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Detecting Extrasolar Planets

How would one detect a planet that orbits a star several light years away? Isnt it as simple as pointing a powerful optical telescope towards a star and just observe? Well, as you might have suspected, it isnt that easy. The light from the planet, which is reflected light, might be billions of times fainter than the source of the light, the star, not to mention that the star itself may be located several light years away.
Despite the vast distances and contrasts in brightness, astronomers have developed techniques that enables them to find planets around other stars. Of the about 150 planets discovered so far, most of them have been found using the radial velocity method. You can read about this method (and others) below.
Some readers will notice that some methods to detect extrasolar planets are simimlar to detecting binary stars.


With the help of astrometry, astronomers study the precise, periodic wobble that a planet induces in the position in the sky of its parent star. This method is old and can be used to find unseen companions, such as planets, brown dwarfs or even faint stars.
Even though the technique is very accurate, equipment to find smaller, earthsized objects hardly exists, it lays in the front edge of technology. Planets as small as 6.6 Earth masses could be detected in an orbit 1 AU from a star the mass of the sun, at a distance of 10 pc (32.6 light years). The space missions GAIA and Space Interferometry Mission (SIM), which will both be placed in orbit around Earth will have technology to find smaller planets.
Astrometry works best when the orbit of the planet around the star is perpendicular to the viewer. If the orbits is edge on, then a shift in the position cannot be measured, this this method will be useless. Also, a planet that orbits far away from its star will be more easily detected (of course, if the star itself is also close to us, the positionshifting will be greater too) as it will cause a greater shift in the position of the star. Though planets observed at greater distances from the star are problemtic, because they will require observations lasting longer than an orbit. In total this could take many years, perhaps decades.
So far, not one single planet has been detected with this method. Other means of detection are favoured today.

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Microlensing is a phenomenon that ocurrs when an object with enough mass (examples: planet, brown dwarf or a low mass star) passes between us and a background star. If for example a planet and a star would happen to pass infront of such a background star, the background star's luminosity would appear to increase (light is bent by the planet-starsystem's gravity). A passage that lasts for long (days, weeks) would indicate that the planet is orbiting at a greater distance than it would have, if the passage only lasted for hours.
This is a very promising and new method, though the chance is slim that a planet-starsystem would pass between us and a background star. For this reason, it is more efficient to study a background with many stars, for example a view towards the galactic centre would provide a significant amount of stars.
Microlensing, which is sometimes called gravitational microlensing was predicted by Einstein. He claimed that light does NOT travel in a straight line, but if it would happen to pass close to a massive object, its direction would change.
Using microlensing, astronomers have discovered a gas giant of three Jupiter masses orbiting a star at an amazing distance of 15 000 light years! Had the gas giant been an Earthsized planet, astronomers would have been able to detect it anyway! Read more...

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Photometric Transit Method

This method uses the fact that when a smaller and less bright object passes infront of a bright object, such as a star, the star appears to fade in luminosity, even if the reduction is very small (typically between 0.01% and 1%) astronomers can detect it. The object passing by could be a star, brown dwarf or a planet. The event could be regarded as an eclipse or an "occultation".
The photometric transit method has an disadvantage in that the star which is being studied needs to be edge-on relative to the line of sight of the observer. Less than 1% of the stars (F-, G and K class dwarfs are the most promising candidate stellar types) would have this kind of desired orbit. If the angle would be different, then the planet would never appear to pass in front of the star. Thus, this method would fail.
Though, the method could work on great distances and astronomers can observe the planet once during each of its orbit in the future, with more accurate instruments. Another advantage this method has is that during the occultation, the composition of the planet's atmosphere could be detected. The study of such an occultation would produce a light curve, which would show how much a star had faded due to the passage of the planet. If the curve is precise enough, it could even reveal the presence of moons around the planet and astronomers would know immediately if the planet is in the habitable zone. So far six planets have ben found using this method.
The future (2007) mission Kepler will be using this photometric transit method to find habitable planets around other stars. The Kepler observatory will be studying about 100 000 stars continuously and simultaneously. For the first time astronomers will be able to make an estimate of the frequency of planets around F-, G and K class dwarf stars.

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Radial velocity or Doppler Spectroscopy

The radial velocity method has been the most successful so far in finding extrasolar planets. The vast of majority of the now known planets have been detected using radial velocity. This method uses the tug a planet exerts on its star. For instance, it's common knowledge that the planets orbit the Sun. The sun seems stationary compared to the planets, while they orbit it. As a matter of fact, the planets themself exert force on the sun, making it too orbit in a common gravitational orbit. The effect is very small, hence we dont notice anything at first glance. Jupiter exerts the strongest force on the Sun (a radial velocity of 12 m/s, in the solarsystem). Earth may have as little effect as only 10 cm/s, over a period of a year. The more massive a planet, and the closer it orbits the star, the higher effect it will have on it. This effect causes the lines in the star's spectrum to shift towards redder wavelengths when the planet is moving away from us, and a shift towards bluer wavelengths when the planet is approaching. This shift is also known as the Doppler shift and the radial velocity method is sometimes called Doppler spectroscopy.
A disadvantage this method has is that it can only determine the minimal mass of an object, and the larger the object, the easier it is to detect it, which makes it difficult to discover Earthsized planets with this method. The true radial velocity can only be found when the planet orbits the star along our line of sight (edge-on). Due to this drawback, astronomers sometimes combine the radial velocity method with astrometric observations (see above for astrometry) to make more precise measurements. This way, astronomers can be more certain that they are actually obsering a planet, rather than a low-mass brown dwarf. This immediately gives rise to the question are all "planets" we have discovered planets, or are they perhaps brown dwarfs? Perhaps the planet around 70 Virginis is a low-mass brown dwarf?

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Direct Detection

Why did I put this part at the end of this page? I wanted to say that even though it is highly unlikely that an extrasolar planet would be directly viewed, it is not impossible for this to happen. The star HD 209458 harbors atleast one gas giant, which might have become the first planet to ever be imaged. It is a "hot Jupiter" and emits infrared light. For more information please visit these links: Universe Today and Sky & Telescope.

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Next: Extrasolar Missions: Detecting Extrasolar Planets.


Quick Links

Astrometry - Illustration (P)
Photometric Transit Method - Illustration (P)
Radial Velocity (Doppler Spectroscopy)

Direct Detection

Astrometry: As seen above, a star without a companion would appear to travel on a straight line compared to the background stars, but if the star has a companion it will not.

This illustration is available upon request, as a print (4000x3000 pixels, 300 dpi), and as a PSD-document so that it can be customized according to your own desire.

Photometric transit method: When an object passes infront of the star its' luminosity will decrease. The planet is not to scale.

This illustration is available upon request, as a print (5000x3000 pixels, 300 dpi), and as a PSD-document so that it can be customized according to your own desire.

Radial velocity: When the planet around the star will be moving away from us, its tug will drag the star away from us too, which will cause a redshift in the light we receive from it. When the planet is approaching it, the same thing will happen to the star, only that the light is shifted towards blue.
The cross in the image is the gravitational center. The dragging of the planet on the star is not to scale.


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