Definition: Exoplanets are the planets that orbit around stars other than sun. The prefix ‘exo’ means something that is far out of our own solar system. Astronomer’s confirmed that more than 4,000exoplanets are orbiting around the distant stars. The 1st possible evidence of extra solar planets was noted in the year of 1917. As of Aug1,2020, there are 4301 confirmed exoplanets in 3176 systems with 703 systems having more than one planet.
Formation and evolution: A planet can be formed within a few to tens of millions of years after the formation of their hosting star.
A planet can only be observed in their present condition but if we observe them in different stages, their evolution is visible to us. We can observe the young photo-planetary disks where planets are still forming to planetary systems of some billions of years old. When the planets are formed in gaseous planetary disk, they accrete hydrogen/ helium gas and later these gases get cooled overtime and gradually some of the gases or the whole is lost in the space. It means that even the planets of our solar system can start off large radius if they are formed much earlier. Kepler 51b which is only twice of earth in mass and like Saturn in size which is almost 100 times in mass of earth.
Hosting stars: There is at least one planet per each hosting star and 1 in 5 of the sun like stars have an earth sized planet in the habitable zone. Most of the exoplanets orbit the host stars like Sun. Lower mass stars like Red Dwarfs/ of special category M, which are not as much as massive to be detected by radial velocity method. Besides, transit method is also used in detecting the exoplanets orbiting other stars than the Sun. Stars with higher metallicity(the abundance of materials of the exoplanets) are likely to have more exoplanets than that of lower metallicity. Basically exoplanets orbit one member of a binary star system but sometimes circumbinary planets are known that orbit both members of a binary star system.
General features:
1) Colour and brightness- In the year of 2013, colour and brightness of some of the exoplanets were predicted for the 1st time. HD 189733b is deep dark, GJ 504b is magenta and Kappa Andromedae are reddish in colour if seen up close. Helium planets are white or grey. The apparent brightness of these exoplanets depend on how far they remain from the hosting stars and how far the observer is and also on the intensity of light reflected from the planets they get from their hosting stars. So, a planet with low albedo appears to be brighter than that of the one with higher albedo far from the star. The darkest exoplanet is TrES-2b. For gas giants, at near infrared wavelengths for massive or young gas giants thermal emission is more than reflection. Some of the exoplanets appear to be very much dark as they reflect only 1% of the total light due to excessive abundance of hydrogen/ helium gases or some chemical reactions in the atmosphere of that planet.
2)Magnetic Field-- A magnetic filed of HD 209458 was predicted from the way hydrogen was evaporating from the planet in the year of 2014 which is the 1st detection of magnetic field of an exoplanet. This is almost as much strong as one tenth of Jupiter’s magnetic field. These magnetic fields are detectable by their auroral radio emissions that could determine the rotation rate of the interior of an exoplanet. Hot Jupiters have larger radius than expected as the stellar wind interact with the magnetosphere of that planet and forms a flow of electric current that goes through the planet and heats it up allowing to expand in radius. In Aug,2018, scientists have that gaseous deuterium transferred into a liquid metallic form. This is a powerful tool for the researchers to to better understand the giant gas planets, such as Jupiter, Saturn and related exoplanets. Such planets are thought to contain a lot of liquid metallic hydrogen responsible for its strong magnetic filed. Though previously declared that magnetic fields of exoplanets may come with increased flares and starpots on their host stars but this was false for the HD 189738 system. In 2019, the magnetic fields of four hot Jupiters were determined and ranged between 20-120 Gauss where it was only 4.3 Gauss for Jupiter itself.
3) Moon--In December,2013, a candidate exomoon of an exoplanet was announced. In October,2018, the evidence of an exomoon orbiting Kepler-1625b was declared.
4) Atmosphere-- The atmosphere of several exoplanets has been observed and the 1st time to be observed was HD 209458b in 2001. In May,2017, glints of light from earth, seen as twinkling from an arbitrary orbiting satellite, were found to be reflected lights from ice crystals in the atmosphere. This technology used to determine this, may be useful in studying the atmosphere of exoplanets and distant worlds.
Clear versus Cloudy atmosphere of two Exoplanets
5) Habitability-- As more numbers of exoplanets are discovered, the study of exoplanetology is being more deep and gradually the possibility of existence of life on exoplanets is also enhancing......life beyond the solar system. At cosmic distances, life can only be detected if it is formed in a planetary scale and modified the planetary environment. As an example, oxygen in the atmosphere of earth is formed due to photosynthesis and other some of the biological and chemical processes. This is an indication of of life on exoplanets. Also a small amount of oxygen may be formed by some non-biological processes. For the existence of life on an exoplanet, it has to orbit the hosting star maintaining a proper distance so that all the planetary-mass objects with substantial atmospheric pressure can support the liquid water on the surface of the exoplanets.
Interesting...
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