In the last few decades, our science and technology have been developed with a very high speed and the speed is increasing exponentially; it has become easier for us to search for life beyond our solar system. Since 1988, more than 2000 exoplanets have detected. So, What are exoplanets? And how do we detected them?
Exoplanets (or Extrasolar planets) are planets that locate outside the Solar system, they orbit around a star other than the Sun.
Because planets don’t emit visible lights or other electromagnetic radiations like stars, it’s more difficult for astrophysicists to detect them. To detect and confirm the existence of an exoplanet, different special methods must be used; sometimes, more than one methods need to be used at the same time.
The first method I’m going to talk about is Transit Method. This is an indirect method to detect an exoplanet by measuring the decrease in intensity of the lights coming from a star in a short period of time. The star is usually monitored, and the intensity of light emitted by star is measure constantly. As the planet orbits around the star, when it’s between the star and the Earth, it blocks some of the lights from the star and cause the brightness of the lights measured sure on the Earth decreases. After a short period when the planet moves away, the brightness of the star get back to normal. Using the change in the intensity of the lights from the star, radius of the planet can also be determined. However, using this method alone isn’t enough to confirm the presence of the planet because there are some other factors that can cause the dip in brightness of the star. One of these factors is that the star in a binary star system, and the brightness of the other star in the system makes intensity of the lights from the star dip periodically. The bellow video simulate how intensity of the star changes when there is a planet in transition between the star and the Earth.
Therefore, Radial velocity method, which is also the most popular method, is used along with Transit method. According to Newton’s law of gravitation, all bodies in a planetary system orbit around a common centre of mass, so not only the planet orbits but the star also orbit the centre of mass. Thus the lights emitted by the star have different wavelengths when they are detected on the Earth, the change in wavelength depends on the fact that the star is moving away or forward the Earth, this phenomena is called doppler effect.
Doppler effect
By measuring the wavelength of the lights emitted by the star at different times and taken the doppler effect into account, astrophysicists can determined the speed of the star when it’s moving away or toward the Earth and also the period and radius of the orbit of the star. Then the presence of other object in the system of that star can be confirm.
There is another method and also the most interesting method, it’s Gravitational Microlensing method. To explain about this method, I’m going to start with Einstein’s general relativity. Einstein’s general relativity states that spacetime curves in the region around a large, massive object. So when lights from a distant star pass through a another star (this is our interested star), they are bent which cause the image of the star that detected by telescope to be distorted. The star acts as an gravitational lens; and if the star has a planet orbiting around it, when the planet is in some particular region near the star, it gravitational field causes the spacetime curves more, and it acts as an additional lens, therefore, the light from the distant star will be distorted more. This happens until the planet moves away from its star.
Lensing effect
In conclusion, there isn’t any particular method that can be used to confirm the presence of an exoplanet. Combinations of different methods must be use to give a reliable result.
Ryan Tran.
