Twenty Worlds: The Extraordinary Story of Planets Around Other Stars (1*, but possibly 5* for some)
I have a backlog of books to write up.
Twenty Worlds is a clear account of difficult concepts explained by means of well-chosen analogies to make them accessible to non-specialists. Enthusiasts of the undoubtedly brilliant scientific reasoning behind astronomical discovery might well give it five stars (there is, after all, no shortage in the universe) but I couldn’t finish it. Unjustly, under my system, that scores only one.
It is a personal starring system, as personal as the giving and receiving of books. You express how much you like a particular book – namely Andrew Cohen and Brian Cox’s The Planets, reviewed previously in November – and receive others perceived to be similar. This isn’t. My expression should have been more specific. What it does is to select twenty planets orbiting distant stars and explain the methods by which they were discovered, and what they tell us. They have names like 51 Peg b. There is little about what these worlds might be like.
There are some interesting ideas. For example, how can we know there are planets around distant stars if our telescopes cannot see them? It turns out, in a sense, that when a planet orbits a star, the star also orbits the planet. It is a bit like a seesaw with you or me on one end and an elephant on the other. It would balance, but the balance point would be very close to the elephant. Strictly, Jupiter does not orbit the sun, it orbits a balance point just above the sun’s surface. The sun orbits the same balance point. Viewed from a distance, Jupiter’s orbit makes the sun appear to wobble towards and away, and from side to side. The earth makes it wobble too, but almost undetectably because the earth is much smaller and lighter than Jupiter. The earth-sun balance point is inside the sun close to its centre. This phenomenon reveals only huge, heavy planets.
But how do we know a star wobbles when we cannot even see that? Another analogy explains it: the Doppler Effect, i.e. the way the sound of a fire engine changes from a higher to a lower pitch as it passes by. This happens because, when the fire engine is moving towards you, the sound waves are closer together than when it is moving away (the waves still travel at the same speed but are emitted from increasingly closer points and then increasingly distant points). The same happens with light. So, if the orbit of a planet causes a distant star to wobble repeatedly towards and away from us, the frequency of the light keeps changing. The spectrum of light from a star contains dark lines where some frequencies have been filtered out by local conditions, and these dark lines will oscillate towards one end and then the other end of the spectrum. That is what we are able to detect.
It gets cleverer, such as detecting planets by variations in the brightness of stars as planets move in front of them, and analysing the spectrum of light emitted by a planet to determine the constituents of its atmosphere. It is even possible to photograph some of these planets: the book includes a picture of four white spots around star number HR 8799 and discusses the imaging techniques that make this possible.
It is a personal starring system, as personal as the giving and receiving of books. You express how much you like a particular book – namely Andrew Cohen and Brian Cox’s The Planets, reviewed previously in November – and receive others perceived to be similar. This isn’t. My expression should have been more specific. What it does is to select twenty planets orbiting distant stars and explain the methods by which they were discovered, and what they tell us. They have names like 51 Peg b. There is little about what these worlds might be like.
There are some interesting ideas. For example, how can we know there are planets around distant stars if our telescopes cannot see them? It turns out, in a sense, that when a planet orbits a star, the star also orbits the planet. It is a bit like a seesaw with you or me on one end and an elephant on the other. It would balance, but the balance point would be very close to the elephant. Strictly, Jupiter does not orbit the sun, it orbits a balance point just above the sun’s surface. The sun orbits the same balance point. Viewed from a distance, Jupiter’s orbit makes the sun appear to wobble towards and away, and from side to side. The earth makes it wobble too, but almost undetectably because the earth is much smaller and lighter than Jupiter. The earth-sun balance point is inside the sun close to its centre. This phenomenon reveals only huge, heavy planets.
But how do we know a star wobbles when we cannot even see that? Another analogy explains it: the Doppler Effect, i.e. the way the sound of a fire engine changes from a higher to a lower pitch as it passes by. This happens because, when the fire engine is moving towards you, the sound waves are closer together than when it is moving away (the waves still travel at the same speed but are emitted from increasingly closer points and then increasingly distant points). The same happens with light. So, if the orbit of a planet causes a distant star to wobble repeatedly towards and away from us, the frequency of the light keeps changing. The spectrum of light from a star contains dark lines where some frequencies have been filtered out by local conditions, and these dark lines will oscillate towards one end and then the other end of the spectrum. That is what we are able to detect.
It gets cleverer, such as detecting planets by variations in the brightness of stars as planets move in front of them, and analysing the spectrum of light emitted by a planet to determine the constituents of its atmosphere. It is even possible to photograph some of these planets: the book includes a picture of four white spots around star number HR 8799 and discusses the imaging techniques that make this possible.
And so it continues. In essence, this is astrophysics without the mathematics. I gave up. Lazy, I know, but just like in a neutron star, electron degeneracy pressure was unable to stop my brain from collapsing in upon itself and pulsing out radio waves.
If you find this fascinating, this may be the book for you.
If you find this fascinating, this may be the book for you.
Key
to star ratings: 5*** wonderful and hope to read again, 5* wonderful,
4* enjoyed it a lot and would recommend, 3* enjoyable/interesting, 2*
didn't enjoy, 1* gave up.
