The search for planets

How we managed to discover planets orbiting other stars is quite a story.

Actually, how we came to find the planets in our own Solar System is an interesting story too: one that covers our entire history, dating back to when we first started to look hard at the sky and to record what we saw.

At night our ancestors saw the stars wheeling across the sky as the hours passed, saw different constellations at different seasons. They also deduced that what constellations they saw was dictated by whether the Sun was in the sky.

The stars were still there, but invisible in the glare. People did note that under certain circumstances, they could see stars during the day, for example, looking up from the bottom of a dark hole or a hollow tower.

They also noted that the patterns of the stars did not change. However, there were some bright star-like objects that moved against the starry background over days, weeks and months.

The Greeks called them wandering stars, or planets. These wandering stars moved to and fro along a strip of sky called the zodiac. They named them Mercury, Venus, Mars, Jupiter and Saturn. All of these planets are relatively easy to spot.

The scenario of seven objects moving around the sky (including the Sun and Moon) remained for a long time. After all, seven was a number of mystical importance. Then, the telescope was invented.

The early telescopes, like the ones used by Galileo, had relatively poor light-collecting power, and even worse, could only easily observe a small piece of sky.

It was a point-and-observe instrument, not good for searching the sky for new things, although, obviously, Galileo and his contemporaries must have done that. What was needed was a revolution in telescope design.

This happened in 1668, when Isaac Newton made the first reflecting telescope. Instead of using a convex lens to collect the light and form the image, he used a concave mirror.

In addition to not having the false colours and other problems of the lenses available at the time, mirrors could be made big, because they could be supported from behind. Large mirrors mean collecting lots of light and detecting fainter objects.

William Herschel made several large reflecting telescopes and used them to search the sky for comets. In 1781, he found a new planet, which was named Uranus.

Newton's theory of gravity made it possible to precisely analyze planets' orbits, and showed that in addition to planets being pulled at by the Sun, they also tug and tweak at each other, slightly changing their orbits.

Measurements of the motions of the outer planets by Urbain Le Verrier led him to conclude there was another large planet out there. In 1846, Johann Gottfried Galle looked at the predicted position and found the culprit, now known as Neptune, a similar body to Uranus.

In 1906, Percival Lowell, who had dedicated most of his life to the study of Mars, did similar studies of the orbits of the outer planets, including Neptune, and concluded their orbits were being perturbed by an unknown, ninth planet.

Lowell failed to find it. Ironically, he had recorded faint images of this unknown planet in 1915, but hadn't noticed. Finally, in 1930, Clyde Tombaugh did.

However, it turned out that Pluto was too small to explain the orbit perturbations used to find it, so this discovery might have been more due to serendipity and dogged searching than science.

Today, we have more powerful and sophisticated instruments than astronomers in the past could have dreamed of. We can search large areas of sky, look for very faint objects and measure small perturbations in orbits.

We are finding many unknown bodies at the outer edges of the Solar System, but so far, no more planets.

• In the early hours, Venus lies low in the northeast
• Mars in the southeast
• Saturn and Jupiter low in the south.
• The Moon be new on the 20th.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.