Astronomy Guide

Exoplanets

Facts and Information

Types of Exoplanets

Gas giants are planets similar to Jupiter and Saturn. Their mass is mostly composed of hydrogen and helium with possibly a dense rocky or metallic core. Exoplanets with a mass of more than 10 times the Earth are classified as gas giants. Due to their size the majority of planets detected outside our solar system are gas giants.

Hot Jupiters are gas giant planets similar in mass to Jupiter but which orbit very close to their parent star. As a result of the close proximity to their star their surface temperatures exceed 700C (1300F). Apart from gas giants Hot Jupiters are the most common type of exoplanet detected.

Super Earths are ‘potentially’ rocky planets with up to 10 times the mass of Earth. The term ‘Super Earth’ simply refers to the mass of the planet and not to any planetary conditions, so some of these may actually be gas dwarfs. The first two exoplanets to be detected were Super Earths orbiting around the pulsar PSR B1257+12.

Free floating planets or orphan planets do not orbit around any star. It is believed that these isolated worlds were somehow ejected from developing systems and now free-float around the galaxy. Although very few have been detected they are believed to very common in our galaxy.

Pulsar planets orbit around Pulsars or Neutron Stars. These super dense, rapidly spinning stars are the core remains of a large star after a supernova explosion. It is highly unlikely that any orbiting planet could survive the blast from a supernova so Pulsar Planets probably formed after the event and now orbit around the dead star.

Ocean Worlds are planets whose surfaces are entirely covered in water. Evidence suggests that these planets originally formed as objects made of ice and rock far from their parent star. As they drifted towards the star they heated up melting the ice and so became covered in oceans. Under the surface the water is so dense its consistency is more like ice.

Chthonian Planets were once gas giants but have migrated far too close to their parent star. As a result their atmospheres have been roasted away, leaving only a rocky or metallic core. It is possible their surface may be covered in molten lava. Due to their similarity to terrestrial planets some Super Earths may actually be Chthonian Planets.

Exo Earths are the Holy Grail of planet hunting. These are Earth like planets that orbit in the habitable zone of its star. This is an area where the temperature would allow water to flow on the planet’s surface and possibly allow life to flourish. To date there have been no confirmed discoveries of any Exo Earths.

Searching for Exoplanets

Doppler Spectroscopy

Doppler Spectroscopy also known as Radial Velocity is the most successful method of finding exoplanets. This technique measures how much a star wobbles due to the gravitational effects of orbiting planets. It is not strictly true to say that planets orbit around stars, they actually both orbit around their common center of mass. As stars are much more massive than planets the common center of mass is very close to the star and very far from the planet, as a result the star has a very slight, circular or elliptical orbit. You can see this demonstrated in the video to the left.

This wobble can be detected by measuring the light from the star as it moves back and forth from Earth. As the star moves towards Earth its light moves very slightly towards the blue part of the spectrum, known as blue-shifting, and as the star moves away its light moves very slightly towards the red part of the spectrum, known as red-shifting. This is called the Doppler Effect. You can see this demonstrated in the video above, if you imagine Earth is below the video then as the star moves away it turns red and as it gets nearer it turns blue. Of course this is an exaggerated version of what actually happens, the shifts in the spectrum are so small only highly sensitive instruments can pick them up. By observing these shifts over a period of time a regular pattern may emerge signalling that a planet or other object is orbiting the star.

Stellar Nurseries

These dark molecular clouds of gas can become stellar nurseries, providing an environment which allows stars to be born. When pockets of the dark nebula have sufficient density, hydrogen molecules will begin to condense, this is possibly triggered by the shockwaves of a nearby supernova explosion. Clumps of hydrogen will grow large enough to begin collapsing under their own weight, heating up and creating the early stage of a star called a Protostar. Over thousands of years the young star continues to grow and heat up until its core is hot enough to allow nuclear fusion, this is when hydrogen atoms are fused together producing enormous amounts of energy. The star is now in its main sequence and will remain in this state for most of its life.

Around 5 billion years ago this process began at the edge of the Milky Way, eventually producing a yellow dwarf star. Out of the large disk of gas and dust that formed around the new star a small rocky planet was created which was in just the right location to allow water to flow on its surface. This was Planet Earth, these conditions allowed life to flourish ultimately leading to the evolution of mankind.

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