– Ujjal Lamichhane
M.Sc. Physics final year
Human beings have a curious mind by birth, hunting every single thing that strikes their mind. It has been a long time when mankind begun to think about its own existence. After the successful Big Bang Theory, the curiosity of the evolution of the Earth as well as the whole Universe has tickled much more brains. Humans have regularly searched the Earth like planets beyond the solar system and the possibility of life in it. The quest of Exoplanets is constantly striking the human mind.
An exoplanet or extra solar planet is a planet that orbits a star other than the Sun. For centuries, philosophers and scientists supposed that extra solar planets existed but there was no way of detecting them. Neither was their frequencies estimated nor were their similarities with the planets of the solar system imagined. In the sixteenth century, the Italian philosopher Giordano Bruno put forward that the fixed stars are similar to the Sun and likewise accompanied by planets. In the eighteenth century, Sir Isaac Newton mentioned the same possibility. But, various detection claims were rejected by the astronomers in nineteenth century. The first confirmed detection came in 1992 with the discovery of several terrestrial mass planets orbiting the Pulsar star. The first confirmation of exoplanet orbiting the main sequence star was made in 1995.
As of 25th August 2015, 1949 planets have been discovered in 1233 planetary systems. There are also rouge planets which do not orbit any star and tends to be considered separately. About 1 in 5 Sun like stars have an Earth sized planet in the habitable zone (where it is possible for liquid water to exist on the surface) with the nearest expected to be 12 light years distance from the Sun. Assuming 200 billion stars in the Milkyway, there are 11 billion potential habitable Earth sized planets in the Milkyway Galaxy alone. The EOD (Exoplanet orbit Database)
table suggested 1569 EOD planets till date and 24 other planets including microlensing and imaged planets thus resulting 1593 confirmed planets. With 3793 unconfirmed Keepler candidates there are total 5344 exoplanets observed till date.
The discovery of exoplanets has intensified interest in the search of extra-terrestrial life. There is special interest in the search of planets that orbits in habitable zone. The study of planetary habitability also considers a wide range of other factors in determining the suitability of planet for hosting life.
Habitable zone and exoplanets in habitable zone:
Habitable zone is the distance from a star where liquid water can exist on the surface of the planet. If a planet is too far from the star, water freezes. Similarly, if the planet is too close to its star, water evaporates. A planet must stay in the habitable zone throughout its orbit in order for water to remain liquid. The distance at which a planet can have water is determined by how much energy is given off by the stars. The distance for habitable zone is measured in astronomical unit (AU), i.e. the average distance between the Sun and the Earth (93 million miles). Our Sun’s habitable zone for larger life forms including the humans is between 0.9 AU to 1.5 AU. However, this distance varies somehow for other stars. For cooler red dwarfs, the habitable zone is so close to the star whereas for very hot blue stars, the habitable zone is further away. But the major thing here to understand is that even if a planet is in the habitable zone throughout its entire orbit, human survival is not guaranteed. It only signifies the possibility of life in such planet.
On November 2013, based on Keepler Space Mission, Astronomers reported 40 billion Earth sized planets orbiting in the habitable zones of Sun-like stars and red dwarf in the Milkyway galaxy alone of which 11 billion may be orbiting Sun-like stars. However, only 10 Earth sized and 21 Super Earth sized (5 to 10 times the Earth sized) potentially habitable exoplanets have been detected in these two decades of time since the detection of first exoplanet.
Is life really possible in exoplanets? Do human beings exist in the other part of universe? This is the greatest quest of astronomers these days. Of all these detected exoplanets in habitable zone, scientists today believe life is possible in 21 exoplanets detected up to now. May be the bigger form of life including humans or human-like creatures do not exist but the possibility of microbial lives cannot be rejected as well. However, besides the rumors spread time and again, not a single clue for any sort of life in any exoplanet has been discovered. Neither any supportive proof to confirm the existence of living organism has been observed. Still the search
for the twins of the Earth and the lives in it continues and many more scientists are still working for it days and nights.
There is numerous Earth like planets in the universe and there is also the habitable zone thus the possibility of life beyond the Earth can be imagined and there can be humans or human like creatures as well but no any solid evidence has been found till date.