It might surprise you to know that the definition of a planet can vary based on where it is situated. To understand what an Exoplanet is, we first need to know what a planet is and the criteria a celestial body needs to meet with so as to be considered as a planet. For an object outside our solar system to be classified as a planet, it must be big enough and should have enough mass that its own gravity forces itself into a roughly spherical shape. On the other hand, objects within our solar system are subject to more criteria before it can be considered a planet. For example, its surface temperature, distance from the Sun, etc. must fall within strict parameters. The planets in our solar system that have sufficient mass and conform into a roughly spherical shape without fulfilling the other criteria are classified as dwarf planets.
Now, what are Exoplanets?
Exoplanets are planets which are located outside the solar system. While there are several types, exoplanets are typically categorized into two types based on their orbit: exoplanets which orbit stars and exoplanets which don’t orbit stars i.e. rogue exoplanets.
Exoplanets can also be classified into several types based on their structure and composition or whether they are able to sustain life or not. Some classes of exoplanets you might have heard of include gas giants, Neptunians, rocky, terestrial planets and super earths. Each type of planet is formed differently which we will be talking about later.
Types of Exoplanets
Gas Giants
A gas giant is a type of planet which is mostly composed of gases such as hydrogen and helium. These planets are massive in size. Jupiter is an excellent example of a gas giant located in our solar system. It is composed of 71% Hydrogen, 24% Helium and 5% of other trace elements.
Gas giants are also known as “failed stars”. This is because they do not have enough mass to allow for nuclear fusion to take place at the core, despite having a similar composition to that of a star. Without sufficient mass, the core temperature and pressure of a planet does not get high enough for hydrogen to fuse, thereby eliminating the possibility of nuclear fusion. So, in simple words, gas giants are “stars” which simply did not have enough mass to start nuclear fusion.
Gas giants do not have a distinct surface like rocky planets, they just have a gaseous atmosphere. First, let’s talk about Jupiter’s core. We don’t actually know what Jupiter’s core is made of; some scientists think that it is made of a solid material whereas others think that it is made of a super-hot liquid material at very high pressure. There is still ongoing research and countless debates on what the core of a gas giant like Jupiter is made of. Researchers believe that Jupiter’s core is at a temperature of about 20,000O°C, covered by a layer of metallic hydrogen. So, you might be wondering what metallic hydrogen is. Metallic hydrogen is a phase of hydrogen between liquid and solid (i.e in a partial liquid state) that conducts electricity. This substance is formed at very high pressures and temperatures, which can easily be found inside a gas giant such as Jupiter.
Outside that layer of metallic hydrogen is a layer of liquid hydrogen. Liquid hydrogen is formed when gaseous hydrogen is exposed to very high pressures and temperatures. The liquid hydrogen is at a slightly lower pressure and temperature compared to the metallic hydrogen. Liquid hydrogen – when exposed to even higher pressures and temperatures – forms metallic hydrogen. After the liquid hydrogen layer, is “normal” gaseous hydrogen which is at a much less pressure and temperature compared to the layer of liquid hydrogen. Outside the layer of gaseous hydrogen are clouds. These clouds can be categorised into 3 different parts based on their composition as bottom clouds, middle clouds and top clouds. The innermost layer of the clouds is the bottom layer, which is thought to be composed of water ice and vapour. The middle layer is thought to be composed of ammonium hydrosulfide crystals whereas the top layer is said to consist of mostly frozen ammonia crystals.
Neptunians
Just as the name suggests, planets similar to Neptune in terms of both size and composition such as Uranus are called Neptunians. All Neptunians are ice giants. Ice giants are different from gas giants because they are composed mostly of ice whereas gas giants are composed of mostly gas. The ice here actually means a frozen solid state of various substances, not water. Some examples of Neptunians or Ice giants in our solar system are Uranus and Neptune itself. Unlike gas giants, ice giants or Neptunians are composed of elements denser than hydrogen such as Oxygen, Sulphur, Carbon, Nitrogen etc.
The composition of Neptunians are typically similar. The core of an ice giant like Neptune is small compared to the size of the entire planet. Its core is thought to be very dense and composed mostly of rock and ice. Outside the core of Neptune is its mantle. Its mantle is thought to be composed of mostly frozen substances such as water, ammonia, methane, etc. Outside the mantle is a very dense atmosphere. It is much denser than the Earth’s atmosphere. The atmosphere here can be divided into two parts: The Inner atmosphere and the Upper atmosphere. As you might have guessed, both layers of the atmosphere consist of more or less the same elements, mostly hydrogen, helium and a little bit of methane.
Fun fact, Neptune appears blue to us because of the presence of methane in the upper atmosphere of Neptune. It absorbs almost all red light and therefore makes the planet look blue. The major difference between the inner atmosphere and the upper atmosphere is that the upper atmosphere has clouds which can be observed by us. On the other hand, we have not yet observed any cloud formation in the inner atmosphere of Neptune.
Rocky Planets
The next type of exoplanets are Rocky/Terrestrial exoplanets. As the name suggests, they are composed of mostly rock and are generally smaller and less massive than gas or ice giants. This is because they are made up of rocks that have higher density compared to gas which is the main component of gas and ice giants. Some of the examples of rocky planets present in our solar system are Mercury, Venus, Earth and Mars. Rocky planets are generally made up of rock, metal and silicate.
In the above image, we can see that the interior of Earth is roughly divided into 5 layers. They are, Inner core, Outer core, Lower mantle, Upper mantle, and the Crust.
First, let’s talk about the Inner core of Earth. The inner core of the Earth is at a temperature of approximately 5,430°C which is about the temperature on the surface of the Sun. The inner core consists mostly of an alloy of Iron(Fe) and Nickel(Ni) which is known as FeNi or NiFe. You might be wondering why the alloy of Iron and Nickel hasn’t melted and this is because its melting point is about 1400°C which is much lower than the temperature at the core which happens due to very high pressure at the inner core of the Earth. This prevents the solid from melting into liquid.
Next is the outer core of the Earth. . It is at a much lower temperature than the inner core. The temperature of the outer core is from about 2,700°C to 4,200°C. Both the inner and the outer cores consist of the alloy of Nickel and Iron but here, it is in the molten liquid state because the temperature is still higher than the melting point but the pressure is not enough to keep the solid from melting into a liquid state.
Outside this is the Mantle. The mantle of the Earth is divided into 2 parts namely lower mantle and upper mantle. First, let’s talk about the lower mantle. The lower mantle is at a temperature of about 2000°C which is less than the temperatures found at the outer core. The pressures found here are also less than those found in the outer core. Both the lower and upper mantles consist of mostly Iron, Silicon, Calcium, Magnesium, Oxygen, etc. Unlike the Outer core, both the mantles are made of solid rock. The main difference between the upper and lower mantle is that Lower pressures and temperatures are found in the upper mantle when compared to the lower mantle. The outermost layer of the Earth is the crust which is a layer of solid rock which encases the upper mantle.
The Super-Earths
Any rocky planet which is bigger than Earth yet smaller than Neptune is called a Super-Earth exoplanet. Unfortunately, there are no examples of any Super-Earths in our Solar system. But there are several others outside our solar system which include GJ 15 ABb, 55 Cancri e and TOI-1075 b. Although we are not yet sure if all the Super Earths that we’ve discovered so far are rocky planets because some could be Gas Giants or Ice giants, as of now, we have discovered more than a thousand Super-Earth exoplanets which is amazing!
Habitable and Non-Habitable Planets
Exoplanets can also be divided on the basis of sustenance of life and also their distance from the parent star. For life to sustain, the first condition is, the surface temperature of the exoplanet must neither be too cold nor too hot so that water in its liquid state can exist. This is because water is very essential for life to exist. Based on the distance between the exoplanet and the parent star, the orbit of a planet can further be classified into 3 zones around the star. If a planet is too close to its parent star, the planet’s surface temperature would be too high. This means that water in its liquid form cannot exist on that planet as it would evaporate into water vapour. If the planet is too far away from its parent star, liquid water cannot exist on the planet’s surface because its surface temperature would be too cold which causes the liquid water to freeze into solid ice. Hence there is a soft spot between these two zones which is known as the habitable zone or goldilocks zone.
Planets which are located in the habitable zone of their parent star have a surface temperature which is neither too cold nor too hot where water can exist in its liquid form. Planets which are located in the habitable zone of their parent star are called Habitable planets. The distance between the Habitable zone and the parent star changes depending on the star, its surface temperature, mass, size and so on. If the star is very big and massive, then the habitable zone will be very far away from the star. Earth is an example of a habitable planet which is located in our solar system.
Formation of Planets
Planet formations happen in different ways. For example, Gas Giants are not formed in the same way as Ice Giants. But before looking into the formation of planets, we will first talk about the formation of the Sun. All stars in the universe, including our Sun, are formed from molecular clouds of gas called nebulae. These nebulae consist of mostly molecular Hydrogen and Helium. The first stage in the formation of the Sun is its protostar phase. During this phase, the Sun will take in the matter from the molecular cloud. Due to gravity of the protostar, some of the matter in the nebula eventually forms a disk around the protostar called the protoplanetary disk. This disk consists of very hot matter like Ice, Gas and dust particles which orbit the protostar.
The matter present in the proto-planetary disk also has mass and the gas and dust particles in the disk are not distributed homogeneously which means that the matter particles are closer to each other in some parts than in the others. This means that the density of the disk varies depending on the part of the disk. And, due to the gravity of the matter present in the disk, the densest parts of the disk slowly start attracting other small lumps of gas and dust particles present in the disk which form small dense(relative to the other parts of the disk) bodies. There will be many such dense bodies in the cloud and isolated bodies continue to grow in size and mass and eventually stop growing until they have absorbed all matter present in their close neighbourhood and they eventually form planets. As mentioned earlier, different types of planets are formed in different ways.
What we have discussed now is the general formation of planets. During the formation of the parent star, small relatively denser lumps of matter are formed. The type of planet formed depends upon what the majority of the lump is made of. If the lump is mainly made up of gases and if that lump eventually becomes a planet, it will be a gas giant. If the lump is mostly made of ice as in frozen solid substances and some amounts of hydrogen and helium, then the planet thus formed will most likely be an Ice giant or a Neptunian and if the lump is mostly made up of rock, then the planet thus formed would most likely be a rocky or a terrestrial planet. If the lump is mostly made up of rocks and is big and massive, the planet formed as a result would most likely be a Super-Earth.
Discovery and Detection of Exoplanets
Michel Mayor and Didier Queloz in January 1995 discovered the first exoplanet 55 Pegasi b which is an exoplanet that orbits the star 55 Pegasi. It is a Gas giant about half the size of Jupiter which orbits relatively close to its parent star. There are many different methods to detect exoplanets. Some of the most common methods are: Radial velocity method(also known as the wobble method), Transit method, Direct imaging method, Gravitational Microlensing and Astrometry.
Radial Velocity Method or The Wobble Method
Let’s begin with what radial velocity is. Radial velocity is the velocity of an object along the line of sight of the observer. This method works by observing the radial velocity of the stars. But, how exactly does this work? Let’s imagine that there is a star called ‘A’ and a planet called ‘B’ which is orbiting the star ‘A’. When B is orbiting A, there is a point of the center of mass located between A and B which is called the barycenter. Both A and B orbit the barycenter of the system. In a star system, the star will have more mass than the planet. This means that the barycenter or the center of mass will be closer to the star than the planet.
When the star orbits the barycenter of the star and its planet, it seems as if the star is “wobbling”. Now, you might have heard of the phenomenon called Doppler shift. If not, just imagine that an observer is standing at the side of the road and an ambulance with the siren on passes by the observer. When the ambulance is coming towards the observer, the sound of the siren would get louder and when the ambulance is moving away from the observer, the sound would seem to fade away. This effect is called the Doppler shift. Similarly, the same effect takes place with light as well. We know that light can act both as a wave and a particle. When light acts as a wave, light has a wavelength. For example, in the visible light spectrum, blue light has a shorter wavelength than red light. Let’s imagine that a star is moving away from us. So, when we observe that star, we will see that the light emitted by the star is red-shifted. This means that the electromagnetic waves get “stretched”. So, when we observe the light emitted by the star, the wavelength of light would be longer than what the star actually emits.
In simple words, redshift is the phenomenon by which the wavelength of light emitted by a star which is moving away from us gets shifted towards the red (ie; towards the longer wavelengths). For example if an orange star is moving away from us at high speeds, when we observe that star we would observe the star’s light to be redshift to the longer wavelengths such as to red or infrared. Now, let’s talk about Blueshift. As you might have guessed, Blueshift is the exact opposite of Redshift. Imagine a yellow-coloured star moving towards us at a very high speed. When we observe the light emitted by the star, we will observe that the light is Blueshifted. This means that the light emitted by the star is being “compressed” to shorter wavelengths such as blue. When a star is moving towards or away from us, the red shift or blue shifts would not be as much as described here. The effect has been exaggerated for the sake of simplicity.
Now, going back to the star orbiting the barycenter. While we observe the star, we will see repeating redshift and blueshift occurring in a pattern at a particular time interval. This means that the star is moving back(redshift) and forth(blueshift) from our point of view. If we observe repeating blue shift and Redshift on a star, it means that the star is orbiting around the barycenter of the star and a planet located in the star system and in that case, we can also figure out the distance between the star and the planet, the mass of the planet and so on. But if we do not observe this repeating pattern, it means that there is no planet orbiting the star and therefore no barycenter for the star to wobble around. If there is a planet orbiting around the star. To discover exoplanets, we continuously observe thousands of stars to check for the phenomena of redshift and blueshift.
Transit Method
The next is the Transit Method. This method works by observing a small decrease in brightness of a star when a planet passes by the star. Let’s imagine that we are observing a star which has a planet orbiting around it. If the orbital plane of the exoplanet is perfectly edge-on to us, the planet will pass directly in front of the star as it orbits around the star.
If this happens while we are observing that star, we will observe a very small decrease in the brightness of the parent star. Using this method, we can figure out the orbital period of the exoplanet, its size, mass, etc. We can also find the composition of the atmosphere in the planet by taking spectrographs of the light from the star which passes through the atmosphere of the exoplanet(the edge of the exoplanet). Most exoplanets have been observed using this method and as of April 2nd 2023, about 3900 exoplanets have been discovered using the transit method.
Direct Imaging Method
In this method, we try to take images of stars directly to see whether any planets are orbiting around the stars. But, how can the planets be seen. Well, if we have a big enough telescope, we can imagine the light of the parent star being reflected by the planet. However, the visible light emitted by the star is a billion times brighter than the light reflected by the planet. But, the infrared light emitted by a star is only a million times brighter than the infrared light from the star which is reflected by the exoplanet.
The main disadvantage of this method is the fact that we can only image the exoplanets directly which are in our relatively close neighbourhood because to observe exoplanets which are further away, we require large telescopes and high angular resolution. Angular resolution is the ability of a telescope to differentiate stars which are very close to each other. If the telescope can differentiate two stars which are very close, then we say that the telescope has a very high angular resolution. If a telescope cannot differentiate between the two stars, we say that the telescope has a low angular resolution.
Gravitational Microlensing
To understand what Gravitational Microlensing is, we first need to understand what spacetime is. Space-time can be defined as a scientific model in which all three dimensions of space(x,y,z) and also a fourth dimension (a.k.a time) was fused to form a 4d manifold. Now, since we have a rough idea of what spacetime is, let’s move forward. We all have studied that light travels in a straight line. That is true but an important fact is that light travels in a straight line with reference to spacetime. A body with mass can bend or curve space-time and this curvature in space-time caused by a body with mass is called gravity. Now, what if spacetime is curved due to a nearby massive body? If an observer located in flat space-time observes the path of light, they will observe that light is travelling in a bent path. Now, let’s look at what Gravitational lensing is.
First, what is a lens? Specifically a Convex lens. A convex lens is a converging lens which means that it refracts light to converge at a point called the focus. Similarly, gravity can also act as a lens. Let’s imagine that there is a very massive galaxy in the foreground and another galaxy in the background. The galaxy in the background is directly behind the very massive galaxy in the foreground. So, what happens is the light from the galaxy in the background bends around the galaxy in the foreground due to the curved spacetime around it and reaches the observer. Although this lens does not converge the light perfectly, it still is a lens.
Einstein Ring
Depending on the alignment between the observer, the foreground object and the background object, the image of the background object formed by the lens can be of various shapes. If all three are perfectly aligned, then something called an Einstein ring which is shown in the above image can be observed. What we are seeing in the above image is basically an image of a galaxy in the foreground being warped around the massive elliptical galaxy which is also in the foreground. Sometimes if the alignment is not perfect, 2 or more images of the galaxy can be formed or the image can be distorted in different ways. So, now let’s talk about gravitational microlensing.
Gravitational microlensing is the same as gravitational lensing but at a much smaller scale. When an exoplanet passes in front of its parent star, a very small gravitational lens is produced in some cases. Sometimes, due to this tiny gravitational lens, we can see a distorted image of the parent star which is behind the exoplanet. The main disadvantage of this method is that we need a telescope with a very high angular resolution to observe the images produced by these tiny gravitational lenses.
Astrometry
The next method of discovering exoplanets is by using astrometry. This method is very similar to the Radial Velocity method. If a star has an exoplanet orbiting around it, the star will be orbiting around the barycenter. Using this method, we make precise observations of the positions of the star. If the star is orbiting around a barycenter, we can observe small wobbles in the position of the star. The wobble of the star is very small. If we don’t observe any wobbles of the star, that means that there is no exoplanet which is orbiting that particular star. So, the major disadvantage is that we require big telescopes which can make very precise observations of the positions of the star.
Reference: NASA
Wanna know more about Gravitational Lensing? Head to the article: What is Space-time? Understanding Gravitational Lensing
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