600 extrasolar planets and counting....

A planet around the star Beta Pictoris: a beautiful example of how we can see actual images of planets around other stars. Whilst we may not be able to glean much information from images such as this, the technique is in its infancy and yet still provides some of the most poignant pictures of the cosmos. (Image credit: ESO/A.-M. Lagrange et al.)

Since the nineteen-nineties, astronomers have been scrutinizing the skies in search of planets that lie outside of our own solar system, orbiting other stars, and as of today, there are 603 of these extrasolar planets which have been confirmed. Descriptively named, they range from the gargantuan CD-35 2722 b, a whopping 31 times as massive as Jupiter, to the celestial weight-watchers weighing in at a mere 2-3 times the mass of the Earth. They have been spotted as nearby as a mere ten years or as distant as twenty five thousand, and we may even have found a planet composed of diamond, but how is it we know these planets are there?

Only a handful of planets outside our solar system have been large enough and close enough to be directly imaged, and astronomers often have to be more subtle to tease out information from these incredibly remote stellar wanderers.

Most extrasolar planets have been discovered by the examining their 'radial velocity'. As a planet orbits a star, it causes the star to wobble. As the star moves away from us, its light is moved towards the red end of the spectrum (red shifted). As it moves back towards us, the light is blue shifted, and this change in the light allows astronomers to determine the mass of the orbiting planet.

If we're lucky, we can spot planets around other suns just by looking at how bright the star is over time. If the path of a planet's orbit brings it between its host star and ourselves, we will observe it periodically dimming and brightening again as the planet first passes in front of the star and then passes away from from our line of sight. Known as the transiting method, this sounds like a synch, but it is far from easy. Often much brighter than the sun, and impossibly distant, these stars are thousands of times wider than their companions. Like looking for a tiny dark needle in a blazing haystack!

More complicated than the radial velocity and transit methods, 'gravitational microlensing' tracks the brightness of an object as its light is interfered with by the gravitational influence of a star or planet that passes in front of it. Although the light from our target planet may be much too dim to see, the way it bends the light from the background star allows us to find and identify it. Not dependent only on visible light, the gravitational microlensing effect allows us to identify objects that emit all kinds of electromagnetic radiation. Verification of gravitational lensing in general was found in 1979 after having been a prediction of Einstein's General Theory of Relativity since 1936.

In 1992, the first planet outside of our solar system was discovered orbiting a pulsar a mere eighteen million billion kilometers away in the constellation Virgo. A pulsar is a small but massively dense object, the leftover remains of a supernova, which rotates sometimes thousands of times per second. These rotations can be timed with incredible precision, meaning that pulsars can be as accurate as some atomic clocks. As the pulsar rotates, it emits beams of electromagnetic radiation which sometimes sweep across the distant Earth. As we see with the radial velocity method, as a planet orbits a star, the star shows tell-tale wobbles. This wobble affects the timing of the beams of radiation from the pulsar, giving away the otherwise impossible hiding place of the orbiting planet.

There have also been twenty four extrasolar planets discovered by direct imaging alone. Whilst not nearly as successful as the radial velocity method, direct imaging may be the most inspiring way of hunting for our planetary neighbours. When I was a child, not that long ago, the question 'are there any planets outside the solar system?' was yet to be answered, but now we can literally see them for ourselves, and detect hundreds more with ingenious technology and cunning know-how. Some planets are baked in ferocious heat, hurtling around dangerously close to their parent stars while others linger out in the cold and dark, taking hundreds and even thousands of years to circle their distant host. The number of extrasolar planets has just passed 600, and it is likely to reach a thousand within the next few years; a number which will grow exponentially in the coming decades. With such a high abundance and a plethora of sizes, types and compositions, it surely only a matter of time before we find a planet significantly like our own. Of course, once we do begin to find earth-like worlds scattered across the galaxy, we will come tantalizing close to finding whether life is extraordinarily rare, or staggeringly common.

GAS

Juno during construction: NASA's Juno spacecraft was launched in August 2011 and will reach its destination, the gas giant Jupiter, in 2016, where it will spend one year mapping the behemoth planet's composition, atmosphere, gravity and magnetic field. (Credit: NASA/JPL-Caltech/LMSS)

Each year over US$100 billion is spent around the globe on space exploration by the space-faring nations of the world. But why....?

1. Technology - Space exploration commands the abilities of some of the finest scientists and engineers in the world. Their invaluable contributions frequently lead to new ideas and innovations that can be utilised by us mere mortals here on Earth. Revolutions in communications, materials, energy, meteorology and countless other disciplines take innovative ideas and concepts from the space arena on a frequent basis. Pushing ourselves beyond the limits of this world can make our lives easier, safer and more enjoyable here on earth.

2. Survival - there are serious risks to humanity both from space-borne dangers and earthly hazards, but greater investment in space exploration offers us the tantalizing prospect of salvation from either, or even both. Firstly, the greater our knowledge of our solar system and the dangers lurking within (or even without) it, the more prepared we can be to defend ourselves. Whether we are under threat from an errant asteroid or solar flare, greater understanding will enable us to combat these dangers in a more capable fashion. Secondly, should we lay waste to our planet by war, disease, famine or climate change, we might one day possess the means to move ourselves to a new home. Governments and private enterprises are already making the first steps toward making this distant fantasy a reality, and it is one day entirely possible that we will owe our very survival to the exploration and understanding of the cosmos around us.

3. Inspiration - Generations of scientists have been swept into their profession by wonder and awe inspired by the great innovators and explorers of their time. The Apollo programme has most likely been one of the most effective programmes that have encouraged young people to develop a keen interest in science, but the Hubble Space Telescope (HST), several generations of Mars rovers and the wonderful images sent back by the Cassini-Huygens mission continue to inspire delight and curiosity in adults and children alike. As was the case with Christopher Columbus, Charles Darwin, Roald Amundsen and countless others, without these great strides in exploration the world would be a very different and altogether less interesting place.

4. Global co-operation - Whilst it is true that the development of space flight was closely tied in with the development of nuclear weapons and other aggressive technologies, it has been, and indeed continues to be, a great drive toward further co-operation between space faring nations. "Despite the continued space competition between the United States and U.S.S.R., Khrushchev sent Kennedy a letter raising the possibility of space cooperation on a modest level [...in 1962]"(http://www.nasa.gov/50th/50th_magazine/coldWarCoOp.html) Then, in 1975 the US and USSR created the Apollo Soyuz Test Project - a primarily symbolic mission that has encouraged co-operation and mutually beneficial exploits in space ever since. Today, the ESA (European Space Agency) is a conglomeration of the efforts of unified countries in space. There have been major contributions to the International Space Station (ISS) by NASA, Roscosmos (Russia), the CSA (Canada), ESA and JAXA (Japan), as well as other contributions from several other nations. If these great success stories continue to grow and develop, it will encourage us to demonstrate better relations on the ground. Space exploration brings us closer to peace.

5. It is MAJORLY cool - on October 3rd 1957 there were no man-made objects in orbit around the earth. 54 years later, we have sent robotic explorers to seven planets , several non-planetary bodies and human beings have stood upon the Moon. We observe the sun with extremely high tech equipment 24 hours per day, 365 days per year, and we are discovering hundreds of planets around other stars. We can describe the inner dimensions of our great galaxy, and witness events in countless billions of others throughout the cosmos. We are finally en route to answering the question: Are we alone? We live in a truly amazing and awe inspiring time. Astronomy and space exploration promise to allow us to discover some of the most profound insights into our lives, and compels us to ask the most audacious of questions. Seriously, it's majorly cool.

The United States of America spends around a tenth of one percent of its annual budget on space exploration, much more than many of its fellows. If we can use this tiny amount to land on the moon, send probes to Mars, Jupiter and Saturn, and peer into the far reaches of the Universe, just think what we could achieve with a meagre on or two percent!

GS