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The Milky Way, our galaxy, is a magnificent sight in the night sky, but we know surprisingly little about it for certain. What is its shape? How many stars does it actually contain? What lies at its centre? The Gaia space telescope will answer these questions, being armed with the most advanced camera to leave our planet, and it will allow us to see our galaxy as we've never seen it before. The Sky at Night visits the factory in Chelmsford that made the astonishing sensor at the heart of the mission.

From unexplained flashes in the night sky to flying saucers, this episode delves into the mysterious world of UFOs. How our drive to explain these bizarre phenomena, and desire to discover little green men, has in fact transformed our understanding of the universe.

For 25 years the Hubble Space Telescope has been showing us the cosmos as we've never seen it before. The team reveals the 'top five' greatest images Hubble has produced, images that have astounded us, transforming our understanding of the universe and our place in it.

The team explores our nearest neighbour Venus, discovering how it formed and how ESA's Venus Express spacecraft has revealed the secrets of its atmosphere. -- How can two such similar planets have become so different? One is the crucible of life, the other an inferno with a surface scorched by raining acid, yet both began as almost identical bodies. With Venus prominent in the sky in May, the team explores our nearest neighbour, discovering how it formed and how ESA's Venus Express spacecraft has revealed the secrets of its atmosphere.

With the exciting news that the Philae lander had woken up on comet 67P, Sky at Night reveals the latest results from the Rosetta comet landing. What have they learnt so far from Philae's onboard instruments? What do the stunning images from Rosetta tell us about the formation and structure of comets? And project scientist Matt Taylor shows how Rosetta is measuring the growing tail of the comet as it hurtles towards the sun.

Maggie Aderin-Pocock and Chris Lintott present the inside story of NASA's groundbreaking visit to Pluto. This is the first time any probe has visited the dwarf planet and Sky at Night has ringside seats, bringing you the entire story and expert insight into the latest images from the New Horizons probe. Sky at Night celebrates its 750th episode with the most exciting space event of 2015.

The team looks at the dynamic nature of the universe, winding its timeline backwards and forwards to reveal how the night sky changes over time. We see how different the night sky looked in the past and how it will be transformed billions of years into the future as the stars migrate and galaxies collide. Broadcast from the Royal Observatory Greenwich, the team explores the latest theories on solar system evolution - how the familiar layout of today's solar system was created by a gravitational dance between the giant planets that left scars we can still see today.

We think of volcanoes as some of the most powerful natural phenomena on earth - but they are nothing compared to the volcanoes we find elsewhere in the solar system. This month's Sky at Night reveals the weird and wonderful world of volcanism on other planets and moons - from the giant extinct volcanoes of Mars to the tantalising possibility of continuing eruptions on Venus, and from the vast sulphur plumes of Io to the mysterious cryovolcanoes of Enceladus.

As we close in on the discovery of the 2,000th planet outside our solar system, or exoplanet, the Sky at Night investigates the techniques that are revealing so much about these alien worlds. The programme asks if we are really any closer to finding another world like our own - a second Earth.

Astronomers have been fascinated by the idea of the Star of Bethlehem for centuries. Did it exist? And if so, what was it? The list of candidates includes some of the most exciting objects in the night sky - supernovae, comets, meteors and unusual alignments of the giant planets. In this surprising and entertaining Christmas special the Sky at Night team go in search of the potential causes of the Star of Bethlehem. The team explore the possibilities, investigating the nature of the phenomena and digging through the historical records including Babylonian clay tablets and ancient Chinese manuscripts, to reconstruct events in the night sky 2,000 years ago. Maggie Aderin-Pocock goes hunting for supernovae using the most powerful laser in Britain, and discovers that these mighty explosions caused by the death of stars can shine brighter than the moon in our sky. Chris Lintott reconstructs the night sky over Jerusalem at the time of Jesus's birth, discovering a once-in-a-millennium conjunction of Saturn and Jupiter that was first suggested as a cause of the star by the great astronomer Johannes Kepler in 1604. Armed with his telescope, Pete Lawrence searches out the features of the night sky we can observe today that may provide clues to the origin of the Star of Bethlehem. Professor Alan Fitzsimmons explains why the sudden appearance of a comet in the night sky has always been seen as an omen of great events on Earth. Dallas Campbell goes in search of the historical and archaeological records that can shed light on the identity of the star. Finding Babylonian tablets in the vaults of the British Museum and ancient Chinese texts that record all the unusual events in the night sky 2,000 years ago, including a bright new star that appeared for 70 days in the year 5BC.