The Science of Discworld Revised Edition

– Earth. The instruments were doing the wiggling at
this
end, not the pulsar at the far end.
    Scarcely had this startling claim of a pulsar planet been withdrawn, however, when Aleksander Wolszczan and Dale Frail announced the discovery of two more planets, both circling pulsar PSR 1257+12. A pulsar solar system with at least two worlds! The way you wiggle when you have two dancing partners is more complex than the way you do it with one, and it’s difficult to mistake such a signal for something generated at the receiving end by the motion of the Earth. So this second discovery seems to be fairly solid, unless there is a way for pulsars to vary their output signals in just such a complex manner without having planets – maybe the radio beam could be a bit wobbly? We can’t go there to find out, so we have to do the best we can from here; and from here it looks good.
    So there do exist planets outside our solar system. But it’s the possibility of life that really makes distant planets interesting, and a pulsar planet with all those x-rays is definitely not a place for anything that wants to be alive for very long. But now conventional stars are turning out to have planets, too. In October 1995 Michel Mayor and Didier Queloz found wobbles in the motion of the star 51 Pegasi that were consistent with a planet of about half Jupiter’s mass. Their observations were confirmed by Geoffrey Marcy and Paul Butler, who found evidence for two more planets – one seven times the mass of Jupiter orbiting 70 Virginis, and one two or three times Jupiter’s mass orbiting 47 Ursae Majoris.
    By 1996 seven such planets had been found. As we write, about 70 extrasolar planets have been detected, either by the wobble method, or by observing the light output from a star and seeing whether it changes as an orbiting planet reflects different amounts of its light. Theoretical calculations show that with improved telescopes, this method might even be able to detect how fast the planet is rotating. Even now, new extrasolar planets are being found virtually every week. The exact number fluctuates because every so often astronomers discover problems with previous measurements that cast doubt on somebody else’s favourite new planet, but the general trend is up. And our nearest sunlike neighbour, epsilon Eridani, is now known to possess an encircling dustcloud, perhaps like our Sun’s Oort cloud, thanks to observations made in 1998 by James Greaves and colleagues. We can’t see any wobbles, though, so if it has planets, their mass must be less than three times that of Jupiter. A year earlier, David Trilling and Robert Brown used observations of a similar dustcloud round 55 Cancri, which does wobble, to show that it has a planet whose mass is at most 1.9 Jupiters. This definitely rules out alternative explanations of the unseen companion, for example that it might be a ‘brown dwarf’ – a failed star.
    Although today’s telescopes cannot detect an alien planet
directly
, future telescopes might. Conventional astronomical telescopes use a big, slightly dish-shaped mirror to focus incoming light, plus lenses and prisms to pick up the image and send it to what used to be an eyepiece for an astronomer to look down, but then became a photographic plate, and is now likely to be a ‘charge-coupled device’ – a sensitive electronic light-detector – hooked up to a computer. A single telescope of conventional design would need a very big mirror indeed to spot a planet round another star – a mirror some 100 yards (100 m) across. The biggest mirror in existence today is one-tenth that size, and to see any detail on the alien world you’d need an even bigger mirror, so none of this is really practicable.
    But you don’t have to use just one telescope.
    A technique known as ‘interferometry’ makes it possible, in principle, to replace a single mirror 100 yards wide by two much smaller mirrors 100 yards apart. Both produce images of the same star or planet, and the incoming light waves that form those images are aligned
very
accurately and combined. The two-mirror system gathers less light than a complete 100-yard mirror would, but it can resolve the same amount of tiny detail. And with modern electronics, very small quantities of incoming light can be amplified. In any case, what you actually do is use dozens of smaller mirrors, together with a lot of clever trickery that keeps them aligned with each other and combines the images