The first image of the Moon was taken by a camera on the Apollo 8 mission in 1968. Since then, we have been taking pictures of it by sending probes there. But when will we get to look at an Earth-like planet? What is it going to look like? How could we know whether we had found Earth 2.0? At the moment we are probably no more than five to 10 years away from seeing something that looks a bit like Earth in images sent back by a satellite mission to a planet orbiting another star. That was the word from Sara Seager, an astrophysicist at the Massachusetts Institute of Technology (MIT) Media Lab who spoke at the British Science Festival on October 19. Seager has spent much of the last 10 years working out just what a picture of another planet taken from another star might show us. This has led her to identify a "transiting alien Earth". A transiting alien Earth is one that is in orbit around another star in a similar orbit to our own, and one whose surface is seen from Earth in a way similar to how our own is seen from Earth. Alien Earths, aliens? The possibility that we might have been discovered this way was first raised by the American author Carl Sagan in his book "Contact". If an alien species from another planet were to land on Earth, Sagan suggested, we might be able to recognise them from their appearance. "To see our world as it truly is, big enough in our imagination to see the whole universe as its spectators, is one of the magic things about the scientific life," Sagan wrote. For a while after Sagan wrote that line of reasoning had given way to a belief in an "alien Earth" or "alien life". These terms didn't actually appear in Sagan's book but were used by astronomers and planetary scientists in the years after he wrote his book to describe bodies with similar characteristics to Earth. One group of scientists in particular have focused on finding a sister planet to Earth in recent years. They are looking for signs of life in a planet that might be orbiting the star HD 189733, some 163 light years away, and just 41% of the mass of Earth. These scientists talk about a "habitable exoplanet" that they believe could be at the right distance from its star to support water at the surface, and not too cold or too hot for life to flourish. Seager thinks we could find a transiting alien Earth much more easily than a "habitable exoplanet" just like our own. "It would be astonishing if there wasn't something that looked like us in the outer solar system," she said. How does the universe form planets? To predict what alien Earths might look like, Seager uses something called a "forward model". This predicts how much light an Earth-like planet would reflect, absorb and transmit, as well as the colours that would appear in a picture of it. She created a table that predicts what we might see in images of the surface of an alien Earth. From the table, we can predict that a transiting alien Earth might have clouds, volcanoes, snow, oceans, ice caps, vegetation and deserts. The planet might even have rings. We can use this information to plot where in the universe these alien worlds might be. This is all on the basis of what we know about how Earth formed. Our planet was formed around 4.6 billion years ago and is 4.6 billion years old. Seager's predictions are based on a model of the universe's history that goes back to the first 500 million years after the Big Bang. She calls this the "reassuringly simple model". It includes six ingredients: hydrogen and helium, which were made in the first three minutes after the Big Bang; dark matter, which wasn't made at that time but is thought to have existed from the beginning of the universe; dark energy, a force that was introduced at the beginning of time, which began to stretch and shrink the universe in a way that was first observed in 1998; and four ingredients, three forces that appear to make up the four fundamental forces of physics – the electromagnetic, strong and weak forces and the nuclear force. Seager's work is an attempt to work out how these properties might have shaped an alien Earth. As the universe got larger it became warmer and denser, and the same forces that created planets early in the universe's history may still be in operation today. Planets evolve So do the planets orbiting these alien stars have the same properties that make our planet an Earth? For the most part, yes. They are made of rock and iron, and they have "enough water" on their surface to form oceans. The temperatures of these planets change with the age of the star that they orbit, and also with their position in the galaxy. But even though our picture of the universe has been simplified in a way that allows scientists to predict how a transiting alien Earth might look, Seager pointed out that there is a lot we don't know about how this universe formed. This is the reason why Seager believes that it is important to keep observing the universe in an attempt to test the "reassuringly simple model". In her words: "It's not just about us." Explore further NASA spacecraft reaches orbit around Jupiter for epic exploration More information: Seager's presentation will be webcast at the Jodrell Bank Astrometric Centre's "Astronomy 2015" event. It will be available on demand to registered delegates from the UK Science Festival. She will discuss exoearths – extraterrestrial planets – also at this event. Find out more about the festival and register to attend on the main website. Seager's presentation will be webcast at the Jodrell Bank Astrometric Centre's "Astronomy 2015" event. It will be available on demand to registered delegates from the UK Science Festival. She will discuss exoearths – extraterrestrial planets – also at this event. Find out more about the festival and register to attend on the main website. www.astronomy2015.org.uk/festival/ This story is published courtesy of The Conversation (under Creative Commons-Attribution/No derivatives).