How Are Exoplanets Detected? Methods and Real-World Examples
Everything including planets, moons, asteroids, and comets revolve around the sun in their orbits. Our solar system consists of eight planets. The Sun is the only star in our solar system. There are millions of stars in this universe and planets revolving around these stars. The planets revolving outside the solar system revolving around a star other than the sun is called an exoplanet. Scientists have discovered many exoplanets using various methods. These methods used to detect the planets outside the solar system are discussed in detail here.
What is an Exoplanet or Extrasolar Planet?
The term exoplanet comes from combining the words exo meaning external and the planet. The word exo and planet are combined to produce the word exoplanet meaning planets outside the solar system. Any planet outside the solar system which is revolving around stars other than the sun is called an exoplanet. The exoplanets are also called extrasolar planets. These exoplanets orbit their own stars with some being part of entire planetary systems. The exoplanets are made up of the same elements by which the planets in our solar system are made up of. Based on the composition and the structure, each exoplanet differs from the others.
The exoplanets are classified into four types. The different categories of exoplanets are as follows:
Giant or Neptune like planets
Hot Jupiters
Super earth
Earth analogs
The giant or Neptune-like planets are large gaseous exoplanets like the gaseous planets in our solar system. Hot Jupiters are other gaseous exoplanets that closely orbit their stars. Since hot Jupiters are closely orbiting their respective sun, the surface temperature of these hot Jupiters is high. Super earth is another category of exoplanets. Super earth is larger than our earth but smaller than the gas giants. Super earth is terrestrial and is made up of primarily rocky or icy materials. The fourth category of exoplanets is earth analogs. As the name suggests, earth analogs are similar to earth in various ways. The similarities of the earth analogs with our earth include size, composition, and distance from the star.
Methods of Detection of Exoplanets
It is very difficult to detect the exoplanets directly. The main reason that makes the exoplanet unable to detect or identify directly is that exoplanets are very faint to detect. This is because of the brightness of the stars that exoplanets are orbiting around. There are various methods used to detect the exoplanets. Scientists use radial velocity methods to detect and study the exoplanet. Let us learn some methods used by astronomers to detect the exoplanet in detail. We will also discuss the principle behind each method.
Radial Velocity Method
Through radial velocity, an exoplanet can be identified outside the solar system. The radial velocity method measures the motion of the host star in response to the gravitational tug by their planets. The first planet discovered outside the solar system is 51 Pegasi b in 1995. The great discovery was made by Swiss astronomers Michel Mayor and Didier Queloz. They were given the Nobel prize in Physics for their discovery.
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The exoplanet and the star exert equal amounts of gravitational force according to Newton's law of gravitation. Even though the exoplanets are small compared to the stars, the gravitational pull of the exoplanet on the star causes the star to wobble. Suppose we are observing the exoplanet orbiting the star. When the position of the exoplanet is between the star and the observer at a particular instant of its orbital motion, the exoplanet pulls the star slightly towards us. When the exoplanet is behind the star, then the exoplanet pulls the star away from us. This causes the wobbling of the star back and forth due to the gravitational tug of the exoplanet on the star. The astronomers look for the wobbling of the stars to identify the exoplanet.
The astronomers use a spectrograph and powerful telescope to identify the wobbling of the stars. The lights coming from distant stars are examined in a spectrograph. The spectrograph contains dark lines or small dark gaps due to the absorption of light by the atmosphere of the star when the light passes through it. When the star moves toward us due to the gravitational tug, these dark lines shift towards the blue region of the spectrum. When the star moves away from us due to the gravitational pull of the exoplanet, the dark lines shift towards the red end of the spectrum. Therefore, this is how astronomers check if any exoplanet is orbiting a distant star by examining the light spectrum of the star. Through the radial velocity method, astronomers can find the size and shape of the orbits of the extrasolar planet orbiting the star.
Transit Method
The transit method uses the concept of shadows to identify the exoplanet. If we are observing a star from the earth which is orbited by an exoplanet, then the exoplanet will regularly pass through the position between the observer and the star. When the position of the exoplanet is between the star and the earth or the observer from the earth, the exoplanet will block a tiny amount of light coming from the star and reaching the observer. Hence the intensity of light from the star is slightly reduced for a period of time due to the current transit of planets.
The current transit of planets and their effects is similar to the case of the solar eclipse. A solar eclipse happens when the position of the moon is in between the sun and the earth. The moon will block a certain amount of light coming from the sun from reaching the earth. Similarly, for a brief period of time, the star looks dimmer. This will help the astronomers to identify that an exoplanet is orbiting the star. If the brightness of the distant star is decreased regularly and repeatedly for a brief period of time, it could be due to the reason that the exoplanet is transiting the star. THis is the principle behind the transit method to identify the exoplanet.
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The transit method is also called transit photometry. Through this method, astronomers can measure the size of the exoplanets as well as the time period of the exoplanet orbiting around the star. Astronomers combine the radial velocity method and transit photometry to obtain accurate measurements of the mass of the exoplanets. The density of the exoplanet can also be measured which helps to identify the composition of the exoplanets. The spectroscopy studies of the transit exoplanets help to identify the gases such as hydrogen, sodium, and methane in the upper atmosphere of giants. In 1999, the exoplanet HD 209458b was identified using transit photometry. The exoplanets orbiting close to the stars are identified easily using the transit photometry method.
Gravitational Microlensing Method
Through gravitational microlensing, exoplanets are identified by astronomers. When one celestial body passes in front of another celestial body from our point of view, The close objects’ gravity can bend and magnify the light of a more distant object. This causes the distant object to appear brighter than normal for a short period of time. This is called gravitational microlensing. This gravitational microlensing is a form of gravitational lensing. Gravitational lensing is a physical phenomenon that occurs when a massive body bends the light coming from the source with respect to the observer. A gravitational lens bends the light by the gravity of the massive body. If the bending of light due to bodies like stars, planets or blackholes, then it is called microlensing.
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If one star passes in front of another distant star, the distant star will temporarily appear brighter for a short period of time. The brightness of the distant star returns to normal once the closer star passes it. But if the star that is passing by is accompanied by an exoplanet, then the brightness of the distant star increases temporarily for the second time as the planet passes in front of it. Therefore astronomers can identify the presence of an exoplanet by observing the distant star’s brightness to intensify twice.
Astrometry Method
Astrometry is the science of precisely measuring the position of an object in the sky. As we have discussed earlier, when an exoplanet is orbiting the star, the exoplanet will exert a gravitational pull on the star which causes the star to slightly move toward the observer on the earth as well as away from the observer depending on the position of the exoplanet. This causes the star to wobble due to the gravitational pull of the exoplanet. So, there is a small relative movement of the star with respect to other stars. This movement with a periodic rhythm can be observed using a powerful telescope. Thereby, astronomers can identify the presence of exoplanets orbiting the star.
Direct Imaging Method
Taking a picture of an exoplanet is the most difficult way because of the reason that exoplanets are outshined by the bright stars. The presence of the exoplanet's faint glow is very very difficult to detect in the bright lights from the stars. In spite of this, astronomers use this technique to discover any exoplanets out there. Astronomers use a black mask called a coronagraph to cover up the star and most of the light coming from the star. If the planet is nearby to the star and is not covered by the coronagraph, then a powerful telescope might be able to detect the reflected light from the exoplanet. This is how the direct imaging technique works.
Exoplanets or extrasolar planets are planets outside the solar system which are orbiting a star other than the sun. Due to the faint glow of these exoplanets in the presence of bright stars, exoplanets are very difficult to identify. Therefore astronomers use different methods to identify the presence of exoplanets. There are five different methods astronomers use to detect and study exoplanets.
The radial velocity method is the most successful method so far to detect the exoplanet. Astronomers can measure the mass of the exoplanets accurately by combining the radial velocity method and the transit photometry method. There are many exoplanets similar to our earth. The study about exoplanets helps us to find out if there is any possibility of planets having extraterrestrial life outside the solar system.
FAQs on Exoplanets: Categories and Physics Behind Their Discovery
1. What is an exoplanet, and how is it different from a planet in our Solar System?
An exoplanet, or extrasolar planet, is a planet that orbits a star outside of our Solar System. The primary difference lies in the host star: planets like Earth, Mars, and Jupiter orbit our Sun, whereas exoplanets orbit other stars. While they are both celestial bodies that orbit a star and have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a nearly round shape, the term 'exoplanet' specifically categorises them as belonging to a different star system.
2. What are the main types of exoplanets discovered so far?
Scientists have classified the thousands of discovered exoplanets into several broad categories based on their size, mass, and composition. The main types include:
- Gas Giants: Large planets composed mostly of hydrogen and helium, similar to Jupiter and Saturn. A common subtype is the 'Hot Jupiter', which orbits extremely close to its parent star.
- Neptune-like: Gaseous or icy planets similar in size to Neptune or Uranus. They have a mix of rock, ice, and thick atmospheres.
- Super-Earths: Planets with a mass higher than Earth’s but substantially below that of the ice giants Uranus and Neptune. They can be rocky, oceanic, or even gaseous.
- Terrestrial: Rocky planets with a size and composition similar to Earth. Finding these within a star's habitable zone is a primary goal for astronomers.
3. How do astronomers detect exoplanets using the radial velocity method?
The radial velocity method, also known as the Doppler wobble method, detects exoplanets indirectly by observing their effect on the host star. As a planet orbits a star, its gravitational pull causes the star to 'wobble' slightly. This wobble changes the star's movement relative to Earth. When the star moves towards us, its light spectrum shifts towards the blue end (blueshift). When it moves away, its light shifts towards the red end (redshift). By measuring these tiny, periodic shifts in the star's light, astronomers can infer the presence, mass, and orbit of an exoplanet.
4. Besides the radial velocity method, what other key techniques are used to find exoplanets?
While the radial velocity method is powerful, several other techniques are crucial for discovering and characterising exoplanets. The most successful is the Transit Method, where astronomers measure the slight, periodic dimming of a star's light as an exoplanet passes in front of it. Another technique is Direct Imaging, which involves directly photographing the exoplanet, though this is extremely challenging due to the overwhelming glare of the host star. A third method is Gravitational Microlensing, which occurs when the gravity of a star and its planet bends the light of a more distant star, acting like a lens and causing a temporary brightening.
5. What makes an exoplanet 'habitable', and what is the 'Goldilocks Zone'?
A 'habitable' exoplanet is one with conditions that could potentially support life as we know it. The most critical requirement is the ability to sustain liquid water on its surface. This leads to the concept of the habitable zone, often called the 'Goldilocks Zone'. It is the orbital region around a star where the temperature is not too hot and not too cold, but 'just right' for liquid water to exist. However, true habitability also depends on other factors like the planet's atmospheric composition, its mass, and protection from the host star's harmful radiation.
6. Are 'Super-Earths' just larger, more habitable versions of our planet?
Not necessarily. The term 'Super-Earth' refers only to a planet's mass and size—typically 1 to 10 times the mass of Earth—not its surface conditions or composition. A Super-Earth could be a barren rocky world, a planet completely covered by a deep ocean (a 'water world'), or even a mini-gas giant with a thick, crushing atmosphere. Its habitability depends entirely on whether it orbits within its star's Goldilocks Zone and possesses a suitable, stable atmosphere, not just on its size.
7. Which exoplanet is closest to Earth, and why is its discovery significant?
The closest known exoplanet to Earth is Proxima Centauri b. It is located approximately 4.2 light-years away and orbits Proxima Centauri, the nearest star to our Sun. Its discovery is highly significant because its relative proximity makes it a prime candidate for detailed observation with current and future telescopes. Studying Proxima b helps astronomers test theories of planet formation around low-mass red dwarf stars and provides a key target in the ongoing search for signs of life beyond our Solar System.
8. Why is the study of exoplanets important for understanding physics and our place in the universe?
The study of exoplanets is fundamentally important for several reasons. Firstly, it allows us to test and refine our theories of planet formation and evolution by observing thousands of other solar systems that are vastly different from our own. Secondly, it drives technological innovation in optics, spectroscopy, and space telescopes. Most profoundly, searching for Earth-like exoplanets in habitable zones is a direct step towards answering one of humanity's oldest questions: Are we alone in the universe? It provides context for how common or rare our Solar System and life on Earth might be.
