Astrophysical processes

Stage one is characterised by the formation of a star at the center of mass of the system. Gravity within the gas (made of mostly hydrogen and helium) leads to clumping of gas and dust into successively larger bodies until a star forms. The remaining matter forms a large protoplanetary disk of gas around the star.

Matter in the disk forms congregations which develop from sub-micron-sized dust particles to meteors and finally early planets. This era lasts for roughly 10 million years.

The disk of gas slowly disappears as the gas clumps form planets and they clear any sizeable matter out of their orbit. Aggregation of dust and grains into successively larger chunks is mostly due to random movement and collisions but can be aided by magnetic attraction and, at larger volumes, gravitational attraction.

This stage is characterised by the formation of rocky planets and more detailed occurrences such as oceans and atmospheres appearing.

Researchers running simulations of solar system formation show that the most consistent result is the generation of a few rocky planets between 0.5 – 2 times the distance from the sun the earth (0.5 – 2 AU) over a period of 100 million years. Collisions of similar sized bodies in similar orbits also often result in one larger body and a small chunk of debris being released, caught in the larger bodies orbit, and developing into a moon.

Rocky planets are thought to form close to the star since most of the gasses are consumed into the star or remain far out in the protoplanetary disk leaving rocky chunks as the majority of the matter nearer the centre of the disk. Also, the high temperatures nearer the centre favour the condensation of rock and metal. The line for defining the type of planets that form is called the “frost line”. The distance of the frost line from the centre of the system is dependent on the size of the star.

Beyond the frost line, temperatures are cooler allowing hydrogen to form ices. The proportion of metals and rocks in the area is also significantly reduced. The increasing mass of the planets and lack of condensation means that the planets form successively larger and larger gas clusters until all of the gas has been absorbed and a gas giant remains, an example being Jupiter.

Contrary to popular perception, stars are not stationary at the centre of their solar system with planets orbiting around them. Instead they exhibit a “wobble”.

This phenomenon occurs because the planet doesn’t actually orbit around its host star, but both the star and the planet orbit around their common centre of mass. This centre of mass is usually found very close to the host star, sometimes even within it, due to the fact stars are a lot more massive than their accompanying planets.

Very precise measurements of extrasolar stars’ motion can show evidence of this wobble, and allows us to infer the presence of an exoplanet.

An example of an instrument currently in use that is using this method is the GAIA spacecraft, ran by the ESA.

The Doppler effect is the alteration of the observed wavelengths of light emitted by a star, due to the star moving towards or away from us. This effect gives rise to the radial velocity method of exoplanet detection.

As an exoplanet orbits its parent star, it causes the star to also wobble (as explained in the Astrometry method), so for us on the earth, or rather through high tech telescopes, it looks like the star is moving away and then back towards us as it completes a whole orbit around the centre of mass.

This slight change in the star’s motion causes very small Doppler shifts, but these shifts are periodic, and so scientists look out for these repeating shifts as they also infer the presence of an exoplanet.

Large planets that orbit very close to their stars (such as Hot Jupiters) are the easiest to detect using this method, as they have a bigger pull on their host star and don’t take long to complete a whole orbit, and therefore it’s a lot easier to spot the repeating pattern.

Transits are probably the easiest method to understand. Transiting planets are very simply those that come directly between their host star and Earth during on their orbit. When the planets do this, they block out a very small amount of the light that we see coming from the star. Again, this pattern is repeated with every orbit, so by looking at the amount of light received from a star over time, we can see these repeated dips and again infer the presence of an exoplanet.

Within the past few years, technology has advanced to the point that allows us to actually directly observe exoplanets themselves, rather than just infer their presence by how they interact with their host star. By using adaptive optics to block out the main source of light from their host star, we can directly observe planets orbiting around a star. This method is best suited to detecting planets in large orbits, away from the glare of the host star.