Important methods of detecting exoplanets

April 02, 2020

Exoplanets (Extrasolar Planets) are heavily bodies that orbit a star, just like our Earth orbiting a star (Sun). The nearest exoplanet from our solar system is Proxima Centauri b by a distance about 4.24 light-years which orbiting a star called Proxima Centauri. 

 Till March 2020, we have 4,141 Exoplanets and there are more than 40 exoplanets discovered in the Habitable Zone ( An orbital region where liquid water will be found and life will exist ).

 Important methods of detecting exoplanets -

 1. Radial-velocity or wobble method :
                                                                                                                   
The sun is not fixed at a position and spining on its axis and every planet revolves around. But the Sun and planets are moves around a common point known as Barycenter. The common point is the center of mass of sun and a planet. Since sun has a higher mass than the planets and so, the center of mass always shifted towards sun. 

Every pair of sun and a planet has a different center of mass which makes planets motion around the sun so complex. Sun moves on an orbital path, so the star wobbling takes place.
  
For small planets, the center of mass may lie inside the sun. And, this phenomenon is used to find the planets of another solar system.

 Now let see how astronomers find exoplanet through method with an example -

 Suppose, you stand on a bus stand and a police car passes then you will notice sounds get high pitched when it comes towards you and low pitched when it goes far from you. This is called Doppler's effect. Similarly, Astronomers observe the star's light spectrum ( Visible spectrum ) and if it gets slightly shifted toward red and blue, then, they get the presence of an exoplanet through this method. When a star comes toward the observer it shifts to blue and it shifts to red when it is going away.
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An artist shows Doppler shift due to wobbling star
Credit - NASA
 Wobble method or radial velocity method is only applicable for big exoplanets because in small exoplanets the shift of the center of mass towards a star is very close to its center (star). So, Astronomers can't detect the spectrum and can't guess any exoplanet. 
If we found any exoplanet through this method then Astronomers can also tell about their mass. 
  

 2. Transit photometry method :


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Transit photometey method
Credit - NASA
   
This method is the most effective way to find an exoplanet. When an exoplanet comes between earth and star(not our sun) then a dips in the brightness of that star can be observe. Astronomers use Photometry ( a curve b/w time and brightness of a star, see fig ) to study dips in brightness.

 This method detects the presence of exoplanets and his light reduction gave us a diameter of an exoplanet.

 ( By sphere volume = 4/3Ï€r^3 and mass from the radial method )

 Density = m/v Calculated

 If the density is higher then we conclude that it is rocky and when less density than a gas giant.

3. Direct Imaging :
                                                                                                       
Direct imaging exoplanets, As his name suggests,involves taking the pictures of the exoplanet but it's not easy due to the large distance. We know exoplanet reflect and absorb light(consisting various wavelength) of a star when we compare star and exoplanet reflected visual wavelength (wavelength visible to humans eye) then the star is billion times brighter than exoplanet but in case of infrared wavelength it is just million. So with this , we can easily capture a photo of an exoplanet near to our solar system.
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It is first direct image of Exoplanet taken by a group of astronomers led by Gael Chauvin in julu 2004
Credit- NaCo, VLT, ESO

 Astronomers use spectrum and brightness to calculate the surface's temperature of an exoplanet.

 4. Microlensing :

 We know that light travels in a straight line but Einstein told us that sometimes it bends when it passes near a higher mass (like a star). Suppose, a big star is situated in a fixed position and when another star with an exoplanet passes between that big star and our earth, then (in the figure) light reaches denser on earth as compared to that light when they are not aligned in a straight line. Of course, this is because of the bending of light. This bending of light occurs due to the gravity of the star(red) and exoplanet. When it moves away the star appears as before from our Earth. This phenomenon occurs for a short interval.
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Astronomers draw a curve between the light of big stars and time. If they detected a small peak over a large peak, then they would got the presence of an exoplanet.

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This method of finding exoplanets is known as Microlensing.
 Few methods still exist, but these Important methods of detecting exoplanets are widely used.

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