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Extra Solar Planets - Known Worlds Round Other Stars


Until very recently, the possibility of planets around other stars was merely scientific conjecture and the mainstay of science fiction. Although in our galaxy of about 300,000,000,000 stars it was a almost a certaintly that our solar system was not unique in forming planets, there was no direct evidence until the mid-nineties when methods were developed to look for celestial bodies smaller than stars.

Methods of Detecting Stars Around Other Planets

The animations on the left demonstrate the difference in the wobble of a parent star depending on whether the orbiting planet is small or large and also how close it is.

If observed from above as with the animations, the circular orbit of the star around the common centre of gravity is seen and this is called Astrometry. This method is unreliable, but the same wobble can be detective using via other means.

If you imagine an abulance driving in rapid circles a short distance away you would hear the pitch of the siren rise and fall depending on its movement towards or away. The same principle applies to light as well as sound, and this Doppler Shift of the light allows the direction to be determined in the same way. Wobbles in the light of the star means that it is being flung around by another massive object, and if no other light source is visible it can only be a massive planet. More complex and disturbed wobbles allow even more planets to be detected around the same star.

For solar systems which appear edge-on to observers on Earth,
the transit of the planet across the face of the star can be used to detect their existance by observing how much dimmer the star becomes. This method, although somewhat unreliable, allows for more data to be gathered about the planet, such as temperature, atmosphere and size.

There is only one method for detecting planets similar to the mass of Earth: Gravitational Microlensing. This is an intricate system whereby a star in the foreground can warp the light of a star in the background. If the foreground star also has a planet then the lensing effect is even greater.

Seeing Planets Around Other Stars

Extra Solar Planet 2M1207bThis image on the right may not look like much but it is very significant in astronomical history. This is the only confirmed image ever made of a planet around another star. The star is a brown dwarf designated 2M1207 and the planet is called 2M1207b and the picture is an infrared image. A handful of other possible sub-stellar objects have been observed in infra-red but may be brown dwarf stars.

The planet 2M1207b is a gas giant, still hot enough to give off detectable infra red, with approximately 20 times the mass of Jupiter. Despite the enormous mass, Jupiter would be about two thirds the diameter of this exo-planet. 2M1207b orbits its parent star a little further out than Pluto orbits our sun. Below is an artist's impression of this huge planet which exists 173 light years from our own solar system.


  Gas Giant Exoplanet 2M1207b

Single Known Gas Giants

illustration below shows three different solar systems (four including our own in grey) all overlaid on top of each other, if their parent stars were all in the same place.

Extra Solar Planets

In red at the centre is the remarkable planet DH 179949 b which orbits so close to its parent star that it only takes three days to complete one orbit. It has the same mass as Jupiter and therefore also likely to be a gas giant. Planets like this are most easily detected from Earth because of their severe influence over the parent star and the regularity with which they orbit. In light blue is planet HD 164427 b which has an eccentric orbit taking it from the equivalent of inside Mercury's orbit, to out near Venus's. In green is Epsilon Reticuli Ab which is a gas giant about 30% more massive than Jupiter.

The Star Syetem of Mu Arae

Star Syetem of Mu Arae

This star system has a yellow star at its centre, slightly bigger than our sun. Very close in orbit is planet Mu Arae d which is about 14 times the mass of the Earth and therefore one of the lightest planets discovered and probably a massive rocky planet. It orbits so close to the star that it only takes 9.5 days to complete a "year" rotation and its surface temperature is estimated to be 600 degrees celcius. Between the equivalent orbits of Venus and Earth lies Mu Arae e and further out is gas giant Mu Arae e, slightly bigger than Jupiter. This planet lies in the habitable zone around the star where liquid water and therefore life may exist. Although life cannot exist on the gas planet itself, it is quite possible that any large moons could be hospitable. Further out is another gas planet Mu Arae c. Along with these known planets it is of course possible that smaller, rocky worlds exist which are too small for our methods to detect.

Star Syetem of HD 188753

As well as quite typical planetary sysems, others have been found which almost defy explanation. Above is the system of HD 188753 which includes not only the gas giant HD 188753 Ab orbiting extremely close to the star but further out are two more stars which orbit each other. It is the only known triple star system which also contains a planet and goes against current theories which suggest gas giants form further out and migrate inwards.

Only a couple of small rocky worlds have been found. As well as the likely Mu Arae d, which is still large by Earth standards, the world OGLE-2005-BLG-390Lb may only be about 5.5 times the mass of the Earth. Unfortunately however as it orbits a red dwarf star at two and a half times the distance Earth orbits the sun, there is no chance of life.

The search goes on for Extra-solar planets and all the time more unusual systems are being found, as well as more Earth-like ones. As time goes on, methods will be refined and our picture of the galaxy will become one of a place filled with multi-planet solar systems like ours.

Click here to learn about recent changes to the view of our solar system.

Images and Text © Gavin Rymill 2006