Variety is the spice of space

A panel of astrophysicists speaking at ESOF 2012 detailed the ongoing search for exoplanets, and explained that our solar system is not necessarily typical of planetary systems in general. Over the course of three presentations that comprised a seminar chaired by Professor Dame Jocelyn Bell Burnell, experts from the University of Warwick, the German Aerospace Centre (DLR) and University College London (UCL) also questioned whether it is sensible to focus all of our attention on the discovery of a ‘twin Earth’.

During the session, the speakers detailed the history of our search for extrasolar planets, outlined the strategies that have been employed in pursuing their discovery, and assessed our current capabilities in terms of identifying these elusive bodies. The University of Warwick’s Professor Don Pollaco opened proceedings, and outlined how the inert nature of exoplanets makes them difficult to spot.

"Planets only reflect light so they are a lot less luminous than their stars," he explained. "When we are looking for an exoplanet, we know that it will be next to a star and a lot less luminous. Right from the beginning we must face this technological problem; looking for an exoplanet is like looking for a candle next to a floodlight."

Professor Pollaco went on to discuss some of the techniques that have been adopted in the pursuit of exoplanets. Forward-thinking scientists began to search for these extrasolar bodies In the 20^th Century, but at this early stage, their techniques were not sufficiently advanced to make significant headway. In 1992, astronomers successfully discovered an exoplanet by studying a pulsar with a radiotelescope. Later, scientists began to survey sun-like stars. To date, Doppler Surveys have been responsible for the bulk of exoplanet discoveries, but Professor Pollaco and his colleagues are working on a new technique that they hope will result in a brisker rate of identification.

"Events such as solar eclipses are known as transits," he said. "We can use transits to measure information that is in our line of sight. From this information – using mostly geometry – we can learn a lot about planetary systems. We have been pioneering this transit technique using expensive CCD cameras to capture images of large parts of the sky. However, if you want to find smaller planets, you have to go into space.

"NASA’s Kepler mission, for example, has produced some marvellous results. When you look at the size distribution of planets, low and behold, most of the planets out there are small. We expected this to be the case. However, one of the most amazing results was that a lot of planets were part of multiple planet systems. This was not expected."

DLR’s Prof Dr Heike Rauer began her presentation by explaining why the logical way in which our solar system functions – whereby small, rocky planets are nearest to the sun, gas giants inhabit the midsection, and ice planets are found farther afield – caused many to assume that other planetary systems had to share its characteristics.

"In the centre of our solar system we have a huge star that produces a lot of heat and radiation," she said. "Obviously, you can’t have condensed ice near to the sun because it’s too hot. Only rock can be condensed in this region because of its very high melting point. Similarly, you would expect a star to blow away all hydrogen and helium from its immediate vicinity, so there shouldn’t be any gas left to form a gas giant. Until about 20 years ago, this was the general consensus concerning our solar system. It all makes sense and it’s all very logical. Of course, we expected all planetary systems to be like this. So what can we know about exoplanets? How typical is our solar system really?

Professor Rauer went on to say that scientists have actually observed many gas giants at similar distances from their stars as the Earth from its sun. Clearly, this is not in keeping with the logical conclusions drawn from the observations of our own solar system. In fact, scientists have observed no planetary systems that mimic the constitution of our own.

"How can we explain how gas giants are able to exist far too close to their stars?" asked Professor Rauer. "Well, we still believe that gas giants are formed at large distances, but that they move inwards via a process known as libration. Again, this is very different to our own solar system."

Professor Rauer concluded by questioning the legitimacy of media reports that astronomers have identified Earth’s twin.

"What we have not found is a second Earth, so if you read another newspaper headline saying that Earth's twin has been discovered, please be critical. It’s beyond the reach of our techniques."

After outlining what astrophysicists understand about the makeup of extrasolar planets, the session’s final speaker, Dr Giovanna Tinetti from UCL, asked whether humanity’s desire to discover Earth’s twin is to some extent, misplaced.

"One of the first exoplanets discovered was a gas giant, and immediately, scientists started to think about finding another Earth," she explained. "I have to say that for a few years, we were all a bit obsessed by this idea. One journalist even wrote that it was sad that we wanted to find a planet similar to our own; that maybe we should be more open-minded. Because of this obsession, the first experiments that attempted to find Earth’s twin were big, technologically challenging and very expensive. I think that they failed because we still have a way to go, and in fact, other experiments started to give us great data, and opened our minds a bit. Maybe, Earth twins are not the only things to look for."

The prospect of exploring the unknown is what makes science so intriguing. Perhaps it is no surprise that when we imagine what we might find, our minds are drawn to the familiar. However, whilst the hunt for Earth’s twin has persisted in the background, astronomers have discovered planetary systems that push the boundaries of what was considered possible. As the speakers made clear, the prospect of finding Earth’s twin is definitely appealing, but variety might just be the spice of space.