The Exoplanets come in different sizes, there are some planets that are smaller than our moon and there are some exoplanets that are bigger than our gas giant, Jupiter. Some exoplanets are hotter than our mercury, but at the same time, some exoplanets are colder and dense. Some exoplanets have more than one host star. There are even some starless rogue planets that are wandering in the lonely seas of space. So what are Exoplanets? How inspecting the exoplanets might help us find Alien life on other planets?
What are Exoplanets?
The planets outside our solar system are called Exoplanets. Thousands of exoplanets have been spotted and observed by Astronomers over many decades. NASA's Kepler Space Telescope had a vital role in finding the exoplanets. There are billions of star systems out there and even more, planets are there, but what we are looking for is a planet that can hold life on it and which is present in the habitable zone of its own star. The habitable zone is the ideal distance from its star so that the planet can hold water on its surface and it must also have an ideal temperature. The Jupiter sized planet orbiting the Epsilon Eridani star system is the nearest exoplanet that could hold life which is 10.5 light-years away.
Are Exoplanets Habitable?
They are already thousands of Exoplanets found in our galaxy and the probability of finding a habitable exoplanet is high. In fact, we have already found many exoplanets. For a planet to be habitable it has to be at an ideal distance from its star and has to have an ideal temperature. The temperature should not be so cold that the water becomes frozen or the temperature should not be so hot that the water will become vapour. The Research which was done by SETI predicted that there could be 300 million potentially habitable planets in our galaxy. The Keplar-452b, the Earth-sized planet is the closest habitable exoplanet which is at the ideal distance from its star.
How are Exoplanets discovered?
Discovering Exoplanets are really tough and astronomers are finding new ways to improve the spotting of Exoplanets. The brightness of the host star around which the exoplanet revolves makes the spotting of the planet difficult. Also, the distance between the exoplanets and the size of the exoplanet makes the detection of exoplanets complicated. Astronomers are using four major ways to detect the planets directly and indirectly:
The Transit method is an indirect way of detecting the presence of Exoplanets around a star system. It was the earliest technique used to spot the exoplanets. In 1999, the first exoplanet (HD209458b) was discovered by the Transit method. The continuous monitoring of the luminosity of the star is done and when the transit occurs when the planet passes in front of its star and the glow of the star is diminished. By this technique, the radius and the revolution time of the planet can be found. The Kepler Telescope has been vital in finding the exoplanet using this procedure as it had already spotted half a million stars in the Cygnus constellation and the 2600 exoplanets surrounding it. As this process does not involve observing the planet, the features of the planet cannot be discovered.
The Wobble method or the Radial Velocity method involves the principle of the Doppler effect. The stars are affected by the gravitational pull of the orbiting planets and it moves ever so slightly which is enough to be detected by our sensitive instruments. The movement of the star can be detected by the change of the colour spectrum of the star. The change of colour spectrum is caused by the Doppler effect. As the star moves towards the observer, the spectrum will be displaced towards the Blue wavelength and as the star moves away from the observer, the spectrum will be displaced towards the Red wavelength. With our highly sensitive space telescope, the change in a star's spectrum could be tracked. But it can only detect the exoplanets, it cannot even measure the size and weight of the planet.
This is one of the promising ways to observe the Exoplanets because as its name infers it directly images the exoplanets. The Transit and Wobble methods are indirect methods to observe the planets and hence the characteristics of the planets remain unknown. But there is no limit in observing via the Direct Imaging method. But it can only be used to observe the planets near our star system. It could also be used the discover Planet 9. Using this method, 100 exoplanets have been seen in the 82 planetary systems. It can help understand the atmosphere of the planet using the infrared rays emitted from the planet and so much useful information could be found. But observing the planet using Direct imaging is very rare using our current technology. Micro-lensing is the most difficult method to spot the Exoplanets and it is particular used to find low mass planets and the planets which revolve around dim stars. It uses the concept of the General Theory of Relativity and bending space-time by the gravitational field applied by the stars, and finds the smaller exoplanets. The Star's gravity makes space bend near it and when another star goes near it, it uses the star as a lens and shines brighter. So if there is an exoplanet around the star, it also acts like a lens and makes the star more luminous. So far 19 exoplanets have been discovered by this approach.
Conclusion
The observation of Exoplanet might lead us to the unlocking of the mystery of Alien life. It will help us answer the question to "Are we alone in the Universe?". It might help us understand the formation of our Earth and how it had evolved. Going forward discovering exoplanets is going to be crucial for finding the mystery of the Cosmos. 
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