Methods for Detecting Exoplanets

Most exoplanets are too far away to be seen with traditional imaging methods. Instead of imaging exoplanets directly, new techniques have been developed to study the effects that a planet has on its parent star, so that it can be identified indirectly. The development of new techniques for detecting farther and smaller exoplanets is one of the newest and most exciting fields in astronomy. The image below shows how many exoplanets have been discovered, and by which method, as of January 2010.

Sara Seager, Drake Deming. Exoplanet Atmospheres. Annu. Rev. Astron. Astrophys. 2010. 48:631–72

Radial Velocity Method

As a planet orbits its star, it appears at first glance that gravity is only pulling on the planet, while the star remains stationary. However, the planet is also producing a gravitational field which pulls on the star as well, causing the star to 'wobble' slightly as the planet orbits. This small wobble is detectable by telescopes from Earth as a shift in the star's colour, due to the Doppler Effect. As the star wobbles, at certain times it is pulled towards the Earth, and other times it is pulled away. When it is being pulled toward the earth, its light waves stack, causing a fequency shift in its emitted light towards the blue end of the spectrum (i.e. the light wavelengths shorten). As the planet is pulled away from the Earth, the lightwaves are spread out, causing a shift towards the red (i.e. the light wavelengths lengthen). By measuring these spectral fluctuations, astronomers ­­can detect planets. This method is best for detecting large planets that orbit close to their stars.

Transit Method

The Transit Method is used to detect planets by measuring a characteristic drop in the apparent brightness of a star when a planet's orbit causes it to cross in front of the star, blocking some of the light from reaching earth. To use this method, astronomers have to look at many stars over long periods of time, since a transit can only happen once every orbit. If astronomers used this method to detect Earth, a transit would only be seen once every year. Another difficulty is that the planet and the star have to be in the same line of sight when viewed from Earth. Most planets orbiting other stars will not pass in front of their star in this way, and so cannot be detected by this method. Also, since planets are much smaller than their parent stars, during a transit the drop in brightness that is detected is less than 0.01% of the star’s total luminosity. With all of these difficulties, this is still one of the best methods for studying and detecting exoplanets. It is even capable of detecting exoplanets in an Earth like orbit; however, due to the line of sight issue, the probability of such a planet orbiting its star in a detectable plane is only 0.43%. The Kepler Space Telescope detects planets using this method.

Direct Imaging

Under the right conditions, an exoplanet can be directly imaged around its parent star. At visible wavelengths, the intensity of a star's light is millions of times greater than its planet's light intensity. However, at infrared wavelengths a star is only a few thousand times brighter than its planet. This contrast is improved further in young solar systems that are newly formed, where the planets are still cooling and emitting more heat themselves than in older solar systems. For a planet to imaged directly, it must also be quite far from its parent star (upwards of fifty times further from its star than the Earth is to the Sun) so that it can be resolved in the the glare of starlight.

Other Methods

Timing Method

The Timing Method was first used to detect planets around pulsars. A pulsar is a star that emits a strong magnetic field, and as it rotates it appears from Earth to be pulsing. This pulsation is highly regular and predictable. When a planet orbits a pulsar, the planet causes it to wobble (similar to the wobble described in the Radial Velocity Method) which disturbs the regular pulsations. This method is so precise that it can detect Pluto sized planets that rotate around pulsars. This method has also been used for certain other stars that are not pulsars. With these stars, the solar luminosity is variable in such a way that it fluctuates regularly and predictably like a pulsar. Orbiting planets pull these stars in the same way, altering their predictable pulsation pattern and enabling them to be detected.

Astrometry Method

The wobble caused by an orbiting planet can also be measured by tracking a star's changing position. The small changes in position are imaged directly, and are more difficult to detect than doppler shifts. Atmospheric interference on Earth also makes this very difficult for ground based telescopes. A new space telescope planned by the European Space Agency, called GAIA, will search for planets using this method.

Gravitational Microlensing

A star or planet is obviously heavy enough that we can easily feel its gravitational influence. However, even light is influenced by gravity. When a distant source of light is viewed from earth, and a star happens to pass in between, light from the source is bent slightly as it passes by the star. This bending of light due to gravity is similar to what happens in a magnifying glass, so it is called gravitational lensing. If the light is bent by a star, for example, scientists can predict very well how light coming from behind the star will be bent. If a planet is orbiting the star, it will cause a small 'blip' in the light bend, which is detectable from earth. These lensing events are relatively rare and difficult to predict, so there are fewer planets detected by this method.