It was not until 1995 that a planet was successfully detected in a foreign solar system around a sun-like star. Since then, thousands of exoplanets have been discovered, and according to scientific estimates, millions if not billions more are still waiting to be discovered. This special describes the methods used in more detail.
According to statistics (as of May 2024), 5,626 extrasolar planets have been discovered, and many thousands more are candidates that have not yet been confirmed. Many of these are in multiple-planet systems, i.e., solar systems in which more than one planet has been discovered, of which our solar system with its 8 planets is a good example.
Well, with billions of stars in our galaxy alone, the Milky Way, 5,626 planets don't seem like a lot. So why haven't more of them been discovered so far?
This is because planets do not emit their own light, and the stars around which an extrasolar planet orbits are much brighter and therefore outshine them. Various methods have therefore been devised to compensate for this shortcoming. Unfortunately, many of these methods work much better for large planets such as Jupiter than for much smaller planets such as Earth.
Furthermore, exoplanets are mainly discovered in our cosmic neighborhood (within 1,000 parsecs) [1 parsec ≈ 3.26 light years], and the discovery of distant planets is rather due to lucky circumstances (e.g., the gravitational lens effect).
1. Radial velocity method
This method was the most successful in the early days of the search for extrasolar planets. The speed at which the star is moving is measured very precisely. If irregularities occur in the movement of the star, this could be an initial indication of the presence of an extrasolar planet. Although planets orbiting a star are held in orbit by the gravitational forces of the star, they also exert a force on the star due to the law of mutual mass attraction. The gravitational interaction between the planet and the star causes the star to move in a complex orbital motion that can be measured by the Doppler effect. The Doppler effect causes a change in the wavelength of the light emitted by the star, which can be measured as a change in the radial velocity of the star.
This can be observed by analyzing the spectrum of the star. According to Doppler (the discoverer of the Doppler effect), when light moves towards us from a star, it is shifted towards the blue spectrum, whereas when it moves away from us, it is shifted towards the red spectrum.
This can be explained by the fact that the light is compressed as it approaches the observer and is spread out as it moves away from the observer.
This process can take several years, as precise observations and measurements of the star are required. The radial velocity method is more sensitive to larger planets, e.g., planets the size of Jupiter, which have a stronger gravitational pull due to their greater mass and radius. However, under favorable conditions, smaller planets, such as super-Earths or even Earth-sized planets, can also be detected. The gravitational pull of a planet depends on its mass, radius, and orbital parameters, so it is a complex relationship.
2. Astrometric method
While the radial velocity method measures the speed of the star very precisely, the astrometric method measures the position and movement of a star in the sky. As explained in the radial velocity method, a planet orbiting a star causes the star to “wobble” slightly as it orbits around the common center of gravity of the system due to the mutual gravitational interaction. This change in position can be detected by astrometric measurements. The method can not only detect planets close to the star but is also suitable for detecting extrasolar planets that are located at a greater distance from their star.
3. Transit method
Thanks to the use of special space telescopes such as Kepler and TESS (Transiting Exoplanet Survey Satellite), the transit method is currently the most successful method of detecting extrasolar planets. When a planet passes by its star, a tiny part of the star's light is blocked, and these changes in the star's brightness spectrum can be measured.
Only very sensitive devices are able to detect this fluctuation in the brightness of a star. As with the other methods, longer observations are necessary to rule out natural phenomena of stellar life. This method is also better at detecting large planets, as Earth-like planets are too small to cause a detectable variation in brightness.
4. Gravitational Microlensing
This method is based on the general theory of relativity postulated by Albert Einstein. According to Einstein, strong gravitational forces can bend space and also light. The light from a distant star therefore does not spread out in a straight line but is bent by massive objects.
As with the radial velocity method and astrometric measurements, gravitational forces play an important role here. When a planet passes in front of its star, its gravitational forces cause a lensing effect. The light rays are bundled, resulting in a short-term increase in the star's brightness spectrum. This method is also very time-consuming and requires careful analysis and follow-up observation of the data.
5. Direct imaging
This method attempts to photograph the planet directly. Since planets shine much fainter than their stars and are often lost in their brightness, this is very difficult. However, advances in technology, such as coronagraphic techniques (which block the star's light) and adaptive optics (which compensate for atmospheric distortions), have made direct observations of exoplanets possible. This method is particularly effective for large planets that are far away from their star.
Note: You can find out a lot more about this topic in my book “Exoplaneten - Die Suche nach einer zweiten Erde”, which is in German.