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 discoveries of any Exo Earths.

Exoplanet Hunting Techniques

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.

Transit Photometry

When a planet transits it is simply passing between Earth and its star, the image on the left is of a Venus transit of the Sun. This event causes the light from the star to dim very slightly which can be detected by sensitive instrumentation. If the dimming occurs regularly and for fixed periods of time then it is probable the star is being orbited by a planet.

The advantage of this method is that it gives more information about the planet than Doppler Spectroscopy. As the size of the star is known any dimming in its luminosity can provide a fairly accurate assessment of the planet’s size. Also the composition of the planet’s atmosphere can be deduced by analyzing the light that gets absorbed by elements as it passes through its atmosphere. By combining the results of Transit Photometry and Doppler Spectroscopy an estimate of the planets size, mass and composition can be gained.

The main issue with this method is that a transit must occur in order for a detection to be made. The observer has to be in the exact alignment and at the right time to detect the transit, this means that this method of detection can only be applied to a very small minority of stars. NASA’s space based Kepler Mission overcomes this issue by scanning more than 100,000 stars simultaneously over a number of years, so far the mission has provided over 1,000 exoplanet candidates.

---

Posters From Solar System Quick