Planet Earth: What is a planet?

The basic definition of a planet is simple enough. A planet is too small and cool to be a star, but massive enough to form a solid globe. Many, such as the big outer planets of our solar system, hide this solid surface beneath a dense atmosphere. All the planets we know are in orbit around a star; however, there could be some that are not. It is possible for gravitational forces in star clusters to fling planets into the cold of deep space, tar from any warming stars, where they would be exceptionally hard to detect. We also know of some planet-sized satellites in our own solar system. For instance, Triton, the biggest satellite of Neptune, is bigger than Pluto. But if you haven't got a badge, you aren't a cop, and if you are orbiting a planet, you can't be a planet yourself.

Studying the solar system

The story of our understanding of planets and the Earth's place among them begins with the ancient Greeks. The word “planet” has its origins in the Greek verb meaning “to wander”. Like all ancient cultures, the ancient Greeks spotted that, in addition to the Moon, there were five objects in the night sky that moved about. They were the planets that we call Mercury, Venus, Mars, Jupiter and Saturn.

If you spend a night staring at the sky, doing it without a telescope like our ancestors had to, you will notice that very little happens. Stars rise, wheel across the sky and set. Come back the next night, and you will see pretty much the same thing, although everything will happen a few minutes differently from the day before until it is back in the same place a year later. This may seem so dull that it is a wonder astronomy ever got going as an occupation for the lively-minded.

It is these five planets that provide most of the variety we see in the naked-eye sky. They come and go, they vary in brightness, and they move about. One, Mercury, is never very tar from the Sun in the sky. Another, Venus, also stays near the Sun but is less tightly bound. The other three can be seen in the full dark of night. They move across the sky – Mars fastest, then Jupiter, then Saturn – in patterns which gave ancient thinkers pause for thought. Like Mercury and Venus, they chase around the same broad path across the sky as the Sun and Moon, an area called the Zodiac, which has the acute drawback of permitting the development of astrology. Their pace varies and sometimes when they are at their brightest, they turn round and loop back for a bit before resuming their forward progress.

But what had these lights moving through the night sky got to do with the Earth beneath our feet? The discovery that these strange movements could be explained was one of the greatest human achievements. It was made mainly by a Pole, Nicolaus Copernicus (1473-1543), and a German, Johannes Kepler (1571-1630), who realized that a simple change in assumptions could make everything clear. The new assumption was that the Earth was a planet just like the others and that they were all in motion around the Sun. This belief had been on the scene since the time of the ancient Greeks, but its proof is one of the great foundation stones of modern science. (No theory I have heard of has ever seriously questioned that the Moon orbits the Earth.)

The idea that the Earth is just one of a number of planets orbiting the Sun is a very fundamental and unsettling one. It removes the Earth and, more importantly, the human race from the centre of the universe. The Christian Church once found this hard to cope with. Today most Christians regard our knowledge of the universe as an affirmation of their faith rather than an attack on it.

Around this time, we also gained our first definite knowledge of other planets. The first person to look at the sky through a telescope was an Englishman, Leonard Digges, in about 1571. His work did not lead to radical new discoveries, although he did memorably compare the surface of the Moon to a pie that his cook had made. This was radical in its own way, since it broke with the accepted wisdom that all celestial objects were perfect because they were at a safe distance from sinful Earth.

Despite being armed with a crude telescope that was less powerful than a good pair of modern binoculars, Galileo Galilei (1564-1642) was much more successful. He discovered that Venus has phases like the Moon, showing different amounts of its lit face to us as its position in the sky changes relative to the Sun, that Jupiter has moons of its own, that great craters mark the Moons surface and that Saturn, well, looks a bit odd. Later and better telescopes showed that this was because it is surrounded by its now-famous rings. Galileos discoveries showed that other planets are round objects a little like the Earth.

One idea that nobody really caught on to was the notion that there might be other planets beyond those known from the earliest times. In 1781, however, a German astronomer based in England, William Herschel (1738-1822), found the planet Uranus while conducting a complete survey of the sky visible from England. This was one of the most mind-changing discoveries in the history of science, showing that the fixed universe of the past was to be replaced in the imagination by a dynamic place where new types of objects might be found.

In 1846 the planet Neptune was discovered (Galileo had seen it in 1612 without realizing its importance), and in 1930, Pluto. While there is an ongoing debate about exactly what is and is not a planet in the outer reaches of the solar system, it is now well established that the Earth is one of a group of planets which resemble each other more or less closely and which, therefore, can help us to understand the Earth itself in more detail.

But are the planets of the solar system representative of the universe as a whole? To get the full picture, and gauge the Earths uniqueness or otherwise as a planet, we need to find out about planets orbiting other stars.


“Exoplanets” sounds cooler than “planets of stars other than the Sun”, and is also less of a mouthful. Much creativity has been applied to detecting them. Initially, scientists attempted to detect slight movements of stars in the sky, which would indicate the influence of a planets gravitation. But this has been tried for over sixty years with very limited success. More recently, astronomers have attempted to detect this movement indirectly, by measuring variations in the spectrum of the stars light reaching the Earth. As the star moves back and forth in the sky, its light gets a little redder or bluer, a change that can be detected with comparative ease. Most exoplanets have been found this way.

Another technique is to look at the “gravitational lensing” effect by which the mass of a planet will bend starlight. This has been increasingly productive. The “transit method” attempts to spot the dimming of a star as a planet passes in front of it as seen from Earth. And it is now becoming possible to look for the planets themselves, using powerful telescopes such as the Spitzer space telescope and the ground-based “Very Large Telescope” in Chile.

The problem is that the most successful of these methods, the search for changes in the spectrum of starlight, has a massive built-in bias. The planets it is most likely to detect are those which are heaviest, and nearest to the stars they orbit That way you get the biggest gravitational pull on the star. Certainly any alien scientist using the technology now in use to detect exoplanets would not be able to spot the solar system by looking at the Sun. But in the next few years, this should begin to change. The lensing method, for example, is able to detect Earth-size planets. NASA plans a mission called Kepler which will probably detect them in abundance with this method. And a new European satellite called Corot should detect hundreds of exoplanets from 2007 onwards by watching for them as they transit the star they orbit.

At the moment, most known exoplanets are much bigger than Jupiter and have orbits far closer to their star than Mercury. This may tell us that our sample is too skewed to be worth worrying about. Such a solar system would be very different from our own, where the giant planets with all the gas are at a cool distance from the Sun while rocky ones with comparatively thin atmospheres, including Earth, are found closer in.

We are already obtaining knowledge of the atmospheres of exoplanets. One, for example, is so close to its star that it is streaming off hydrogen gas into space. It also contains sodium, carbon and oxygen – enough to get the biologists excited back on Earth.

Large telescopes now being planned – with names such as Owl, the Overwhelmingly Large Telescope – will be able to outgun existing telescopes in space and on the ground by some orders of magnitude. They should be able to get proper spectra of exoplanets and even produce simple images of them, as well as allowing us to decide whether the big planets we are detecting today are typical. At that point our ideas about the Earths uniqueness will face a new test.