HOW ARE PLANETS OUTSIDE THE SOLAR SYSTEM (EXOPLANETS) DETECTED AND STUDIED?

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Life beyond Earth has always been an interesting subject for every astronomy lover. Just like Earth, the Solar System has 7 other planets (and most probably one more planet!). There are about 100 billion stars in the Milky Way galaxy. The formation of stars is an extremely chaotic process but after the star is formed, there is something interesting that happens. The dust left over after the star formation (nebula) eventually leads to the formation of planets by a process called accretion. Thus, there is a strong possibility that almost every star has a planet system around it. However, it is not easy to observe the planets outside the solar system. This is because, the only way the planets can emit light is by reflecting the light they receive from their parent star (which is very low) and also because they are very small in size as compared to the stars.

Thus, stars even millions of light years away can be observed using a powerful telescope such as the Hubble Space Telescope but planets even in the closest star system Alpha Centauri star system cannot be easily observed. (The planet Proxima b in the Alpha Centauri star system was detected by measurements. NASA recently held a news conference in which it made announcements about the TRAPPIST-1 solar system that has 7 planets much similar in size to Earth orbiting a red dwarf star (much cooler than our Sun) about 40 light years away from Earth!


Credit: NASA/JPL


It also pointed out some interesting features about these planets apart from their sizes such as;

1. Their time period of revolution around their parent star.
2. The fact that these planets are rocky (most of them).
3. Their distance from their parent star, etc.

How can NASA assure all this without even observing these planets directly that are at a distance of 378400000000000 km from us?
Well, this is the power of Science!

What NASA or any other team of astronomers does is that it observes the parent star using a powerful telescope. The star is easily visible (as discussed before) and its intensity can be continuously monitored. As the planet orbiting the star moves in front of the star, there is a dip in the intensity of light received on Earth. The planet will arrive in front of the star at the same position as before after one complete revolution around the star thus causing same amount of dip that it produced earlier after the time equal to its time period of revolution.

This dip in intensity will obviously depend upon the size of the planet. Larger the planet, more will be the decrease in the intensity of light received by us. Thus by observing the dip in intensity and its frequency of occurrence, we can get to know about the size and time period of revolution of the planet as shown in image below:
Credit: NASA/JPL
As we can see, the planet marked b has same magnitude of dip produced after 1.5 days. Thus by measuring the magnitude of decrease in star intensity, the size of planet can easily be determined and as can be seen, the time period for planet b is 1.5 days.
Now, by knowing the time period and mass of parent star, we can calculate the distance of planet from the star using Kepler's Law:
Where T = time period around star (known)
G= Universal Gravitational constant (known)
M= Mass of parent star (known- can be found by many methods)
r- Distance between planet and star.

Thus distance can easily be calculated. Knowing the distance of the planet from the star and intensity of star, we can predict if the planet lies in the habitable zone or NOT.

Now, having calculated size of planet and its time period and distance, we need to measure mass of the planets.

The mass of the planets can be determined in many ways.

If the planets are relatively closer to one another in the star system, there will be perturbations in their orbits due to gravitational interaction with each other. This perturbation can provide an indirect estimate for their masses. Besides, as we know that both the planets and stars revolve around the barycenter (center of mass) of each other, the movement of the star AND the wobble produced in the star by the planet can be measured to give an estimate about the mass of the planet. The wobbling of star is shown in the animation below:
Source: Wikipedia
Having known the mass of planet and size of planet, the density of planet can be found out,

Density = Mass/ Volume (size)
Knowing the density, we can estimate the nature of the planet.

1. If the density of planet is closer to planets like Mercury, Venus, Earth or Mars, it is a rocky planet.

2. If its density is closer to that of Saturn, Jupiter, Uranus or Neptune, it is a gaseous planet.

3. If it has density relatively closer to 1 g/cc (density of water), the planet may be entirely made of water (with a solid core).

Such interesting estimates can be made!

These are some of the most common ways of detecting and studying the exoplanets. Other methods such as Gravitational Micro-lensing (check gravitational lens here) direct imaging (for planets larger than Jupiter that are closer to Solar system, we can directly capture them in Infrared wavelengths), Polarimetry etc can also be used.

So now you realize the amount of hard-work put by researchers and astronomers to provide us with so many interesting facts about the distant exoplanets!!!

Hope you had a good time reading this article. If you love astronomy, read other articles in our blog too! Till then Happy stargazing. :)
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