Catherine Zentile

A Whole New World

The search for extra-solar planets

The search for planets outside our solar system, also known as exoplanets, has been of interest for a long, long time. Are there other planets like the Earth? Or are we alone? At the turn of the twentieth century in France, the Guzman Prize of 100,000 Francs was offered for the first contact with an extra-terrestrial species; naturally it was felt prudent to exclude any contact with Martians because that was thought to be too easy!

For scientists today the first step in answering these questions is to look for other planets in other systems around other stars. The problem is that they're very difficult to detect. If you looked at the solar system from the outside, the Sun’s brightness would be a billion times stronger than the Earth’s. Moreover, when you look for exoplanets, not only are you looking for something that is one billionth as bright as the star it orbits, it is also very close to it. It's like trying to see someone holding a lit torch on a neighbouring hilltop in the middle of the day. For this reason, the main way of detecting exoplanets, known as the radial velocity method, doesn't look at the planet at all (see Figure 2). Instead astronomers observe the star to see if it wobbles. If it does, this could be due to the gravitational pull of an orbiting planet.

Fiddly as it sounds, this radial velocity approach (left) has been so successful that it has detected 257 out of the 271 confirmed sightings, including helping researchers to pick out a newborn baby planet called Hydrae. This was a huge surprise since, prior to now, no planet had ever been detected around a star as young as TW Hydrae, which is only 10 million years old.

What this reveals is that the formation of gas giants like Jupiter and Hydrae can happen very quickly, but how? Scientists have found that young stars are surrounded by rings of dust and gas known as proto-planetary disks. Over time gravity pulls together the particles in these disks to form tiny grains, which slowly join up to spawn embryonic planets called planetessimals. Some of these subsequently grow into mature planets and any left-over debris forms rocky bodies such as asteroids.

The whole process is known as the nebular hypothesis, which was first proposed in the eighteenth century, although the precise details of how it works are still a matter of scientific debate and this is where the newborn planet, Hydrae, can help: "Here we're seeing for the first time a planet within the disk from which it was formed," explains Cambridge University astronomer Dr Mark Wyatt.

"We know a lot about the structure of the disk in the vicinity of the planet, which means that we can begin to study how a planet interacts with a disk. This is important because it is thought that this planet formed at 4 astronomical units (1 A.U. is the distance between the sun and the Earth, or about 93 million miles) and it then moved to its current location at 0.04 A.U. through the interaction with the disk. This interaction is something which, until now, has only been possible to study through computer simulations."

So the discovery of Hydrae is helping to confirm long-held astronomical suspicions, and as Wyatt points out, "we stand a much better chance of understanding how these planets formed by seeing them in their birthplace. More generally it shows that planets can be detected in protoplanetary disks and so we can expect to see more of these discoveries in the coming years."

So if you want to look for exoplanets, just where should you begin to look in the sky? "There are two main factors to consider here,” explains Cambridge-based astrophysicist Professor Chris Haniff. "The first question you have to ask yourself is what types environments would my instrument be most suited to detect planets in? That limits the number of targets you have to study straight away. It also depends on the scientific questions that you want to answer. The group looking at this newborn planet wanted to answer the question "when do planets form?" so they deliberately picked stars that were young."

But why do we look for exoplanets? Is it so that we can make first contact? Or maybe it’s because we want to find a new Earth to replace our own bruised habitat? Mark Wyatt is philosophical about it, "this discovery does put our solar system in some kind of context but it also emphasises just how diverse and interesting the universe is." And what about the future of exoplanet detection? NASA’s Terrestrial Planet Finder and ESA’s DARWIN space mission will launch in 2015. Unlike the majority of planet detection techniques they aim to directly observe exoplanets surrounding nearby stars, meaning that they are better equipped to obtain information about the nature of the planet: whether it is Earth-like or habitable, and to search for traces of life. So perhaps it won’t be another century before someone claims the Guzman Prize after all...

- February 2008

About the Author

Catherine Zentile is a member of the Quantum Matter research group in the Cavendish Laboratory at Cambridge University.