There's something impressive about sticking your neck out in a big way, then being proven wrong almost at once. Astronomy and aviation seem to be particularly rich fields for this.

For instance, since ancient times six planets were known. They were Mercury, Venus, Mars, Jupiter, Saturn, and the Earth-Moon system. Then in 1781, William Herschel discovered a seventh one, Uranus. (He originally wanted to call it "George's Star," after King George III, but fortunately he was talked out of it.) After Herschel's discovery, the philosopher, Hegel, declared that seven planets were all there could ever be. He had proved it, he said, by strictly logical arguments. Unfortunately for Hegel, the minor planet Ceres was found soon afterwards, in 1801.

Hegel's views on the discovery are not recorded, though he could well have grumbled that although Ceres was a planet, it was a very small one. He had to live with Ceres for the remaining thirty years of his life, although he died before the discovery of the planet Neptune, in 1846, or Pluto, in 1930.

You might think that people would have learned from Hegel's misfortune. But no. In 1844 another philosopher, Auguste Comte, made the flat assertion that we could never learn anything about the composition of distant stars or planets. He was a little luckier than Hegel, because he died in 1857 - three years before the invention of the spectroscope made it possible to say a great deal about far-off stars and planets.

Other scientists have not been much more fortunate. Lord Kelvin, in 1892, declared that heavier-than-air flying machines were impossible. The most famous American astronomer of his day, Simon Newcomb, said the same thing in 1900. The Wright brothers made him eat his words just three years later. In 1920, a New York Times editorial made an equivalent assertion about the impossibility of a rocket ever rising above the Earth's atmosphere. In 1957, the British Astronomer Royal, Sir Richard Woolley, firmly and famously declared, "Space travel is utter bilge."

What have we learned from all this? Not quite enough. Less than thirty years ago, reputable astronomers were saying that although there might well be planets around other stars, we could never know of their existence. The closeness of the planet to the star, and the weakness of its reflected light compared to the star's own brilliance, meant that no telescope would ever be able to pick out the tiny gleam of the planet against the star's glare.

Today we know of dozens of planets orbiting distant stars. How is this possible, since we have still never actually seen one?

We use a method that has been understood in principle for more than three hundred years, ever since Isaac Newton described the law of gravity. Although we normally say that a planet goes around a star, it is more accurate to say that the two of them orbit around each other. The presence of the planet introduces changes in the position of the star - very small ones, because the planet is so much lighter than the star, but possibly enough for the star's movement to be detected.

We can attempt that detection in two different ways. First, we can look for small changes in the star's apparent position in the sky relative to other stars, with a period equal to the period of the planetary year. The change will be tiny, but if the planet is large, the movement of the star may be big enough to measure. This method sounds good, but it has not so far been successful.

The other, and successful, method of detection also relies on the fact that the star and planet orbit around each other. However, in this case we look for a shift in the wavelength of the light that we receive from the star. When a star is approaching us because the planet is moving away from us, the starlight will be slightly shifted toward the blue part of the visible spectrum. When the star is moving away from us because the planet is approaching us, the star's light will be shifted toward the red part of the spectrum.

Both these shifts are caused by something known as the Doppler effect, which is exactly the same phenomenon as causes a police car's siren to sound higher in pitch when it races toward you, and lower in pitch when it has gone past and is moving away from you. The tiny shift in the star's light spectrum caused by the Doppler effect allows us to infer the existence of a planet. Coupled with the information about the period of the changes, we can also estimate the mass of the planet.

Since both methods of detection depend for their success on the planet's mass being an appreciable fraction of the star's mass, it is no surprise that so far we have been able to detect only massive planets, several hundred times as big as the Earth. This does not mean, of course, that only big planets exist around other stars. The instruments that we have are just not sensitive enough to detect small planets.

In the past few months, a third method has been used that comes much closer to direct observation. If a star possesses a planet, sometimes it may pass directly between us and the star. When that happens, the dark planet will cut off some of the star's light. It will be only a small fraction of the total, but we can look for the dip in light intensity during the transit of the planet across the star's bright face. Such a dip was observed, but again the planet was a large one, far bigger than the Earth.

So other stars do have planets, and every month we learn of new examples. However, the question of life elsewhere in the universe makes us ask something rather different. There are planets around other stars, but are any of these Earth-like planets, in both size and other properties?

I don't know. Nobody does. However, with the specters of Hegel, Comte, Kelvin, and Newcomb peering over our shoulders, I am certainly not going to suggest that we will never know.

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