Planet Earth has been home to humankind for over 200,000 years, but with a population of 7.3 billion and counting and limited resources, this planet might not support us forever. Professor Stephen Hawking thinks the human species will have to populate a new planet within 100 years if it is to survive. With climate change, pollution, deforestation, pandemics and population growth, our own planet is becoming increasingly precarious. In this landmark film Professor Hawking, alongside engineer and radio astronomy expert Professor Danielle George and a former student, Christophe Galfard, join forces to find out if, and how, humans can reach for the stars and relocate to different planets. Travelling the globe, they meet top scientists, technologists and engineers who are working to answer our biggest questions: is there another planet out there that we could call home? How will we travel across the vast distances of space to get there? How will we survive the journey? And how will we set up a new human civilization on an alien world? Travelling the globe, they meet top scientists, technologists and engineers who are working to answer our biggest questions: is there another planet out there that we could call home? How will we travel across the vast distances of space to get there? How will we survive the journey? And how will we set up a new human civilization on an alien world? Taking in the latest advances in astronomy, biology and rocket technology from the Atacama Desert to the wilds of the Arctic, viewers will discover a whole world of cutting edge research. This programme shows that Professor Hawking’s ambition isn’t as fantastical as it sounds - and that science fiction is closer to science fact than we ever thought.

Chaos theory has a bad name, conjuring up images of unpredictable weather, economic crashes and science gone wrong. But there is a fascinating and hidden side to Chaos, one that scientists are only now beginning to understand. It turns out that chaos theory answers a question that mankind has asked for millennia - how did we get here? In this documentary, Professor Jim Al-Khalili sets out to uncover one of the great mysteries of science - how does a universe that starts off as dust end up with intelligent life? How does order emerge from disorder? It's a mindbending, counterintuitive and for many people a deeply troubling idea. But Professor Al-Khalili reveals the science behind much of beauty and structure in the natural world and discovers that far from it being magic or an act of God, it is in fact an intrinsic part of the laws of physics. Amazingly, it turns out that the mathematics of chaos can explain how and why the universe creates exquisite order and pattern. And the best thing is that one doesn't need to be a scientist to understand it. The natural world is full of awe-inspiring examples of the way nature transforms simplicity into complexity. From trees to clouds to humans - after watching this film you'll never be able to look at the world in the same way again.

Horizon discovers what your cat really gets up to when it leaves the cat flap. In a groundbreaking experiment, 50 cats from a village in Surrey are tagged with GPS collars and their every movement is recorded, day and night, as they hunt in our backyards and patrol the garden fences and hedgerows. Cats are fitted with specially developed cat-cams which reveal their unique view of our world. You may think you understand your pet, but their secret life is more surprising than we thought.

Silk is the invertebrates' great invention, used in a range of ways from from the protective stalks of lacewing eggs to the amazing hanging threads of New Zealand's 'glow worms'. Spiders, though, have taken silk-spinning to extremes. The common wolf spider has no web, but the female is a gentle parent, encasing her eggs in silk and carrying the precious bundle wherever she goes. The bolas spider uses a ball of sticky silk soaked in a copy of moth pheromone to lure its prey. Millions of communal spiders live and feed together in a vast, towering web - an arachnophobe's nightmare.

Fourth episode examines how plants either share environments harmoniously or compete for dominance within them. Attenborough highlights the 1987 hurricane and the devastation it caused. However, for some species, it was that opportunity for which they had lain dormant for many years. The space left by uprooted trees is soon filled by others who move relatively swiftly towards the light. The oak is one of the strongest and longest-lived, and other, lesser plants nearby must wait until the spring to flourish before the light above is extinguished by leaves. Tropical forests are green throughout the year, so brute force is needed for a successful climb to the top of the canopy: the rattan is an example that has the longest stem of any plant. As its name suggests, the strangler fig 'throttles' its host by growing around it and cutting off essential water and light. Some can take advantage of a fallen tree by setting down roots on the now horizontal trunk and getting nutriment from the surrounding moss and the fungi on the dead bark. The mountain ash (eucalyptus regnans) grows so tall, that regeneration becomes a considerable problem. It is easily flammable, so its solution is to shed its seeds during a forest fire and sacrifice itself. It therefore relies on the periodic near-destruction of its surroundings in order to survive. Attenborough observes that catastrophes such as fire and drought, while initially detrimental to wildlife, eventually allow for deserted habitats to be reborn.

The Southern Ocean, which circles the globe without being blocked by land, is home to the Antarctic Circumpolar Current (ACC), the longest of the world's ocean currents. Also known as the "channel", it connects the Indian, Atlantic and Pacific Ocean basins and exerts a powerful influence over the Earth's climate. The ACC carries 150 times more water around Antarctica than the flow of all the world's rivers combined.