Physics behind finding Exoplanets

Ever since humanity has looked up into the night sky, we have wondered , are we alone in this universe? Or is there life beyond Earth? This question has bothered us since ages.

It’s so glad to know that it is the first time in human history, that now we have the technological means to find the answer of this question.

Out of many approaches one is to find another Earth like planet; a planet with just the right temperature, density, atmosphere and other conditions which makes planet potentially habitable.

As eight planets of our solar system orbits the sun similarly alien planets orbit other stars. So planets that orbit stars other than our sun are called Exoplanets (Extra-solar planets).

But at first place, how do we detect new planets outside our solar system

They are so far and out shined by the star they orbit. Thus, direct imaging is so difficult. So what we do is use indirect methods. We measure the effects planet makes on its host star. That is where the beautiful physics comes in.

Out of many, two most popular and productive methods are

  1. Wobble method ( also called Radial velocity method)
  2. Transit method


The star wobbles when it has planet around it. So to know if star has a planet, check if it’s wobbling.

Time to get into physics.

Grab a stick or scale and try to balance it using a finger. The point above your finger at which scale gets balanced is a point from which both sides have equal mass. Such a balancing point is called the center of mass. Every point in the system moves around the center of mass .

Let’s have some fun!

Imagine you throw up a bottle whose center of mass is somewhere in the middle.

If you notice carefully, you will see that both base and top of the bottle moves around the center of mass. Thus both ends follow the same orbit.

Now comes the fun part. Suppose you attach a rock at the base of the bottle, so the center of mass shifts downwards towards the heavier part (base).

Again, throw it up. Physics still stays the same. When both ends move around the center of mass, heavier part (base) moves less than the lighter part (top). In other words, heavier part makes smaller orbit while lighter part makes larger orbit around center of mass .

This is exactly how stars and planets work

Compare the heavier part with the star and light one with the planet.

Planet stays in orbit due to star’s gravity.  Planet also have graviational pull on star ( extremely less but yes it does). So planet tugs on star due to its own mass. 

If there were no planets, center of mass would be at the center of star but when star hosts planet center of mass gets little shifted towards the planet/s (although center of mass still remains inside the star)

In orbital mechanics, this balancing point is called Barycentre.

Both star and planet orbits the barycentre. Star’s orbit is very small and that is what we call the star is wobbling.

Fun fact : our sun is wobbling at 9 m/s due to Jupiter ( largest planet of our solar system).

Stars are so far, How can we detect this little wobble?

That is where doppler effect comes to our help. Here’s how.

Have you noticed the sound of ambulance passing by? You must have. First you hear a high pitch sound and when it passes by you hear low pitch sound. That is called Doppler effect.

This effect can help us detect planets outside our solar system. Wondering how?

This is how Doppler effect explains your experience –

Sound is a wave. Whenever a wave producing source is moving (for e.g ambulance) , waves at the front gets bunched up (and you hear high pitch sound) , and waves loosens at the back (that’s why you hear low pitch sound).

Like sound, same thing happens with light emitted by stars.

When star has planet/s around it, it wobbles or jiggles i.e move in small orbits around the center of mass. While making those orbits, for half circle it will be moving away from Earth and for another half circle it will be moving towards Earth.

That is where the Doppler effect comes in. We know whenever wave producing source is moving ( star) waves the front gets bunched up (and we see high frequency light) and waves at the back gets loosened (and we see low frequency light) . Our telescopes can measure that change in wavelength.

So whenever there is periodic shift in wavelength of starlight, it means star is wobbling. And woohoo! Star is wobbling so it has planet/s around it.

This method is called wobble method or radial velocity method and is the 2nd most productive method after transit method. We have found more than 700 exoplanets by wobble method (as of 2018).

It can tell us more..

Along with detection, this method can give us more information.

We can measure how long does star take to orbit once, which is same as the time taken by the planet to complete a orbit. And by Kepler’s third law -T3  ∝  R (where T is orbital period of planet and R is its distance from center of mass), we can know how far the planet is from the center and thus how hot it is. 

Also more the velocity of star, heavier the planet is.

So temperature and mass of planet can roughly be estimated.  


Another productive method is transit method.

What is transit?

The passage of any planet between star and earth is called a transit.

So during that planet blocks out some of the star’s light reaching earth; and telescopes can measure that decrease in brightness of star. Dip in light curve is obtained. If that forms a regular pattern, we can claim that’s an exoplanet. Bigger the dip, bigger the planet is.

We have found more than 3000 planets by transit method.

What’s there in Atmosphere?

Also one more important thing is happening. Some of the star light passes through the

planet’s atmosphere and reaches our telescopes. That light is very precious as it contains

information about elements/molecules present there.

How can we decode that information? Here’s how

Identity of every element is it’s spectrum.

Any light is a combination of many light waves. When light is shined on an element , it

absorbs specific part of that light, forming gaps which appears as black lines in the


Imagine elements as little cartoons and this images below describes the whole story.

Different element absorbs different parts of light. So when the starlight is broken down into its components, we can know which part is missing and gain knowledge about elements present in the planet’s atmosphere. We can be aware if planet’s atmosphere has oxygen, water vapours or what else?

Thus both the methods can give us estimate about temperature, mass, size and atmosphere composition of the planet; which are primary parameters of planet to support life. From size and mass we can deduce density of the planet which tells us if the planet is rocky or gaseous? 

Till now we have almost 4000 exoplanets; some of them are even potentially habitable. Many more space missions are coming up for further hunt of exoplanets. We are so lucky to live in this era of humanity, where such awesome things are happening.

Stay Curious!


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