The Sky at Night - Season 59 / Year 2015

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.

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 Sky at Night team look at cosmic explosions. They explore the beautiful but potentially deadly outbursts of our very own star - the sun - and the most violent and energetic events in the universe, gamma ray bursts.

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.

The Sky at Night team go in search of what could of caused the Star of Bethlehem. Explored are theories that include supernovae, comets, meteors and a unusual alignment of the giant planets.