The search for extra-solar planets

Figure 1: An artist's view of a planet around a red dwarf star.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 below). 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.

Figure 2: The radial velocity methodFiddly 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.

Article by Catherine Zentile.

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