How We Find Exoplanets

Click to enlarge > Alien world with water and methane An artist's impression of hot Jupiter HD 189733b. Credit: ESA, NASA and G. Tinetti (University College London, UK & ESA)

By their very nature, exoplanets are extremely hard to observe. They are, to begin with, very small compared to most stars. Because of their size they are also dim, since their mass is too small to initiate the fusion reaction in their core that makes stars "burn." As a result, whereas most stars burn brightly, planets are lit almost entirely by their star's reflected light. To top it all, planets are usually positioned close to the stars which they orbit. Inevitably, these small and dim objects are completely swallowed up by the brilliance of the bright-burning stars that are their neighbors.

Because of this unalterable reality, almost none of the planets detected to date were found by direct observation. Rather, they were found indirectly, through the effect their presence has on their home star. These effects are extremely minute, and until recently were too small to be detected from Earth. Only in the mid 1990's were instruments developed that were sensitive enough to record the telltale signs that indicate the presence of a planet orbiting a star.

Each of the different methods for finding exoplanets searches for one of the observable effects the planet has on its home star:

The Radial Velocity Method, also known as the spectroscopic or Doppler method, looks for tiny shifts in the light spectrum of a star that indicate that it is moving ever so slightly. When it is moving more towards Earth the star's spectrum shifts slightly towards the blue, and when it is moving more away from us it shifts towards the red. If the spectrum shifts occur at fixed regular intervals it indicates that the star is rocking back and forth at a fixed rate. This can mean that the star is moving to the tug of an invisible planet that pulls it back and forth as it orbits. The majority of exoplanets discovered to date were found by this method.

Click here to learn more about the radial velocity method, its advantages and its limitations.

The Transit Photometry Method looks for a slight temporary dimming of a star that repeats itself over and over again. If the dimming occurs at regular intervals and lasts a fixed length of time it often means that a planet is "transiting" the star, passing between it and the Earth every time it completes an orbit, blocking out part of the star's light. Because it depends on extremely precise measurements of light, transit photometry is very susceptible to atmospheric interference, and the true potential of the method will only be realized from space, with missions such as COROT and Kepler. Many planet-hunters believe that transit photometry will eventually over take the radial velocity method as the leading technique for finding exoplanets. Click here to learn more about transit photometry, its advantages and limitations.

Microlensing relies on a remarkable effect predicted by Einstein's general theory of relativity. When a star is positioned precisely behind another star when viewed from Earth, the closer star "lenses" the light from the distant star, causing it to appear as much as 1000 times brighter than normal. This effect usually lasts for a few weeks until the two stars move out of alignment, the lensing ceases, and the stars appear as dim as they normally would. But if the lensing star is orbited by a planet then it too adds its mass to the lensing effect, causing a unique second spike of brightness lasting several hours or days. When a second spike of brightness is observed and measured in a microlensing event, it is a very strong indication that a planet is orbiting the lensing star. Microlensing is the only method capable of detecting planets at truly great distances of tens of thousands of lightyears. Unfortunately planets discovered this way can only be observed once before they and their stars are most often lost to astronomers, unlikely ever to be seen again.

Click here to learn more about microlensing, its advantages and limitations.

The Astrometry Method relies on extremely accurate measurements of a star's position in the sky to determine whether it is moving to the tug of an orbiting planet. Unlike the radial velocity and transit photometry approaches that work best when the plane of the planetary system is "edge on" when viewed from Earth, astrometry works best when the system is "face on" to an observer from Earth. In that configuration the full extent of a star's motion in the sky can be measured from Earth. Astrometry's future potential is enormous, and astronomers predict that it will be able to detect Earth-sized planets in Earth-like orbits around their stars. Like transit photometry, however, it is highly sensitive to atmospheric interference and its full potential will likely be realized in space. The European Gaia mission, scheduled to launch in 2011, and NASA's SIM mission, currently scheduled for 2015, will both use astrometry to search for terrestrial exoplanets.

Click here to learn more about astrometry, its advantages and limitations.

Pulsar Timing is the method used in 1992 to detect the first confirmed exoplanets, orbiting pulsar PSR B1257+12. Since then a handful of additional pulsar planets have been found by this method. Pulsars are rapidly rotating neutron stars -- the small but enormously dense remnants of massive stars that had exploded in a supernova. As they rotate pulsars emit a powerful beam of electromagnetic radiation that is detected on Earth as a regular and precisely timed pulse. Known pulsars have a rate that ranges from a few miliseconds to several seconds, depending on the speed of the star's rotation. Slight regular variations in the timing of the pulses indicate that the pulsar is moving back and forth, rocking to the tug of an orbiting planet. By precisely measuring the irregularities in the timing of the pulsars astronomers can deduce the orbit as well as the mass of the planet. The method is so sensitive that it can detect planets as small as one tenth the mass of Earth. However, the method applies only to pulsars, which are relatively rare celestial objects. Furthermore planets orbiting these dead stars with their high radiation are very unlikely to be hosts for life.

Direct imaging is the exception to the rule that exoplanets are detected by indirect methods. There is something magical for humans about the ability to actually "see" an object, and inthat respect direct imaging is the holy grail of planet hunting. "Seeing is believing" goes the saying: There is just no substitute to actually seeing a faraway planet. Unfortunately because exoplanets are small and dim, they are easily lost in the brilliant glare of their stars, making direct imaging extremely difficult. There are, however, exceptional circumstances in which it is possible to observe an exoplanet with a telescope, and several such planets have been directly imaged to date.

Click here to learn more about direct imaging, its advantages and limitations.