An artist's impression of an Earth like planet outside our solar system.
Exoplanets also known as extrasolar planets, are planets which exist outside our solar system.
There have been over 4,000 confirmed discoveries of exoplanets.
In 1992 two planets orbiting a distant pulsar became the first exoplanets to be detected.
In 1995 51 Pegasi B was the first exoplanet to be discovered orbiting a sun like star.
Since 2009 NASA's Kepler Misson has detected almost 7,000 possible or confirmed planets using its space based telescope.
The habitable zone is an area around a star which is at just the right temperature to allow liquid water to exist on a planet's surface.
Several possible Earth like planets have been detected orbiting in the habitable zone of stars.
Two small Earth sized planets have been detected in our nearest stellar neighbor Alpha Centauri, it is possible that one of them may be habitable.
Only two exoplanets have ever been directly observed using Earth based telescopes.
Exoplanets - The Search for Life
Could there possibly be another Earth like planet in our galaxy?
Are we alone in the Universe? This is a question mankind has pondered for centuries, but only now do we have the technology to begin to answer that age old question.
The search is truly on to find another Earth like world but as yet we're still the only known planet in the Universe where life exists.
New evidence being compiled suggests that its highly unlikely that Earth is unique, it is estimated there are habitable planets around 10 to 20% of the stars in our galaxy.
When you consider that there are at least 200 billion stars in the Milky Way then it could possibly contain billions of worlds similar to our own. The problem with Earth sized planets
is that they're very small, making them extremely difficult to detect, even so scientists are predicting that its only a matter of time before we have a confirmed
discovery of an Earth like planet somewhere in our galaxy.
Types of Exoplanets
Gas and Ice Giants
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. Ice giants are similar to Uranus and Neptune, they have much less
hydrogen and helium in their composition and more heavier elements.
Around two thirds of the planets that have been discovered outside our solar system are either gas or ice 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).
It seems that Hot Jupiters are very common in our galaxy, which is not what astronomers expected as gas giants orbit in cold regions of our solar system.
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
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.
Water 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.
These are planets that have a similar mass, composition, radius and atmosphere to Earth and 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. Although
several Earth sized planets have been discovered orbiting in the habitable zone around stars their composition and atmospheric conditions are unknown.
So to date there have been no confirmed discoveries of any Exo Earths.
Exoplanet Hunting Techniques
Radial Velocity also known as Doppler Spectroscopy 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 right.
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.
The planet Venus transiting the sun
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 Radial Velocity. 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 the Transit Method and Radial Velocity 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 overcame this issue by scanning more than 100,000 stars simultaneously over a number of years, during it's mission the Kepler space telescope provided thousands of exoplanet candidates.