Methods to find an Exoplanet

 

Extrasolar planets or simply exoplanets are planets found outside the solar system. These are very interesting bodies as they have the same routine as our home planet ie orbiting the centre star. However, there are many exoplanets that do not orbit any star. They are known as Rogue planets. There are possibly more than a billion rogue planets in the Milky Way. Exoplanets were first predicted in 1917 but were not discovered till 1992. Many exoplanets were discovered to have the same composition as the Earth. Hence there might be possible of life there (here life may include small microbes and not giant aliens as seen in fiction). There are about 11 billion habitable Earth-like exoplanets in the Milky Way. 

Till now 4,719 exoplanets are discovered, out of the 772 were part of a system (like the Solar System) having at least three planets. If we want to find extraterrestrial life then exoplanets are the way to find them. If there are 11 billion habitable exoplanets in our home galaxy then we must say that "We are not alone". Exoplanets are very far from the Earth, the nearest is 4.2 light-years away orbiting the Proxima Centauri. Most of the exoplanets discovered are within the Milky Way. Finding an exoplanet is not an easy job. Planets are not the source of light, they just reflect the light. This makes them even more difficult to find. One cannot directly find an exoplanet, just like nebulae or galaxies. Astronomers have developed 5 major indirect methods to find an exoplanet.

(1) Gravitational Microlensing:

In 1992, Abraham Leeb and Andy Gould proposed that gravitational microlensing might be one of the ways to find an exoplanet. Gravitational lensing was the first method capable of finding an exoplanet. Earlier gravitational microlensing was used only to find the binary system of stars. Lensing happens only when two stars are almost perfectly aligned for the observer. When two stars are aligned, the light of the background star is intensified or magnified by the foreground star. 

If the foreground star has a planet orbiting it, then the planet's own gravitational field affects the lensing. Hundreds are stars are continuously monitored to detect lensing. Gravitational lensing lasts for days or weeks, hence this method is widely used to detect exoplanets. This method is more suitable to find planets lying between the galactic centre and the Earth because the galactic centre provides a large number of background stars for lensing. 

Gravitational Microlensing is a one-shot mission. This means two stars can be aligned for only one time. It might take millions of years for the same stars to align again or they might never align again in the future. Also, this method can only detect the presence of the planet and its mass. Its position in orbit, distance from that star etc cannot be detected from this method. 

The reason why it is a widely used method is that low mass planets can be easily detected through lensing. Also, planets very far from the star or at perihelion (farthest distance from the star in its orbit) can also be detected. Many exoplanets discovered by this method has an orbit larger than Neptune or Pluto. 

Many robotic telescopes are developed at different places around the globe by different space organisations only to detect the lensing. These telescopes are continuously in search of lensing.

(2) Radial Velocity:

Until 2012, radial velocity was the most accurate method to find an exoplanet. When an exoplanet moves in its orbit around the centre star, its move far or away from the Earth, this leads to variations in its radial velocity with respect to Earth. This method is also known as Doppler spectroscopy. In this method, the centre star's speed around the system's centre of mass is measured. However, in the planet-star system, the movement of the star is very negligible. For example in the Earth-Sun system, the sun moves the only 9cm/s due to the gravity of Earth. However, with high accuracy spectrometers, the radial velocity of such minute movements of a star can also be detected. 

For Earth-sized planets, this method is applicable for only several light-years (150 to 160). But for Jovian sized planets, this method is applicable for several thousand light-years. With the use of modern spectrometers, the accuracy of this method has also increased. 

This method is suited best to find exoplanets with low sized stars. Because small-sized stars show more relative movement in the planet star system. Also, smaller size tends to slower rotation. Slow rotations generate high accuracy spectral lines. Hence small-sized stars make the hunt for planets easier.

The problem arises when there is a multi-planet system. If there is more than one planet in the planet star system then Doppler spectrography may produce false and inaccurate signals. However, in a multi-planet system, the planet closest to the star and with a smaller orbit produces more accurate signals.

(3) Direct imaging:

As the planet is not the source of light, it is very hard (sometimes impossible) to directly capture the image of the planet. Also, the light from the parent star makes it even more difficult. But the infrared thermal imaging makes it easier to detect a planet. Infrared thermal images can easily capture the reflected light from the planet. 

A special instrument called 'Coronagraph' is attached to the telescope to block the light from the star. Mass of the planet can be detected by this method. Cooler thermal images mean the low mass planet. Sometimes, the planet's radius can also be predicted from the brightness. If the planet is closer to the star and the size is larger (Jovian sized) then it is easier to detect the planet through thermal imaging.

(4) Astrometry:

Astrometry is the oldest method used to search exoplanets. In this method, the movement of the star is constantly observed. If the star has a planet orbiting it, then the star itself will move in a small orbit due to the mutual gravity of the star-planet system. It was William Herschel who first observed that a star named 70 Ophiuchi was moving in a small orbit due to the influence of another body. However, the movement of 70 Ophiuchi was due to the binary star system and not the exoplanet. But similar calculations are used to determine the existence of exoplanet.

One of the main advantages of this method is used for both, larger and smaller orbit exoplanets. However, this method takes a lot of time to completely show the existence of an exoplanet. Because the planet takes a very long time to orbit and the star itself moves very slowly in its orbit, astrometry requires a long consistent observation, sometimes lasting for decades to verify the existence of the planet. 

The combination of radial velocity and astrometry is used to determine the various properties of the planet, like the size of the planet, its orbit, its mass etc. 

(5) Transit Photometric:

Photometric is the very obvious method to search the exoplanet. This method includes observing and collecting the radiations from the star. When any planet while orbiting comes between the spectroscope and star, the intensity of radiations coming from the star decreases. The change in intensity depends on the radius and shape of the planet. This method is also used to determine the radius of an exoplanet. 

But this method has two major disadvantages. First, to observe the transit the planet and star must be perfectly aligned. There is little probability to observe the star in such a configuration. Second, there a high probability of false reading. When the transit happens, without proper observations we cannot determine whether the radiations are blocked by an exoplanet or by any other object. Hence to precisely measure the transit, the radial velocity method or orbital brightness method is used.

If the size of the planet is very small compared to the parent star then there will a very negligible transit. 

There are still many methods to determine the exoplanets. But these 5 are used most widely by astronomers. The accuracy of these methods is very high compared to other methods.