# **EXOPLANET DETECTION TECHNIQUES**

## INTRODUCTION:

### What are *exoplanets* ?

An exoplanet is any planet beyond our solar system. Most of them orbit other stars, but some free-floating exoplanets, called rogue planets, are untethered to any star.

### Why do we need to detect them?

We search for exoplanets in order to understand planet formation and evolution, search for life beyond Earth, compare planetary systems, find clues about Earth’s future, expand human horizons, and drive technological and scientific advancement.

### Why are they difficult to detect?

Exoplanets are difficult to detect because they are too faint and close to their much brighter host stars, which effectively drown out their light. This makes direct imaging nearly impossible, so astronomers mostly rely on indirect methods.

## MAIN DETECTION TECHNIQUES:

### 1) RADIAL VELOCITY

Consider the gravitational interaction between a star and a planet. The star is a massive object with a really powerful gravitational field, whereas, on the other hand, the planet is much smaller, with a whole lot less gravity. That's why planets orbit stars and not the other way around.But even though the planet is small, it still has some gravitational force. It still has an effect on its host star, even if that effect is much less pronounced than the one the star has on the planet.

Look at the below animation.


At first glance, things look normal. There's a big star and a small planet, and the small planet orbits the big star. You've probably seen this many times.
But check out the star. See how it's moving a little bit, too? The effect is exaggerated for this animation, but that's what actually happens in space. The planet's gravity causes the star to 'wobble'around a little bit.
As you might imagine, the bigger the planet, the bigger the effect it has on its star. Small planets, like Earth, make their stars only wobble a tiny bit. Bigger planets, like Jupiter, have a much stronger effect.

Now, what has this to do with exoplanet detection? Wholaa! Here's our hint: **A star's 'wobble' can tell us if a star has planets, how many there are, and how big they are.**

Wobbling stars are great for finding exoplanets, but how do we see the wobbling stars?
The method used is one called 'Doppler shift'. It's named after the physicist who figured it out about 150 years ago.
Energy - sound, radio waves, heat, and light - moves in waves. Like the waves you see in the animation below.


Those waves can be stretched and squeezed, based on the movement of the object that's producing them. when an object that emits energy (like an ambulance speaker or a massive, burning star) moves closer to you, the waves bunch up and squish together. And when the object is moving away, the waves stretch out.
Those changes in the wavelength change how we perceive the energy that we're seeing or listening to. As sound waves scrunch together, they sound higher in pitch. And when visible light waves scrunch together, they look more blue in color.
When sound waves stretch out, they sound lower in pitch. And when visible light waves stretch out, they make an object look more reddish.
This change in color is called 'redshift', and scientists can use it to see if an object in the sky is moving towards us or farther away.
You can see this method working in the animation above. The planet causes the star to wobble around in its orbit, and as the planet moves to and fro, the light waves compress together and then stretch out, changing the color of the light we see.

The radial velocity method was one of the first successful ways to find exoplanets, and continues to be one of the most productive methods. Often, this method will be used to confirm planets found with other methods - an extra step that can prove a planet exists. 1131 planets have been discovered using this method till date.
Lots of astronomers and telescopes around the world use this method to discover exoplanets, but two notable observatories where this work happens are the Keck Telescopes in Hawaii and the La Silla Observatory in Chile.

Keck Telescopes



La Silla Observatory



### 2) TRANSIT METHOD

When a planet passes directly between an observer and the star it orbits, it blocks some of that star's light. For a brief period of time, that star actually gets dimmer (somewhat similar to solar eclipse). It's a tiny change, but it's enough to clue astronomers into the presence of an exoplanet around a distant star.

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The graph you see being drawn on the left side of the animation is what astronomers call a 'light curve'. It's a chart of the level of light being observed from the star. When a planet passes in front of the star and blocks some of its light, the light curve indicates this drop in brightness.


The size and length of a transit can tell us a lot about the planet that's causing the transit. Bigger planets block more light, so they create deeper light curves. You can see that in the animation below.

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Also, the farther away a planet is, the longer it takes to orbit and pass in front of its star. So the longer a transit event lasts, the farther away that planet is from its star.

light curves get complicated when more planets are transiting a star. The combined light curves can give us the same information as a single one, it just takes more work from astronomers to pick out each planet in the data. This is illustrated in the below animation.

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The transit method isn't just useful for finding planets, it can also give us information about the composition of a planet's atmosphere or its temperature.
When an exoplanet passes in front of its star, some of the starlight passes through its atmosphere. Scientists can analyze the colors of this light in order to get valuable clues about its composition. Using this method, they've found everything from methane to water vapor on other planets.

The transit method has been spectacularly successful at finding new exoplanets. 4437 planets have been discovered using this method till date. NASA's Kepler mission, which hunted for planets using the transit method from 2009 - 2013, found thousands of possible exoplanet discoveries and gave astronomers valuable information about the distribution of exoplanets in the galaxy.

Kepler's View of the Galaxy

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### 3) DIRECT IMAGING

Exoplanets are far away, and they are millions of times dimmer than the stars they orbit. So, unsurprisingly, taking pictures of them the same way you'd take pictures of, say Jupiter or Venus, is exceedingly hard.
New techniques and rapidly-advancing technology are making it happen.
The major problem astronomers face in trying to directly image exoplanets is that the stars they orbit are millions of times brighter than their planets. Any light reflected off of the planet or heat radiation from the planet itself is drowned out by the massive amounts of radiation coming from its host star. It's like trying to find a flea in a lightbulb, or a firefly flitting around a spotlight.

On a bright day, you might use a pair of sunglasses, or a car's sun visor, or maybe just your hand to block the glare of the sun so that you can see other things.
This is the same principle behind the instruments designed to directly image exoplanets. They use various techniques to block out the light of stars that might have planets orbiting them. Once the glare of the star is reduced, they can get a better look at objects around the star that might be exoplanets. This is illustrated in the below video.

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There are two main methods astronomers use to block the light of a star:

CORONOGRAPHY:


This method uses a device inside a telescope to block light from a star before it reaches the telescope's detector. Coronagraphs are built as internal add-ons to telescopes, and are now being used to directly image exoplanets from ground-based observatories.

STARSHADE:


It is a device that's positioned to block light from a star before it even enters a telescope. For a space-based telescope looking for exoplanets, a starshade would be a separate spacecraft, designed to position itself at just the right distance and angle to block starlight from the star astronomers were observing.

Direct imaging is still in its beginning stages as an exoplanet-finding method, 84 planets have been discovered using this method till date, but there are high hopes that it will eventually be a key tool for finding and characterizing exoplanets. Future direct-imaging instruments might be able to take photos of exoplanets that would allow us to identify atmospheric patterns, oceans, and landmasses.

### 4) GRAVITATIONAL MICROLENSING

Among his many insights, Albert Einstein rethought the concept of gravity, defining it less as a mysterious attraction between objects and more as a geometric property of spacetime.
In other words, big objects warp the fabric of space. This effect causes light to distort and change direction when affected by the gravity of a massive object, like a star or a planet.

This change of direction can cause some pretty interesting things to happen. Sometimes, gravity can bend and focus light like a lens in a magnifying glass or pair of glasses.

Gravitational microlensing happens when a star or planet's gravity focuses the light of another, more distant star, in a way that makes it temporarily seem brighter.

In the animation below, you can see the rays of light from the more distant star bend around the exoplanet and then the exoplanet's star. In the same way that a magnifying glass can focus the sun's light onto a tiny, very bright spot on a piece of paper, the gravity of the planet and the star focus the light rays of the distant star onto the observer.

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The graph on the left indicates the changing brightness of the distant star as its light is lensed and focused onto the observer. The star starts to get brighter, then there's a brief blip of brightness from the lensing action of the planet.

The light levels fall after the planet is lensed but they continue to increase because of the continued lensing action of the star. Once the lensing star moves out of the optimum position, the brightness of the more distant star fades away.

To an astronomer, a lensing event looks like a distant star that gets gradually brighter over the space of a month or so, then fades away. If a planet happens to be lensed, it looks like a brief blip of light that happens during this brightening and dimming process.

Astronomers can't predict when or where these lensing events will happen. So they have to watch large parts of the sky over a long period of time. When they record a star getting brighter and then dimming in the pattern of lensing objects, they analyze the data to get information about the estimated size of the star.

Sometimes, free-floating planets in space, ones that don't orbit a star, will cause quick microlensing events that astronomers will record. These events give us an idea of how common these so-called 'rogue' planets are in the galaxy. The animation below shows what that kind of event looks like to a telescope.
250 Planets have been discovered by this method till date.


### 5) ASTROMETRY

Doppler shifts aren't the only way astronomers can find stars that are wobbling due to the gravity of their planets. The wobble can also be visible as changes in the star's apparent position in the sky.
In other words, scientists can actually detect the star's position wiggling around in space.
Astrometry, as this method is called, is still amazingly hard to do. Stars wobble such a minute distance that it's very difficult to accurately detect the wobble from planets, especially small ones the size of Earth.
In order to track the movement of these stars, scientists take a series of images of a star and some of the other stars that are near it in the sky. In each picture, they compare the distances between these reference stars and the star they're checking for exoplanets.
If the target star has moved in relation to the other stars, astronomers can analyze that movement for signs of exoplanets.
Astrometry requires extremely precise optics, and is especially hard to do from the Earth's surface because our atmosphere distorts and bends light.
5 planets have been discovered using this method till date.

## SOME OF NASA'S SPACE TELESCOPES INVOLVED IN EXOPLANET DISCOVERY:

### Hubble

NASA's Hubble Space Telescope, which marked its 30th anniversary in orbit in 2020, was a pioneer in the search for planets around other stars; Hubble even has been used to make some of the earliest profiles of exoplanet atmospheres.


### Kepler and K2

Another space explorer, NASA's Kepler Space Telescope, made history with its discovery of thousands of exoplanets, searching for tiny dips in starlight as planets crossed the faces of their stars. In its first mission, from 2009 to 2013, Kepler monitored more than 150,000 stars, watching for "transits" — these tiny dips in starlight as planets crossed in front of their stars. The first mission ended in 2013 when technical problems caused the spacecraft to lose much of its pointing ability. In 2014, it began its second mission, dubbed K2, and continued discovering exoplanets despite its diminished directional capability. Decommissioned in 2018, Kepler remains credited with discovering the most exoplanets of any mission so far — more than 2,600. Researchers are still finding planets in Kepler’s data and will continue to for years.


### Spitzer

Spitzer probed the heavens in the infrared portion of the spectrum, capturing images of newborn stars nestled inside thick clouds of dust along with millions of other images. The space telescope was retired in 2020, although — like with Kepler — the data it gathered will be mined by scientists for years to come, likely yielding a continuing stream of discovery.


## CONCLUSION

In summary, astronomers have developed ingenious methods to detect worlds beyond our solar system — from monitoring tiny dips in starlight to tracking subtle stellar wobbles. Each technique offers unique insights, and together they’ve revealed thousands of exoplanets, including some that resemble Earth. These discoveries not only deepen our understanding of planetary systems but also bring us closer to answering the age-old question of whether life exists elsewhere in the universe. As new telescopes and missions come online, we stand on the edge of a new era in cosmic exploration.