting out the several different kinds of reasoning involved in the detection of exoplanets.

Planets don't exactly orbit their stars. Both star and planet orbit their common center of mass, which means that as a given be-planeted star moves across the sky (relative to us) it gets a little bit further from us and a little bit closer, in a regular rhythm. Analysis of the spectrum of light from that star can show this to-and-fro movement, and thus the mass of the planet in orbit. This amazing step-by-step inference is, to me, more interesting than directly taking a picture of something.

The other main technique for finding exoplanets is to watch them transit, that is, to pass between us and their star. This method is familiar to earthlings from watching transits of Venus; if we lived on Jupiter, we would be able to see transits of the Earth and of Mars. But while you can take a photo of a transit of Venus (or could, a few years ago; the next isn't till 2117), photographing a transit of an exoplanet from Earth is impossible. The star itself is only a dot; you can't see a dot cross a dot. But you can see the star grow fainter and brighter again, and when that happens regularly enough, you can infer the existence of a transiting planet. (In fact, the new exomoon was detected by noticing irregularities in the transit of its planet.) When a planet is detected via transit, we can figure its size (from the percentage of starlight it blocks) but have no idea of its mass. Fortunately, for some exoplanets, we know about them both ways and know both.

It is actually possible to take direct images of exoplanets, if they are large enough compared to their stars, far enough away from those stars, and if you image them in infrared wavelengths, using special devices to mask the starlight. They still look like dots. But they are dots that nobody saw till very recently.

A few other techniques can reveal the presence of exoplanets, notably gravitational lensing that bends starlight around massive obstacles (i.e. the planets). But thousands have been discovered using radial velocity measurements (the back-and-forth spectral shifts) and transit measurements. Goldsmith explains that these planets are wildly various. Some orbit improbably close to their stars. Most are huge, which is certainly an artifact of our limited ability to sense them: we see a lot of huge planets because we can't see the smaller ones.

Not many of the known exoplanets look like good bets to harbor life, for that reason. Large planets tend to be like Jupiter, big balls of roiling gas; and the ones we have discovered tend to be much closer to their suns than Jupiter is to ours, making them both vaporous and torrid. Astronomers are keen to find rocky planets that orbit relatively dim stars – simply because those are the most Earthlike places we're likely to see. If we lived light-years from the Sun, we would likely never suspect the existence of Earth.

A bit of Fermi's paradox enters Goldsmith's book. While extraterrestrial intelligence is not his main topic, it's unavoidable when we're talking about planets far, far away. Even as we step up our study of exo-solar-systems, we continue to draw blanks when it comes to picking up alien broadcasts. Where is everybody? It remains possible that the Solar System and our own Earth are far more unusual than we think. Which only means that they are about as unusual as we used to think them, two or three hundred years ago. But exceptionalism is a risky proposition, especially given the size of the Universe and the vanishingly negligible portion of it we have yet explored.

Goldsmith, Donald. Exoplanets: Hidden worlds and the quest for extraterrestrial life. Cambridge, MA: Harvard University Press, 2018. QB 820 .G64