Ana appreciates the importance of the public's understanding of science, and the realisation that astronomy is a world-wide endeavour. She is in an excellent position to promote cutting edge research, being coordinator of Hands-on Universe in Spain, part of a consortium creating a global education network. She also manages space telescope and ultraviolet astronomy projects.

We are all familiar with planets — after all, we live on one! But how planetary systems are created was a conundrum for scientists, and many of the greatest minds have puzzled over the answer. Now we are in a position to describe the incredible processes that led to planets such as our own Earth being formed.

The Birth of Planetary Systems

Planetary systems and stars are both born in the same turmoil of cosmic gas and dust. Their creation is led by the force of gravity but gently controlled by the rotation of gas around the galaxy's centre. This subtle effect is responsible for forming rotating disks around the young growing suns; planets are the leftovers of these disks once the star has finally formed.

Born in turmoil

The birthplace of planets is particularly fascinating for the giant gas planets like Jupiter. They formed far from the star in a smooth and cool environment at temperatures below freezing point, from dust grains surrounded by icy crusts and highly volatile gas like hydrogen and helium. However, rocky planets like the Earth endured a much harsher environment. As they were close to the building star, they were subjected to the violent and energetic environment produced by the star formation engine: lightening carrying incredible energy, irradiation by particles moving at speeds close to that of light, as well as UV and X-ray irradiation...

As a result most of the gas and ices were removed and planets had to be formed by the sticking of small silicate and carbonate particles, gradually building up to planetary sizes.

So it seems that the very existence of planets like ours was under the control of this powerful engine. But how did it work? Scientists have agreed to think of it like a gigantic hydraulic power plant. This may sound a bit strange but the similarities will surprise you!

A cosmic power plant

How does a hydraulic plant work? Gravitational energy is stored by confining water within a reservoir. This energy is released when the gate opens and water is thrown over the blades of a large wheel called a turbine. The wheel turns because of the water, exactly like a strong gust turning a windmill. The next step is to turn this energy into the electricity that powers our homes. To do this we'll add a couple of ingredients: a shaft with magnets and some copper coils.

As the turbine turns the shaft it rotates the magnets. This produces an electric current in the copper coils. This effect was first discovered in 1831 by Michael Faraday when he measured how electric current changed in a loop of wire as a magnet swept in. It's quite simple and really easy to reproduce at home! This is the clever manner in which we on Earth transform gravitational energy into electric power. The amount of power generated depends on the amount of water flowing on the turbine; more water, more electricity!

So what has a hydroelectric power plant got to do with the creation of a star? First you have to imagine the disk where planets are built as a gigantic reservoir and the growing young sun as a gigantic magnetised turbine. Let's put some numbers on it: this stellar turbine has a radius of some 100 000 km and it is accommodating a matter flow of about 2000 tons per second. Matter falls into this stellar turbine at a velocity of about 1 million kilometres per hour! In a year, the growing star eats about a billionth of the mass of our Sun. The growing sun's magnetic field is about a thousand times stronger than the field that moves needles within our compasses. So these young suns are already behaving as gigantic magnets on top of which matter falls. So far so good, but where is the copper coil? And where does the electric energy go? The answer is that there are no wires, instead the currents are only controlled by gravity and the electromagnetic fields themselves generate. No wonder this problem needed so many clever people to understand it!

Young stars, burning bright

The astronomical observations of young stars are helping a lot in this hard labour. Today we know that about 10% of the gravitational energy is transformed into the ejection of beams of gas aligned with the rotation axis of the stellar turbine. These beams move at velocities comparable to those of the matter falling onto the star. They carry a significant fraction of the mass of the disk reservoir away from the young forming star. We called them protostellar jets and some of the most beautiful astronomical images are from them.

Close to the star, the environment is very harsh. Lightening, gigantic eruptions suddenly releasing a hundredth of the solar luminosity as X-ray and UV radiation are produced. This action takes place within a region that may extend as far as Mercury's orbit. Modern space experiments (such as the SOHO and Cluster missions) are showing splendid pictures of the Sun's corona and its beautiful butterfly pattern. The corona has a temperature of ten million degrees and radiates strongly in hard X-rays. In the growing Sun, a similar but much larger structure was irradiating the disk; the butterfly wing tips could reach Mercury's orbit thus acting as enormous heating panels irradiating the disk, as far as the Earth's orbit. This is the reason why neither ices nor volatiles can survive within the inner region of the disks. This is why terrestrial planets are solid and built of very robust molecules such as silicates or carbonates.

There is however, another intriguing point to it. We all know that excess UV radiation harms our bodies. The reason is that our molecules, the molecules of life, are extremely sensitive to ultraviolet radiation. They react to it, and in the presence of UV radiation the organic chemistry is accelerated. This is bad for us, but it is of great importance to understand how life grew in our planet, or perhaps even in other forming solar systems in the Galaxy.

Exciting times

We are just beginning to grasp our origins but it is clear that understanding this powerful engine that controlled the birth of our planet and the appearance of large organic molecules, and maybe the life, it is of extraordinary importance. Major clues are hidden in this knowledge. Exciting times are waiting ahead with the new ultraviolet telescopes that will allow us to peer into these fascinating engines.

This feature article was written as part of the Cosmic Diary Cornerstone project for the International Year of Astronomy 2009. To find out more, check out www.cosmicdiary.org and www.astronomy2009.org.