When we think of living outside earth, we mostly dream of living in a spaceship, or another planet. But nobody thinks of flying all day and all night wearing the same spacesuits. So the first thing that comes to our mind when we plan to leave our home and travel somewhere is resources. We ensure that the place of visit has got enough water to drink, food to eat and most importantly air to breath. This is the exact thing we are doing by searching for exoplanets (planets apart from those in solar system). But there are reasons which make this field of research so interesting. We humans are making sure that we are not alone in this boisterous universe. this area is being explored using another technique where we try to receive alien signals in the form of electromagnetic waves (using SETI-Search for ExtraTerrestrial Intelligence) and also send signal to be detected by other species out there (passively by the daily use of the signals in the form radio, television etc. which travel to space). But what seems more logical from the above reasons is the formation of everything. By observing how an earth or Pluto like planets form. The constant monitoring of newly found exoplanets also proves or disproves the existing theories and also creates new theories about how the universe governs.
Kepler-7 b exoplanet is located about 3009.4 light years away from our solar system, then how do the scientists find such details of the planet using telescopes? Well it turns out that there are other methods used to detect the presence of exoplanets. Those are-
1. Radial Velocity
Light can be both particle and wave. But to experience a term called "Doppler Effect", we consider the wave character of light. We all must have here the sound of ambulance changing as it crosses us. as it comes towards us, we hear a sound of high frequency and as it goes away from us, we hear a sound of low frequency. This occurs due to Doppler Effect. While coming towards us, the sound waves from the ambulance squeeze and hence the wavelength becomes narrower. So we hear a high frequency sound. Similarly the wavelength increase as the sound waves stretch out as the ambulance goes away from us. in the case of light, when the object emitting light comes towards us, we observe a "blue shift" that is we see the light coming from that object to be of higher frequency (for comparison violet has the highest frequency in visible spectrum while red has the lowest frequency) and when that object recedes we observe a "red shift".
Every planet orbits its parent star because of gravitational force. But being lighter, the planet undergoes rotation and not the star. But the planet also exerts enough force to displace the star from its position. More the weight of the planet more is the wobbling of the star from the center. This wobble makes the star come towards the earth for some time and then go away from us, ultimately changing the frequency frequently. By observing this Doppler Effect we come to the conclusion that the star we are observing has planet or planets depending on the wobble. 801 planets have been discovered till now using this method (https://exoplanets.nasa.gov/alien-worlds/ways-to-find-a-planet/#/1).
2. Transit method
The phenomenon of solar eclipse is observed when moon comes in between the earth and sun. The sun's brightness slowly decreases until it becomes minimum and then increases as the moon passes it. The same idea is used in transit method where the star being observed loses its brightness for some time and then regains it. This drop in intensity tells us that the star has a planet. In case of more than one planet, the drop in intensity is complex and involves calculations to find the number of planets of that star. The additional advantage of this method is that we can know about the atmospheric structure of the planets (that is whether the planet has oxygen, hydrogen, water vapor etc. or not). This happens as part of the star's light passes through the atmosphere of its planet before it reaches us. Due to absorption by the gases, we obtain the "absorption spectra" which is unique for each gas. Being the most successful method among all other methods, a total of 3161 planets have been discovered using it (https://exoplanets.nasa.gov/alien-worlds/ways-to-find-a-planet/#/2).
3. Direct Imaging
Looking at the sun directly doesn’t allow so to see the objects near sun. Similarly, the exoplanets when observed directly using a telescope are not visible due to the brightness of their host star. Hence we use devices like coronographs which block the light coming from star and allow us to observe objects near the star. 49 exoplanets have been observed using this method (https://exoplanets.nasa.gov/alien-worlds/ways-to-find-a-planet/#/3).
4. Gravitational Microlensing
Like every object with mass, gravity also influences light and causes it to bend around massive objects like stars, galaxies etc. the light from a distant star is bend by other massive object coming in between that star and earth and we see the star getting brighter for a temporary period just like the magnifying glass which can focus the beams from the sun onto a paper to make it look brighter. Hence observing this temporary increase in brightness can lead to the conclusion that the massive object is a star or a planet or both depending on the period of the increase. 89 exoplanets have been observed using this method (https://exoplanets.nasa.gov/alien-worlds/ways-to-find-a-planet/#/4).
5. Astrometry
The wobbling of the star not only causes Doppler shift but also changes the position of the star in the night sky. By comparing series of pictures of the same part of the sky, one can compare the shift in position of the star being observed with respect to reference stars. This displacement is used to calculate the presence of planets of the star and their properties like mass, distance etc. The difficulty of using this method can be very well known by the fact that only 1 planet has been discovered using this method (https://exoplanets.nasa.gov/alien-worlds/ways-to-find-a-planet/#/5).
The other methods include Reflection/Emission Modulations, Relativistic beaming, Ellipsoidal variations, Pulsar timing, Variable star timing, Transit timing, Eclipsing binary minima timing, Polarimetry, Flare and variability echo detection, Magnetospheric radio emissions, Auroral radio emissions, Optical interferometry, Modified interferometry etc.
Now the question comes how we know that these planets are earth like (habitable). The term goldilocks zone does the work. We know that liquid water and right temperature range is very much essential for earthly life to exist. Hence this zone is a small space around a star where the presence of planets assures the possibility of finding suitable environment for life forms to exist. This zone need not be at the same distance as we are from our sun. This is because the heat of the sun depends on the type of star like it is smaller for red stars and larger for blue stars. For example the Kepler-22 b orbits its star Kepler-22 in the habitable zone. Hence it is expected to have oceans in it just like on earth.
The discovery of new planets is continuing with lots of possibilities being added every time with the addition of new inventions. But it would be interesting to ponder over the fact that aliens might not be as similar to us as we think. There might be some aliens on the planet right next to us but we may not have the right equipment to detect them. Keep on looking for them!
Barnali Das, IISER, Kolkata
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