During the last decade, detecting exoplanets (planets outside our Solar System), has become one of the hottest research areas in Astronomy, due to the high number of planets that have been discovered. The main reason for this was NASA’s Kepler and now TESS, while new instrumentation on ground based telescopes, together with a variety of methods detecting exoplanets, has also given fruitful results. When it comes in detecting planets, one needs to keep in mind the following:
- Planets are small and very faint sources, since they only reflect light that they receive from their host star.
- A star on the other hand, can be billions of times brighter than the surrounding planets, thus outshining them.
- The distance that these planetary systems are located, makes detection of planets even more difficult.
Below, the most commonly used methods for detecting exoplanets are presented. These methods are divided into two categories. The first category includes the direct methods, which means obtaining spectra, or images of the planets. The second are indirect methods, which means that you observe a star and its properties from where you can reveal effects of the presence of planets.
The methods that will be presented below are:
- Exoplanet transit
- Radial velocity
- Direct imaging
- Gravitational microlensing
Additionally, we will see how fruitful each method has been.
Exoplanet Transit method
Everyone is familiar with the concept of the Solar eclipse. This happens when the Earth, the Moon and the Sun are aligned and the Moon cast its shadow on Earth, blocking sunlight partially, or fully. A transit is something similar and it happens when a planet passes in front of a star.
In order to see a planetary transit, a series of frames from the target star are needed for measuring its brightness. If a transit takes place, the planet (or planets) reduces the apparent brightness of the star and by measuring the brightness reduction, the radius of the planet can be estimated. Additionally, measuring the period of the transit occurrence, will allow us to estimate the orbital period of the planet. Note that, in order to be able to see this the system must be edge on, as you can see in the animated video below.
This method was used by the Kepler probe, and it can be used from ground based telescopes too. Additionally, the same method is used by NASA’s current planet hunter TESS (Transiting Exoplanet Survey Satellite). So far through transit around 3200 planets have been detected.
Radial velocity – Doppler shifting
This method depends on two concepts. The first is gravity, while the second is Doppler shifting. From Newton’s universal law of gravity and Kepler’s first law, we know that a planet orbits around a star in an elliptical orbit. The balance point between the two objects is known as the center of mass and it is closer to the more massive body. Thus, since a star is much more massive than a planet, the center of mass is located within the star. A planet, even though it has very low mass with respect to the star, still has an effect on the massive star, thus pulling it towards it. This makes the star wobble around the center of mass, which in turn causes variations in the velocity that the star moves away, or towards the Earth.
To understand better what is actually this wobble consider the sound waves which can be squeezed, or stretched and this is based on the movement of the object that produces them. Take as an example the sound from the siren of an ambulance. In this case when the ambulance moves towards you the sound wave that is caused by the siren gets squeezed, while it gets stretched when the ambulance moves away. This effect is known as Doppler shift. In the case of a planet-star system, the velocity shift (variation) is small, but it is still measurable. As you can see in the animated video the star wobbles around the centre mass. The gravitational pulling of the planet makes the wave from the observed light of the star to stretch or squeezed, depending on its position.
The wobbling effect depends on three parameters. The first is the distance between the planet and the host star, since we know from Newton’s law of gravity that the gravitational force between two objects is inversely proportional to the square of their distance. Also, a larger planet will make a star wobble more. Finally, the less massive a star is the higher is the wobbling from the orbiting planet. A star’s wobble provides information on the mass of the planet, the orbital period and the eccentricity of the orbit. This method is quite effective and it is used from ground based telescopes and has resulted in the discovery of around 800 planets.
Direct imaging of exoplanets
As the term suggests, the task is to obtain a direct image of the planet. The challenge here is obvious. The star will always outsign the planet, or the surrounding planets. Thus, in order to take direct images of planets, the light of the host star must be masked out. The technique that is currently used is called coronagraph. As you can see in the animated video, once the host star is masked out, you can observe the planet. With this method a device inside the telescope blocks the light from the target star before it reaches the telescope. Through this method around 50 planets have been found.
Gravitational microlensing from planets
Albert Einstein redefined the concept of gravity. Thus, instead of just considering gravity as a force between objects, it is a geometric property of space-time. To view this simply, consider a fabric that is stretched. If an object is placed on it, it bends due to its presence. Gravity has the same nature, thus objects tend to warp the “fabric” of space-time around them. Gravity can also affect light, thus when light passes nearby a massive object its path is distorted. In the animation you see that light rays from the star bend around the planet.
So as a magnifying lens can focus Solar rays into a smaller area, the gravitational effect of the planet does the same to the light rays of the host star, making the star appear brighter to the observer. This can be seen on the graph on the left, where you see that as the planet (or planets) move away from our field of view, the observed light fades away. This method requires a series of frames in order to measure such an effect, but still around 90 planets have been detected from ground based telescopes.
We went through the most commonly used methods for detecting planets outside our Solar System. What makes it hard to detect even more planets has to do with the fact that exoplanets are at great distances, they are very dim and the large amount of data that is required, in order to detect a planet. So far the most fruitful method is the planetary transit. Of course as instrumentation technology progresses, we will be able to build much more sensitive instruments and combined with the next generation of both ground-based and space-based telescopes we will be able to detect even more planets. The quest for Earth-like planets continues, which will eventually allow us to find a planet that can support life. This of course may open a lot of doors, since we may also detect life somewhere else in our Galaxy.
All animations in the article are courtesy of NASA (https://exoplanets.nasa.gov/alien-worlds/ways-to-find-a-planet/)