Using that evidence, we come up with an idea called a model of what is going on.

We use the model to make predictions, and see with more evidence if we've got it right.

We might have to modify our model, or throw it out and start again. Or we might just have got it right.

We share our model with other scientists, who often in a process called peer review try and find problems with it. Sometimes it takes years to find a problem with a model, which then turns out to be completely wrong.

One Doctor Who writer, Douglas Adams, explained it like this: the scientific method 'rests on the premise that any idea is there to be attacked. If it withstands the attack then it lives to fight another day and if it doesn't withstand the attack then down it goes.'

After Galileo's discovery and people being able to check it for themselves science became a fashionable hobby. In some countries, there were clubs where you could go for a night out to watch the latest experiments be performed. One such club, the Royal Society, was founded in 1660. A few years later, a debate among its members about the movements of the planets prompted Isaac Newton to write up his own ideas.

Newton was a genius, but he's been described as a sorcerer as much as a scientist. It's thought that it was his interest in magic and the ability to apply force from a distance that led him to come up with the theory of gravity. But, in the scientific method, it doesn't matter where his ideas came from, only that they fitted the evidence. And gravity more or less did.

Very simply, Newton worked out that bodies in the universe for example, planets are drawn towards each other, and the strength of the attraction is related to both their combined ma.s.s and how far they are apart. His explanation was complex, but he supplied a simple mathematical formula that allowed people to test if he was right.

Science was still fashionable a century later when the musician William Herschel caught the bug for astronomy. In 1781, from his garden in Bath, he spotted through his telescope a star that didn't behave like the others. At first, he thought it was a comet, but then realised he'd discovered an entirely new planet Ura.n.u.s.

But the movement of Ura.n.u.s didn't quite match Newton's simple formula. Which meant either Newton was wrong or that another large object was out there in s.p.a.ce, and the effect of its gravitational pull on Ura.n.u.s was the reason its...o...b..t was odd. Sure enough, in 1846 another new planet was discovered Neptune. It's...o...b..t was odd, too, so astronomers searched for another planet, even further out in s.p.a.ce. Pluto was discovered in 1930 and was still too small to explain the odd orbit. It was discovered in the 1990s that we had overestimated Neptune's ma.s.s; the correct numbers matched the oddness of the orbit.

There were problems with the theory of gravity closer to home, too. Mercury the closest planet to the Sun was never quite where Newton's formula said it should be. Then in 1916 Albert Einstein suggested that the Sun was so big it warped s.p.a.ce and time, bending it like a lens in a telescope bends light. In 1919, new observations of Mercury showed it exactly where Einstein not Newton said it would be, proving Einstein's theory of relativity.

When Doctor Who landed on its first alien world in December 1963 (Skaro, home planet of the Daleks), we knew in reality of nine planets, all of them in orbit round our Sun. Pluto was a planet the Doctor even calls it one when he visits in The Sun Makers (1977). But we still didn't know what a planet was. So, what changed?

In November 2003, astronomers spotted what was reported in the press as the tenth planet Sedna. More discoveries soon followed: Haumea in 2004, Eris and Makemake in 2005. Were these all planets, too? Eris seemed to be larger than Pluto, but if we made the decision based on size then surely Ceres counted, too. Ceres, discovered in 1801, is the largest rock in the asteroid belt between Mars and Jupiter and had originally been thought to be a planet before more of the asteroid belt was discovered.

So, after much debate, in 2006 the International Astronomical Union pa.s.sed Resolution 5A, setting out three things that define a planet. A planet: Is in orbit round the Sun Is big enough that its own gravity makes it (nearly) spherical Doesn't share its...o...b..tal path with other objects.

The Tenth Planet and Vulcan Before Einstein came up with relativity, there were other theories about why Mercury didn't move exactly as Newton's formula said it should. As with the odd orbits of Ura.n.u.s and Neptune, it was suggested that there might be another, as yet undiscovered planet affecting its gravity. But why hadn't we seen this other planet?

One explanation was that this planet was so close to the Sun that it was hidden by its glare. That's why the theoretical planet was named Vulcan after the Roman G.o.d of fires and forges (as we learnt in The Fires of Pompeii (2008), Vulcan is also where we get the word 'volcano'). Another explanation was that Vulcan was always on the far side of the Sun from us, moving round it at the same speed that we were. The only way that could happen was if it shared our orbit a twin planet, just like Earth, always hidden from view.

Though Vulcan turned out not to exist, it influenced two Doctor Who stories in 1966. In The Tenth Planet, Earth's long-lost twin, Mondas, is home to the Cybermen. The very next story, The Power of the Daleks, is set on a planet called Vulcan. By coincidence, that same year a new science fiction series began on American television. Star Trek's Mr Spock was also from a planet called Vulcan.

Ceres, Pluto, Haumea, Makemake and Eris were recognised as 'dwarf planets' though the IAU was clear that that meant they are not planets.

At the same time we were losing one planet, we were discovering thousands more.

In 1992, astronomers announced the discovery of two planets round a star called PSR 1257+12. As of February 2015, we've discovered 1,523 planets, while the s.p.a.ce observatory named after Kepler has found a further 3,303 that await confirmation by more tests. We found these planets by measuring tiny but regular wobbles in the light from distant stars. Using Kepler and Newton's laws, we can use these wobbles to work out the size of the planets and how far they are from their suns.

That means we also know a little about what some of these planets might be like. There are 'hot Jupiters' huge planets that orbit very close to their stars. It's thought there could be planets made of carbon perhaps like the diamond planet in Midnight (2008). And there seem to be planets like Earth.

'The Doctor won't tolerate anyone deliberately playing havoc with his favourite planet.'

The Master (about the Sixth Doctor), The Mark of the Rani (1985)

As we've learnt about other planets, we've also learnt about our own. We've not (yet) found life anywhere else in the universe, so what makes Earth so special?

The Earth has lots of features that make it good for life. It has had a stable orbit for billions of years, which the Moon proportionately large compared to other planets' moons helps regulate. We have a ready supply of energy from the Sun. Our magnetic field protects the planet from high energy particles that can be harmful to life. Earth is rich in carbon as the Doctor tells Sarah in The Hand of Fear (1976), we are carbon-based life forms. The planet is big enough that its gravity keeps hold of its atmosphere, which keeps Earth nicely warm and allows us to breathe.

With all these things going for it, it's no wonder so many alien races (in Doctor Who) want to conquer Earth. But there's something else, too.

We know that oxygen isn't essential for life as we've found microorganisms living on Earth that don't need it. But there's one thing all life we know of depends on: liquid water. Oceans make up seventy per cent of the Earth's surface.

For water to be in liquid form, a planet needs to be just the right distance from its sun. Too close and the water evaporates, while too far away and the water freezes. We call the right distance the 'habitable zone'. About one in five of the planets Kepler has found fit this category. Kepler has only looked in a small region of s.p.a.ce but, from what it has found, scientists think there might be as many as 40 billion Earth-like planets in just our galaxy. These planets could have liquid water on them, and that means they could have life.

Is there life on these planets? Venus is in the habitable zone around our Sun, but its thick clouds of carbon dioxide make it extremely hot. Could life still exist there anyway? At the moment, we simply don't know. As yet, we haven't found any.

What might alien life be like? In Doctor Who, we've met all kinds of strange life forms fish people, b.u.t.terfly people, slug people, rock people, gas people, water people, even viruses that can talk and people who exist in only two dimensions. In reality, life on Earth is extremely varied and diverse. Anything seems possible. In fact, the chances are that alien life will turn out to be something we haven't even thought of.

Think about that. Discovering the first alien life would mean we're not alone in the universe. But it could also mean we have to redefine what life actually is.

All we have to do is find it. And that means us getting out into s.p.a.ce...

The Solar System according to Doctor Who * The Sun * Mercury * Venus has metal seas (Marco Polo (1964)), flowers (The Wheel in s.p.a.ce (1968)), spearmint (The Shakespeare Code (2007)) and a creature called a Shanghorn (The Green Death (1973)). The Doctor is skilled in Venusian aikido, first seen in Inferno (1970).

* Earth the Doctor's favourite planet.

* Mars the Doctor visits in The Waters of Mars (2009). Indigenous species include the Ice Warriors and the Flood. The Osirans also built at least one pyramid on Mars in the time of the ancient Egyptians (Pyramids of Mars (1975)).

* Unnamed 'fifth' planet the home planet of the alien Fendahl was time-looped by the Time Lords, possibly creating the asteroid belt. The Doctor says it was 107 million miles out (from Earth?) and broke up 12 million years ago (Image of the Fendahl (1978)).

* Jupiter British astronaut Guy Crayford is rescued by the Kraals while trapped in orbit around Jupiter (The Android Invasion (1975)). At some point in the future, the 'planet' of gold, Neo Phobos, moves into orbit around Jupiter as one of its moons and is renamed 'Voga' (Revenge of the Cybermen (1975)).

* Saturn the Doctor is infected by a talking virus while on Saturn's moon, t.i.tan (The Invisible Enemy (1977). It seems t.i.tan is destroyed at the end of the story.

* Ura.n.u.s the only source of the rare mineral taranium, used to build a time destructor (The Daleks' Master Plan (19651966).

* Neptune * Pluto the Doctor calls it a planet when he visits in The Sun Makers (1977). One of its inhabitants, Mandrell, doesn't believe there is life on any other planets.

* Mondas Earth's long-lost twin planet drifted 'to the edge of s.p.a.ce' before returning in 1986 and exploding (The Tenth Planet (1966)).

* Planet 14 used as a base by the Cybermen in The Invasion (1968) it's not stated that Planet 14 is in the Solar System, but since the Cybermen come from the tenth planet (Mondas), it is possible they've colonised other, outer worlds.

* Ca.s.sius according to K-9 in The Sun Makers, Pluto was thought to be the 'outermost body' of the Solar System until the discovery of Ca.s.sius.

It was like this every time they landed recently: the Doctor, whizzing his scarf around his neck and jamming his hat atop his curly mop of hair, would throw caution to the wind, fling open the doors and charge out into the sunlight like an excited child.

It was almost inevitable, really, that he'd sniff out the first sign of trouble, and then mire them both in it. Sarah had come to expect nothing less. This, she realised, was his modus operandi, his idea of having fun. She, on the other hand, wasn't quite so convinced.

There was no discussing it, though. Each time she'd tried to raise the issue, suggesting even a preparatory glance at the scanner before hurtling headlong into... whatever, he would just nod sagely and say, 'Yes, I see precisely what you mean. Terrible business. You'd better have another jelly baby,' before handing her the paper bag and carrying on as if the conversation was over.

This time had been no exception. He'd bustled them out of the TARDIS in an unnecessary hurry, urged her on into the strange wooded glade, and marched them straight into oncoming danger.

Still, Sarah had to admit even she hadn't been expecting a rabid, mutant sheep.

Presently, they were crouched behind a bush while the Doctor tried to keep the thing distracted with the trailing end of his scarf. Sarah peered at it through the foliage. It was the strangest sheep she had ever seen if it could even be called a sheep. It was overgrown (at least twice the size of a normal sheep) and clearly carnivorous (she could tell this from the way it had tried to bite her calves as they ran). It had wild, flitting eyes, and elongated canines that protruded over its bottom lip. Its wool coat was wiry and matted with dried blood that Sarah suspected wasn't the sheep's. It snorted in frustration and made another lunge for the Doctor's scarf.

'Remember I told you that, besides Earth, oak trees didn't grow anywhere else in the galaxy?' Tentatively, so as not to attract the attention of the sheep, the Doctor patted the bough of a nearby tree. It was barely broader than his hand, yet still towered far above their heads, its uppermost branches disappearing into a canopy of green.

Sarah, exasperated, nodded in the affirmative. 'Yes. This was just before we were shot at by a gang of homicidal androids, if I remember correctly.'

He seemed to ignore her qualification. 'Well, I was wrong.' He beamed at her encouragingly. 'They grow here, too. Wherever here is.' He tugged absentmindedly on his scarf, and the sheep gave a ferocious growl. 'Watch out, I think he's grumpy with us.'

'These are not oak trees,' said Sarah. 'Look at them. They're all tall and spindly.'

'Ah, but then, so would you be if you'd spent your formative years in a place like this. It's all about gravity, you see. Don't you feel a little lighter?'

Sarah thought about this for a moment. 'Well, yes, I suppose I do. But what's that got to do wi-' she stopped abruptly at the sight of the sheep, which had followed the Doctor's scarf around to their side of the bush and was now preparing to launch another attack. Ribbons of saliva dripped from its open jaws. It looked positively monstrous. 'Um... Doctor...'

A horn blared suddenly, somewhere in the near vicinity, and the wooden shaft of a spear came whistling through the air and struck the earth a few inches from the Doctor's left boot, burying its tip in the loam. Sarah stifled a scream.

The sheep, terrified by this sudden incursion, dropped the end of the Doctor's scarf, turned about, and fled into the trees.

'Well, that's a bit of a ewe-turn,' said the Doctor, winding in the end of his scarf and looking forlornly at the macerated end. He flicked it resignedly over his shoulder.

By now, Sarah could hear the thunder of approaching footsteps. She parted the bush and peered through. Around thirty men and women dressed in rags and carrying primitive-looking weapons were forming a wide semi-circle around them. They were humanoid, but looked... odd. Like the trees, they were tall and willowy, at least eight feet tall, with thin, gangly limbs. Their skin tones were pale and washed out, like they hadn't seen enough sunlight, and their noses were almost flat upon their faces, little more than wrinkles of flesh protecting tiny vents.

The Doctor stood, coming out from behind the bush with his hand extended. 'How very nice to meet you. I'm the Doctor. This is Sarah. We're new around these parts.' He grinned, flashing his teeth.

The tribes people glared at him, still brandishing their motley a.s.sortment of weapons.

'No?' said the Doctor. He shrugged and lowered his proffered hand. 'I should work a little on your hospitality, if I were you. All those spears, you'll scare off the tourists.'

A tribesman in a bright headdress stepped forward, jabbing his weapon in the Doctor's direction. Sarah, who had now come around the bush to join the Doctor, could see that it resembled a garden spade.

'Oh, now that's very interesting,' said the Doctor. 'Very interesting indeed. A spade, a hoe, a laser cutter... What fascinating weapons you all have, Mr...?'

'Euripides,' said the man. His voice was guttural and deep, and, although the accent was thick and unfamiliar, he spoke clearly in a form they could understand. 'I am Euripides, and you, Doctor and Sarah, are our prisoners.'

'Oh, why aren't I surprised?' muttered Sarah. 'It would be nice if, just once, we were greeted by people who wanted to give us tea.'

The Doctor laughed. 'Let me guess, Euripides. You're going to take us to your leader. Good.'

Euripides jabbed his spade at them. 'You will come with us to the Temple of Life,' he said.

'Oh, if you insist,' said the Doctor, tipping the brim of his hat. 'Lead on, lead on!'

The Doctor and Sarah were led at spear point through the flora, following trails that felt more like the claustrophobic tunnels of a military bunker than any natural environment. They walked in single file, with little conversation, and although they must have trudged through the undergrowth for over a mile, the going wasn't too tiresome, given the reduced gravity.