Humans have been gazing at the stars since times immemorial. Once we understood what stars were, and that our planet together with others circled one such star, it was only a small step to think that there must be other planets outside of our Solar System. But only in the last 25 years have we been able to start finding these so-called exoplanets. Astronomer Donald Goldsmith here promises, and delivers, an introduction that even an astronomy novice such as myself could understand and thoroughly enjoy.
“Exoplanets: Hidden Worlds and the Quest for Extraterrestrial Life“, written by Donald Goldsmith, published by Harvard University Press in September 2018 (hardback, 254 pages)
I first touched on exoplanets in my review of astrobiologist Charles Cockell’s book The Equations of Life: The Hidden Rules Shaping Evolution, who provided a very brief introduction to them as a little aside. Their existence and the remote possibility of finding worlds inhabited by other lifeforms has captured the public imagination. Just in the last two years there have been at least three other popular science book on the topic (The Planet Factory: Exoplanets and the Search for a Second Earth, Exoplanets: Diamond Worlds, Super Earths, Pulsar Planets, and the New Search for Life Beyond Our Solar System, and One of Ten Billion Earths: How we Learn about our Planet’s Past and Future from Distant Exoplanets). So where do you start? Though I haven’t read these books yet, what I can tell you is that Goldsmith here provides an inclusive overview.
After familiarising the reader with the vast distances involved and the units astronomers use, including Astronomical Units, light years and parsecs (yes, the one George Lucas famously got wrong), the first half of the book dives into the how. Given that even the nearest stars are very far away, and planets emit no light, how, indeed, do you find them?
“Given that even the nearest stars are very far away, and planets emit no light, how, indeed, do you find them?”
Two indirect methods have been particularly successful and are explained here in-depth by Goldsmith: radial velocity measurements and transit observations.
The former relies on the fact that as a planet circles a star, it exerts a small gravitational tug on the star, causing both the star and the planet to turn around a shared centre of mass that is not exactly at the centre of the star, but somewhere towards its edge or even outside of it. In effect, the star wobbles, and this wobble can be observed in changes in the wavelength of the light that reaches us. Remember the Doppler effect? How a siren sounds higher in tone when an ambulance approaches you, and lower in tone when it moves away from you? It is the same for light. The differences are minute, but with sensitive enough equipment these shifts in wavelength can be measured.
The second method, pioneered by NASA’s now-retired Kepler space telescope, is perhaps easier to grasp: a planet moving in front of its star causes a temporary blip in the intensity of the star’s light. If these observed blips are regular enough you have a candidate exoplanet. Of course, this only works when a planet’s orbit passes in front of a star from our vantage point, but there are plenty of stars out there for which this is true.
“What we have discovered and catalogued so far has very much surprised everyone […] our Solar System is far from the only possible configuration. “
These two methods have led to the discovery of thousands of exoplanets. But Goldsmith would not be thorough if he did not also explain other methods, such as direct observations using the infrared radiation emitted by planets, gravitational lensing (the bending of light’s path due to the proximity of massive objects – one of those counterintuitive phenomena Einstein predicted), orbital brightness modulation (where a star’s observed brightness fluctuates ever so slightly by light reflected off nearby orbiting planets), changes in light polarization (also due to reflection off a planet), and several other obscure methods, some of which have so far been unsuccessful. I found his explanations here clear, and they gave several of those “aha” moments. For example when he explains adaptive optics, an engineering solution employed in modern telescopes to correct for image distortion caused by Earth’s atmosphere (so that is why I keep seeing photos of telescopes shooting a laser beam into the sky).
Throughout, Goldsmith is careful to point out the limitations and biases of current methodologies. Despite their success, the resolution of most methods is still so poor that we can only find planets (much) bigger than Earth. Technological improvements are continuing apace though, and new observatories, whether on Earth or in orbit, are finding smaller and smaller planets.
What we have discovered and catalogued so far has very much surprised everyone. Goldsmith takes the reader through an eye-opening tour of the weird and wonderful planetary systems; giant planets racing around stars at a fraction of the distance between our Sun and Mercury, planets around binary stars, planets young and old, big and small, dense and fluffy… our Solar System is far from the only possible configuration. Goldsmith adds a very interesting chapter on how this affects our existing theories of star and planet formation.
“For the moment, the exoplanet search is largely a cataloguing exercise and we have gathered only the most basic of [information].”
But it is not all hard facts. Goldsmith permits himself plenty of informed speculation towards the end of the book as he ponders which planets are likely to be friendly to life as we know it. For the moment, the exoplanet search is largely a cataloguing exercise and we have gathered only the most basic of estimates on sizes, masses, distances to stars, and orbital periods. Ongoing and planned projects such as ESA’s PLATO mission and NASA’s James Webb Space Telescope will hopefully change this and Goldsmith gives an informative overview of future missions and the technical challenges that need to be overcome to improve our detection methods. He happily veers into near science fiction when speculating about future missions that will send nanoprobes into interstellar space to pass by the nearest discovered exoplanets, and the hypothetical possibility of sending humans on interstellar missions.
Goldsmith lightens up his writing with the occasional wry observation on the quirks of his profession, and quotes from fellow astronomers who he has interviewed. There are places where I would have liked a few more illustrations to explain certain principles, although the ones that have been included are useful and clear, having been carefully redrawn for this book.
The strong suit of the book is its solid writing though: Goldsmith goes into plenty of technical detail but he never lost me. The book strikes the right balance between starting from first principles for readers without a background in astronomy or astrophysics in a way that is not patronising, while delivering plenty of technical details and caveats about what we know so far. Above all, he transmits the sheer awe that this fast-moving field inspires.