Analysis Of The Techniques Of Exoplanet Detection

Introduction

Until 1990, we were unaware of the existence of planets outside our solar system. But in thelast two decades thousands of exoplanets have been detected. The first two exoplanets weredetected in 1992 but to our surprise they were orbiting a pulsar instead of a sun-like star. Few years later, in 1995 swiss astronomers Mayor and Queloz detected a a planet around asun-like star. Surprisingly, the mass of this planet was estimated to be that of a Jupiterand it was orbiting it’s host every 4. 3 days which means that it’s more nearer to its starthan mercury is to sun. It was quite unusual at the time because the only knowledge thatwe had was that of our own solar system. This exoplanet system was intriguingly oppositeto what we expected based on our own solar system. So its pretty evident that formationof exoplanetary systems are more chaotic as compared to our own orderly solar nebulaemodel. Migration of gas giants towards their hosts and huge abundance of small rocky planetsin inner orbits. These variation are quite bewildering and baffling at the same time.

Till now, several techniques have been developed to detect exoplanets. Like, Doppler spec-troscopy (radial velcoity method), direct imaging, astrometry and transit observation are someof the frequently used ones. As expected, each one of these mentioned techniques has it’spros and cons. For instance, Radial velocity and transit methods are preferred when detect-ing planets that orbits very close to their hosts stars and are larger in size or mass whilephotometry and direct imaging works more aptly for planets in wider orbits. As of September2018, we have 3851 confirmed planets in 2871 systems with 636 systems having more thanone planet. Let’s talk a bit more about these techniques.

Astrometry

Its an indirect technique, used to detect planet, which requires strict measurements of vari-ations in the position of the star with respect to its nearby stars. RV method observes theregular jiggle in star’s position while astrometry observes the periodic alteration of positionof the star in sky. the gravitational pull of planet over its host is very low but strong enoughto pull its star from its position. With the advancement of telescopes this variations canbe observed very precisely. If the star sways with periodic rythm than it’s obvious that anunseen companion probably a planet is inducing these effects.

Transit Photometry

Transit photometry has proved itself to be a very successful method till date. It occurs whena planet passes between star and earth. So basically its looks for a tiny drop in brightnessof the star when a planet passes in front of the star. If this feature is periodic than we cansay that star has a companion perhaps a planet. But for this method to work, a transit mustoccur that means a planet ahs to pass between it’s star and earth. but sadly for most ofextra-solar planets it never happens because of their orientation relative to ours.

Direct Imaging

Direct imaging, the name says it all, is way to find the planet directly. But it isn’t as easy asit seems and probably the most difficult technique to search planets. in most cases, next toimpossible. It employs observation in infrared wavelengths because in visible range it’s likelooking for a firefly near a spotlight from miles away. In infrared, even sun is only 100 timemore luminous than jupiter. So we cover up the star and most of its light via coronagraph. Remeber this method works only for planets that very far from their hots, so if the planetis sufficiently far fro it’s host than keen eyed telescopes would able to detect it. Significantnumber of extrasolar planets have been discovered with this technique.

Radial Velocity Method

The Radial Velocity Method or Doppler spectroscopy is a frequently used way to find planetsfar away from solar system. This technique brings into consideration the fact that a star neverstays completely stationary if its accompanied by companions. Any planet despite being tinycompared to its parent star exerts a gravitation tug over it which is almost negligible yetrobust enough to topple the star from its place. This tug pulls the star towards or awayfrom the observer and hence making the star to wobble. The high resolution spectrographmounted over the telescope strictly observes this disturbance by examining the star’s light.