THE PANORAMIC UNIVERSE - Edition II

We are pleased to introduce you to the second part of our series, where we take you into the depths of the universe and enable you to have an understanding of fundamental concepts.

This episode includes the following topics.

• Galaxies
• Galaxy Formation
• Type of Galaxies
• Galaxy Merge
• The Birth Of A Star
• Nebulas
• The Death of A Star
• Black Holes
• Black Hole in M87
• The Anatomy of a Black Hole
• Solar System & Formation
• Five Dwarf Planets
• Orbits

Galaxies

A galaxy, in astrophysical terms, can be defined as a large conglomerate of various cosmic entities such as stars, remnants of stellar phenomena, interstellar gas, particulate matter known as cosmic dust, as well as the elusive dark matter. These diverse components are held together in a coherent structure by the omnipresent force of gravity. A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. Galaxies, averaging an estimated 100 million stars, range in size from dwarfs with less than a hundred million stars, to the largest galaxies known – super-giants with one hundred trillion stars, each orbiting its galaxy's center of mass. Most of the mass in a typical galaxy is in the form of dark matter, with only a few percent of that mass visible in the form of stars and nebulae. Supermassive black holes are a common feature at the center of galaxies. These black holes are celestial entities of immense mass and density, so much so that nothing, not even light, can escape their gravitational pull once it reaches the event horizon, which is the boundary marking the point of no return. The supermassive black holes at the centers of galaxies are believed to play a crucial role in galactic evolution and the dynamics of stellar orbits.

Interestingly, in a typical galaxy, the majority of its mass is not accounted for by the visible components like stars and nebulae. Instead, the bulk of a galaxy's mass comes from dark matter, a mysterious form of matter that does not interact with electromagnetic radiation, making it invisible to current detection methods. Only a minuscule portion of a galaxy's mass, possibly just a few percent, can be attributed to the visible matter in the form of stars, nebulae, and other observable interstellar materials.

Galaxies have magnetic fields of their own. They are strong enough to be dynamically important, as they:

• Drive mass inflow into the centers of galaxies
• Modify the formation of spiral arms
• Can affect the rotation of gas in the galaxies' outer regions
• Provide the transport of angular momentum required for the collapse of gas clouds, and hence the formation of new stars

Galaxies do not exist in isolation in the cosmos. They are bound together by the unifying force of gravity into larger structures known as groups and clusters of galaxies. A galaxy group is a smaller system typically consisting of a few dozen galaxies gravitationally bound together, while a galaxy cluster is a larger assembly that can include hundreds or even thousands of galaxies. These groupings of galaxies further coalesce into colossal cosmic structures known as superclusters. These are vast regions of space, some spanning hundreds of millions of light-years, where galaxy clusters are found in higher concentration. These structures are the largest known gravitationally bound formations in the universe, illustrating the complex hierarchical structure of the cosmos on the grandest scales.

Galaxy Formation

Two predominant theories exist in the field of astrophysics to explain the genesis of the earliest galaxies in the universe. Both theories offer plausible explanations, and the actual mechanism of galactic formation might indeed encompass aspects of both these models.

The first model proposes that the formation of galaxies was initiated by the gravitational collapse of gigantic clouds composed of gas and dust. According to this theory, these colossal, primordial clouds of gas and dust - a form of matter collectively referred to as the interstellar medium - started to contract under the influence of their own gravitational force. As these clouds collapsed, the increasing pressure and temperature conditions triggered the process of nuclear fusion, thus leading to the birth of stars. Over time, these star-forming regions grew and eventually formed the gravitational cores of the earliest galaxies.

This model, in essence, portrays galaxies as evolving from the 'bottom-up', with smaller structures such as stars forming first, which then coalesce to form larger structures - the galaxies. This process of hierarchical clustering and merging continues over billions of years, leading to the diverse range of galaxies we observe in the current universe.

The Hubble Space Telescope has photographed many such lumps, which may be the precursors to modern galaxies. According to this theory, most of the early large galaxies were spirals. But over time, many spirals merged to form ellipticals. The galaxy-formation process has not stopped. Our universe continues to evolve.

Evidence for the appearance of galaxies very early in the Universe's history was found in 2006, when it was discovered that the galaxy IOK-1 has an unusually high redshift of 6.96, corresponding to just 750 million years after the Big Bang and making it the most distant and earliest-to-form galaxy seen at that?time. Current models of the formation of galaxies in the early universe are based on the ΛCDM model. About 300,000 years after the big bang, atoms of hydrogen and helium began to form, in an event called recombination. Nearly all the hydrogen was neutral and readily absorbed light, and no stars had yet formed. As a result, this period has been called the "dark ages". It was from density fluctuations in this primordial matter that larger structures began to appear. As a result, masses of baryonic matter started to condense within cold dark matter halos. These primordial structures eventually became the galaxies we see today.

Type of Galaxies

After looking at galaxy formations and their fundamental stages, perhaps the most important question is the types of galaxies and the reasons underlying them.

What kind of galaxies are there?

1) Spiral Galaxies

Spiral galaxies appear as flat, blue-white disks of stars, gas and dust with yellowish bulges in their centers. These galaxies are divided into two groups: normal spirals and barred spirals. In barred spirals, the bar of stars runs through the central bulge. The arms of barred spirals usually start at the end of the bar instead of from the bulge. Spirals are actively forming stars and comprise a large fraction of all the galaxies in the local universe.

• Normal spirals galaxies :?A spiral galaxy typically has a rotating disc with spiral 'arms' that curve out from a dense central region.
• Barred spirals galaxies : A barred spiral Galaxy is a spiral Galaxy with a central bar-shaped structure made of stars. Bars are found in up to 65% of spiral galaxies. They affect the motions of stars, dust and gas. It is believed that bars act a bit like a funnel, pulling matter into the bulge from disk.

Spiral galaxies are distinguishable due to features like a central bulge, spiral arms, and a disk. However, within spiral galaxies there are subcategories based on how noticeable these common features are. The subcategories are Sa, Sb, Sc, barred, and S0 (lenticular).

• Sa: these types of galaxies have their arms wrapped closely around the center and are predominantly made of space dust and gas. The central bulge for these types of galaxies is noticeably larger compared to other subcategories.
• Sb: these types of galaxies have more defined spiral arms than Sa galaxies, but less so than Sc. The arms are not so tightly wrapped around the central disk, which is considered "moderate" in size.
• Sc: the arms on these types of spirals are loosely wrapped around their centers and the disks appear smaller. This subcategory has the highest abundance of gas and dust freely available to make stars.
• Barred: a majority of spiral galaxies have a prominent bar-like feature that connects the spiral arms through the center of the disk. This bar is easily defined and is thought to form as the spiral galaxy ages.
• S0 (Lenticular): this is the late stage of a spiral galaxy's life where its shape is no longer distinguishable. The notable spiral arms have started to disappear, and it begins to appear more elliptical.

2) Elliptical?Galaxies

There are ordinary, giant, and dwarf elliptical galaxies. As the names imply, giant versions are massive galaxies that can be hundreds of thousands of light-years across. Dwarf versions are small elliptical galaxies that are only a few hundred light-years across.

Despite which kind it is, there are specific elliptical galaxy characteristics and elliptical galaxy descriptions:

• They have an ellipsoid shape, though it can vary from almost a perfect sphere to an elongated oval.
• The stars in an elliptical galaxy are old, some of the oldest in the universe.
• The orbit of the stars is random and elongated, affecting the shape of the galaxy. The stars do not orbit in a disc that is on the same plane, as they do in spiral galaxies.
• Due to the old stars they contain, these galaxies have a redder color.
• Elliptical galaxies are much dimmer than other galaxies and far away, making them difficult to see. A deep and narrow telescope is needed to see them so there is more time to collect light from these dim galaxies.
• Very little gas and dust are contained in these galaxies.
• Elliptical galaxies tend to exist in galaxy clusters, where many galaxies are close together and merge, forming new elliptical galaxies or adding to old ones.
• At the core of an elliptical galaxy is a supermassive black hole.

3) Irregular Galaxies

An irregular galaxy is the catchall name given to any galaxy that does not neatly fit into one of the categories of the Hubble classification scheme. They have no defined shape nor structure and may have formed from collisions, close encounters with other galaxies or violent internal activity. Astronomers often see irregular galaxies as they peer deeply into the universe, which is equivalent to looking back in time. These galaxies are abundant in the early universe, before spirals and ellipticals developed.

There’s only one irregular galaxy in the Messier catalog of objects, and that’s M82; also known as the Cigar Galaxy. It’s located in the constellation Ursa Major about 12 million light-years away, and is famous for its heavy amounts of star formation.

• Irregular galaxies are mostly made of hydrogen gas, and this is important for the formation of new stars.
• Irregular galaxies are thought to have once been elliptical or spiral galaxies that lost their shape.

Galaxy Merge

Galaxy mergers can occur when two (or more) galaxies collide. They are the most violent type of galaxy interaction. The gravitational interactions between galaxies and the friction between the gas and dust have major effects on the galaxies involved. The exact effects of such mergers depend on a wide variety of parameters such as collision angles, speeds, and relative size/composition, and are currently an extremely active area of research. Galaxy mergers are important because the merger rate is a fundamental measurement of galaxy evolution.

Galaxy mergers can be classified into distinct groups due to the properties of the merging galaxies, such as their number, their comparative size and their gas richness.

1) By Number

• Binary merger: Two interacting galaxies merge.
• Multiple merger: Three or more galaxies merge.

2) By Size

• Minor merger: A merger is minor if one of the galaxies is significantly larger than the other(s). The larger galaxy will often "eat" the smaller, absorbing most of its gas and stars with little other significant effect on the larger galaxy. Our home galaxy, the Milky Way, is thought to be currently absorbing several smaller galaxies in this fashion, such as the Canis Major Dwarf Galaxy, and possibly the Magellanic Clouds. The Virgo Stellar Stream is thought to be the remains of a dwarf galaxy that has been mostly merged with the Milky Way.
• Major merger: A merger of two spiral galaxies that are approximately the same size is major; if they collide at appropriate angles and speeds, they will likely merge in a fashion that drives away much of the dust and gas through a variety of feedback mechanisms that often include a stage in which there are active galactic nuclei. This is thought to be the driving force behind many quasars. The end result is an elliptical galaxy, and many astronomers hypothesize that this is the primary mechanism that creates ellipticals.

3) By Gas Richness

• Wet merger: A wet merger is between gas-rich galaxies ("blue" galaxies). Wet mergers typically produce a large amount of star formation, transform disc galaxies into elliptical galaxies and trigger quasar activity.?
• Dry merger: A merger between gas-poor galaxies ("red" galaxies) is called?dry. Dry mergers typically do not greatly change the galaxies'?star formation?rates, but can play an important role in increasing?stellar mass.
• Damp merger: A?damp merger?occurs between the same two galaxy-types mentioned above ("blue" and "red" galaxies), if there is enough gas to fuel?significant star formation?but not enough to form?globular clusters.
• Mixed merger: A?mixed merger?occurs when gas-rich and gas-poor galaxies ("blue" and "red" galaxies) merge.

Astronomers have estimated the?Milky Way Galaxy?will collide with the?Andromeda Galaxy?in about 4.5 billion years. It is thought that the two?spiral galaxies?will eventually merge to become an?elliptical galaxy?or perhaps a large?disk galaxy.

The Birth Of A Star

In the vast expanses of the Universe, a diverse range of star types exist, including, but not limited to, Protostars and Red Supergiants. They can be taxonomically categorized based on several defining attributes, such as their mass and temperature, which are crucial determinants of a star's life cycle and eventual fate. One method of categorization is based on a star's evolutionary stage, from the initial Protostar phase - where gravitational forces collapse clouds of gas and dust to form a hot core - to the late stages of stellar evolution such as the Red Supergiant phase - when a star has exhausted its hydrogen fuel and expanded to immense sizes while its surface temperature decreases, causing it to appear red.

Stars can also be classified according to their spectral properties, which essentially refer to the specific wavelengths of light they absorb due to the presence of different chemical elements in their atmospheres. This absorption forms a unique pattern called a star's spectrum, analogous to a cosmic fingerprint, providing valuable information about the star's chemical composition. Another parameter used in star classification is brightness, often referred to as apparent magnitude when observed from Earth. This can give us insights into a star's luminosity, or intrinsic brightness, which when combined with the star's distance, allows us to ascertain the amount of energy it emits.

Lastly, by determining a star's spectral class - a categorization based on the temperature and spectral characteristics of a star - astronomers can glean significant information about the star's physical properties and evolutionary state. A well-known classification scheme is the Morgan–Keenan (MK) system, which uses the letters O, B, A, F, G, K, and M, a sequence from the hottest to the coolest stars, often remembered by the mnemonic "Oh Be A Fine Girl/Guy, Kiss Me".

All these combined data - the star's mass, temperature, brightness, spectra, and spectral class - enable astronomers to piece together a comprehensive understanding of each star, from its current state to its past evolution and potential future.

Seven main stages of a star:

• Giant Gas Cloud
• Protostar
• T-Tauri Phase
• Main Sequence
• Red Giant
• The Fusion of Heavier Elements
• Supernovae and Planetary Nebulae

Now let's discuss some stages in detail.

Stage 1: A star originates from a large cloud of gas. The temperature in the cloud is low enough for the synthesis of molecules. The Orion cloud complex in the Orion system is an example of a star in this stage of life. Stars are born in a region of high density Nebula, and condenses into a huge globule of gas and dust and contracts under its own gravity.

Stage 2: When the gas particles in the molecular cloud run into each other, heat energy is produced. This results in the formation of a warm clump of molecules referred to as the Protostar. The creation of Protostars can be seen through infrared vision as the Protostars are warmer than other materials in the molecular cloud. Several Protostars can be formed in one cloud, depending on the size of the molecular cloud.

Stage 3: A T-Tauri star begins when materials stop falling into the Protostar and release tremendous amounts of energy. The mean temperature of the Tauri star isn’t enough to support nuclear fusion at its core. The T-Tauri star lasts for about 100 million years, following which it enters the most extended phase of development. T Tauri stars generally increase their rotation rates as they age, through contraction and spin-up, as they conserve angular momentum.

Stage 4: Main Sequence stars are young stars. They are powered by the fusion of hydrogen (H) into helium (He) in their cores, a process that requires temperatures of more than 10 million Kelvin. Around 90 percent of the stars in the Universe are main-sequence stars, including our sun. The main sequence stars typically range from between one-tenth to 200 times the Sun’s mass. A star in the main sequence is in a state of hydrostatic equilibrium. Gravity is pulling the star inward, and the light pressure from all the fusion reactions in the star are pushing outward. The inward and outward forces balance one another out, and the star maintains a spherical shape. Stars in the main sequence will have a size that depends on their mass, which defines the amount of gravity pulling them inward.

Stage 5: A star converts hydrogen atoms into helium over its course of life at its core. Eventually, the hydrogen fuel runs out, and the internal reaction stops. Without the reactions occurring at the core, a star contracts inward through gravity causing it to expand. As it expands, the star first becomes a subgiant star and then a red giant. Red giants have cooler surfaces than the main-sequence star, and because of this, they appear red than yellow.

Stage 6: Helium molecules fuse at the core, as the star expands. The energy of this reaction prevents the core from collapsing. The core shrinks and begins fusing carbon, once the helium fusion ends. This process repeats until iron appears at the core. The iron fusion reaction absorbs energy, which causes the core to collapse. This implosion transforms massive stars into a supernova while smaller stars like the sun contract into white dwarfs.

Stage 7: Most of the star material is blasted away into space, but the core implodes into a neutron star or a singularity known as the black hole. Less massive stars don’t explode, their cores contract instead into a tiny, hot star known as the white dwarf while the outer material drifts away. Stars tinier than the sun, don’t have enough mass to burn with anything but a red glow during their main sequence. These red dwarves are difficult to spot. But, these may be the most common stars that can burn for trillions of years.

Massive stars have a mass 3x times that of the Sun. Some are 50x that of the Sun. It is beneficial to separately address these formation and development stages for Massive Stars.

Stage 1: Massive stars evolve in a simlar way to a small stars until it reaces its main sequence stage.The stars shine steadily until the hydrogen has fused to form helium ( it takes billions of years in a small star, but only millions in a massive star).

Stage 2: The massive star then becomes a Red Supergiant and starts of with a helium core surrounded by a shell of cooling, expanding gas.

Stage 3: In the next million years a series of nuclear reactions occur forming different elements in shells around the iron core.

Stage 4: The core collapses in less than a second, causing an explosion called a?Supernova, in which a shock wave blows of the outer layers of the star. (The actual supernova shines brighter than the entire galaxy for a short time).

Stage 5: Sometimes the core survives the explosion. If the surviving core is between 1.5 - 3 solar masses it contracts to become a a tiny, very dense Neutron Star. If the core is much greater than 3 solar masses, the core contracts to become a Black Hole.

Nebulas

There are a variety of formation mechanisms for the different types of nebulae. Some nebulae form from gas that is already in the interstellar medium while others are produced by stars. Examples of the former case are giant molecular clouds, the coldest, densest phase of interstellar gas, which can form by the cooling and condensation of more diffuse gas. Examples of the latter case are planetary nebulae formed from material shed by a star in late stages of its stellar evolution. Other nebulae form as the result of supernova explosions; the death throes of massive, short-lived stars. The materials thrown off from the supernova explosion are then ionized by the energy and the compact object that its core produces. One of the best examples of this is the Crab Nebula, in Taurus. The supernova event was recorded in the year 1054 and is labeled SN 1054. The compact object that was created after the explosion lies in the center of the Crab Nebula and its core is now a neutron star.

Still other nebulae form as planetary nebulae. This is the final stage of a low-mass star's life, like Earth's Sun. Stars with a mass up to 8–10 solar masses evolve into red giants and slowly lose their outer layers during pulsations in their atmospheres. When a star has lost enough material, its temperature increases and the ultraviolet radiation it emits can ionize the surrounding nebula that it has thrown off. The Sun will produce a planetary nebula and its core will remain behind in the form of a white dwarf.

The Death of A Star

Stars essentially operate on a balance between two main forces: the immense gravitational pressure pulling everything inward and the nuclear force pushing everything outward as hydrogen atoms in its core fuse to create helium in a process known as nuclear fusion. This balance maintains the star's stability for the majority of its life, also known as its main sequence.

However, when a star exhausts its hydrogen fuel, the nuclear forces decrease, but gravity continues to act. Depending on the mass of the star, several things can happen:

• For lower-mass stars (like our Sun), they will expand into a red giant, in which the outer layers will eventually escape into space, and leave behind an Earth-sized, hot core called a white dwarf.
• For stars that are about 8-10 times the mass of the Sun, they go through stages of fusing heavier and heavier elements until they reach iron. Since fusing iron consumes energy rather than releasing it, the star suddenly loses its battle against gravity and the core collapses very quickly. This collapse causes a shock wave that makes the outer layers of the star explode in a supernova. The core that's left behind becomes a neutron star or, if the star was massive enough, a black hole.

In both cases, the dying of a star leads to the spreading of elements heavier than hydrogen and helium into space. These elements are the building blocks for new stars, planets, and even life, hence why we often say that we are all made of "star stuff."

Black Holes

Black holes are indeed fascinating and extreme cosmic phenomena, a testament to the strange and often counterintuitive physics that govern the universe. The core concept of a black hole is gravity so intense that nothing, not even light, can escape its grasp. This is the result of a star that was massive enough to collapse under its own gravity at the end of its life, leading to a singularity – a point of infinite density. The boundary surrounding this point, within which escape is impossible, is the event horizon. The size of this event horizon, or Schwarzschild radius, is directly proportional to the mass of the collapsed star.

While black holes do not emit light, they can still be observed through the effects of their gravitational pull on surrounding matter. For instance, stars orbiting around an unseen point suggest the presence of a black hole, as was the case for Sagittarius A*, the supermassive black hole at the center of the Milky Way. Non-stellar black holes can form in other ways too. Some might form from large accumulations of interstellar gas collapsing under their own gravity. There's also the theory proposed by Stephen Hawking about primordial black holes formed during the big bang. These tiny black holes could have the mass of an asteroid or less and, according to Hawking's theory, slowly lose mass over time due to a process now known as Hawking radiation. The study of black holes continues to reveal profound insights into the nature of space, time, and gravity, and raises fascinating questions about the nature of the universe itself.

Black Hole in M87

Black hole at the center of the massive galaxy M87, about 55 million light-years from Earth, as imaged by the Event Horizon Telescope (EHT). The black hole is 6.5 billion times more massive than the Sun. This image was the first direct visual evidence of a supermassive black hole and its shadow. The ring is brighter on one side because the black hole is rotating, and thus material on the side of the black hole turning toward Earth has its emission boosted by the Doppler effect.

The shadow of the black hole is about five and a half times larger than the event horizon, the boundary marking the black hole's limits, where the escape velocity is equal to the speed of light. This image was released in 2019 and created from data collected in 2017.

Black Holes Formation

Scientists know about one way that black holes form, but there may be others. One way to make a black hole is to have a massive star collapse at the end of its life. Prof. Subrahmanyan Chandrasekhar was the first to calculate that when a massive star burns up all its fuel, it will collapse. The idea was ridiculed at first, but other scientists calculated that the star continues forever to fall inward toward its center—thus creating what we called a black hole. Black holes can grow more massive over time as they “eat” gas, stars, planets and even other black holes!

There’s another type of black hole called a supermassive black hole. These are way too massive to have been created by one star collapsing; it’s still a mystery how they form. Black holes can eat other black holes, so it’s possible that the supermassive ones are made of many small black holes merged together. “Or perhaps these big black holes were especially hungry, and ate so much of their surroundings that they grew to enormous size,” said Prof. Holz. But we can see these supermassive black holes formed very early on in the universe—maybe too early to have been made by stars getting old enough to collapse—so it’s possible there’s some other way to make a black hole that we don’t know about yet.

The Anatomy of a Black Hole

Black holes, with their extreme gravitational fields and intriguing properties, are among the most fascinating objects in the universe.

Here's a summary of those concepts:

• Event Horizon: This is the boundary of a black hole, beyond which nothing, including light, can escape its gravitational pull.
• Accretion Disk: This is a disk of gas, dust, and other matter that spirals into a black hole, getting hotter and emitting light as it does so. This disk is the primary source of observable light from a black hole.
• Event Horizon Shadow: Due to the extreme gravity of a black hole, the light from behind it is bent, causing a dark shadow to appear around the event horizon. This shadow is about twice the size of the event horizon itself.
• Photon Sphere: This is the region where light orbits the black hole. The light from the accretion disk passing near this sphere can be caught in multiple orbits before escaping, creating thin, faint rings of light.
• Doppler Beaming: The accretion disk of a black hole is rotating extremely fast. As a result, light from the side of the disk moving towards us appears brighter and bluer (blue-shifted), while light from the side moving away from us appears dimmer and redder (red-shifted). This is a relativistic effect known as Doppler beaming.
• Corona: The region around the black hole where magnetic fields create a cloud of extremely hot and turbulent gas. This corona emits high-energy X-rays.
• Particle Jets: Some black holes emit jets of particles at nearly the speed of light. These jets are ejected in opposite directions from near the black hole's poles. The process that creates these jets is not fully understood.
• Singularity: At the very center of a black hole, according to general relativity, is a point of infinite density, known as a singularity. This is the final destination for anything that falls into the black hole. The nature of the singularity is still a topic of ongoing research in physics.

These concepts are a part of our current understanding of black holes based on general relativity and observations.

Solar System & Formation

Our solar system formed about 4.5 billion years ago from a dense cloud of interstellar gas and dust. The cloud collapsed, possibly due to the shockwave of a nearby exploding star, called a supernova. When this dust cloud collapsed, it formed a solar nebula – a spinning, swirling disk of material. At the center, gravity pulled more and more material in. Eventually, the pressure in the core was so great that hydrogen atoms began to combine and form helium, releasing a tremendous amount of energy. With that, our Sun was born, and it eventually amassed more than 99% of the available matter. Matter farther out in the disk was also clumping together. These clumps smashed into one another, forming larger and larger objects. Some of them grew big enough for their gravity to shape them into spheres, becoming planets, dwarf planets, and large moons. In other cases, planets did not form: the asteroid belt is made of bits and pieces of the early solar system that could never quite come together into a planet. Other smaller leftover pieces became asteroids, comets, meteoroids, and small, irregular moons.

The order and arrangement of the planets and other bodies in our solar system is due to the way the solar system formed. Nearest to the Sun, only rocky material could withstand the heat when the solar system was young. For this reason, the first four planets – Mercury, Venus, Earth, and Mars – are terrestrial planets. They are all small with solid, rocky surfaces. Meanwhile, materials we are used to seeing as ice, liquid, or gas settled in the outer regions of the young solar system. Gravity pulled these materials together, and that is where we find gas giants Jupiter and Saturn, and the ice giants Uranus and Neptune.

The most recent definition of a planet was adopted by the International Astronomical Union in 2006. It says a planet must do three things:

• It must orbit a star (in our cosmic neighborhood, the Sun).
• It must be big enough to have enough gravity to force it into a spherical shape.
• It must be big enough that its gravity cleared away any other objects of a similar size near its orbit around the Sun.

A planet is a large object that orbits a star. To be a planet, an object must be massive enough for gravity to have squeezed it into a spherical, or round, shape,. It must also be large enough for gravity to have swept up any rocky or icy objects from its path, or orbit, around the star.

Earth is one of eight planets that circle the star we call the sun. Together, the sun, the planets, and smaller objects such as moons make up our solar system.

The four planets closest to the sun—Mercury, Venus, Earth, and Mars—are called terrestrial planets. These planets are solid and rocky like Earth (terra means “earth” in Latin). Earth is the largest of the four terrestrial planets, and Mercury is the smallest. All are surrounded by a layer of gas, or atmosphere. Their atmospheres vary in density from Mercury’s extremely thin atmosphere to Venus’, which is thick with clouds of sulfuric acid.The four planets that are more distant from the sun—Jupiter, Saturn, Uranus, and Neptune—are called gas giants. Gas giants are huge compared with Earth, and they do not have solid surfaces. They are big balls of gas. Jupiter and Saturn are composed mostly of hydrogen and helium. Uranus and Neptune have greater proportions of water vapor, ammonia, and methane. Each of the four gas giants also has a ring system. A planet’s rings are made of ice, dust, and small rocks. Saturn’s ring system is the largest. Every planet except Mercury and Venus has at least one natural satellite, or moon. A planet’s moon orbits it as it revolves around the sun. Jupiter, Saturn, and Uranus each have dozens of moons.

• Mercury is the closest planet to the Sun. Despite being the smallest planet from the Solar System, this planet consists of about 70% metallic and 30% silicate material leading to its high density and thus placing it as the second densest planet. Mercury doesn’t have any known satellites.

• Venus is the second planet from the Sun and the sixth-largest. Together with Mercury, they are the only planets without a satellite, even though Mercury is closer to the sun, Venus is the hottest planet. The atmosphere consists mainly of carbon dioxide 96.5% and 3.5% nitrogen with traces of other gases, most notably sulfur dioxide. About 80% of the Venusian surface is covered by smooth, volcanic plains, consisting of 70% plains with ridges and 10% smooth or lobate plains.

• Earth is the third planet from the Sun and our home planet. Earth is the fifth largest planet of the Solar System. The surface of Earth is covered by water, around 71%, only 29% of Earth’s surface is covered by land. Earth has the greatest density out of all the planets in our Solar System. This means it is very compact. The mixture of gases commonly known as air are nitrogen, oxygen, argon, and carbon dioxide. Earth has only one satellite – the Moon, but it also has a couple of temporal artificial satellites.

• Mars is the fourth planet from the Sun and the second-smallest planet with a thin atmosphere, having the surface features reminiscent both of the impact craters of the Moon, and the valleys, deserts and polar ice caps of Earth. It is the most widely searched planet for life. It lacks an active plate tectonic system. Mars doesn’t have a magnetic field but certain areas are highly magnetized. Mars has two moons named Phobos and Deimos. Mars has the tallest volcano/mountain in the entire Solar System. Its atmosphere is mostly composed of carbon dioxide, nitrogen and argon gases.

• Jupiter is the fifth planet from the Sun and the largest planet of the Solar System. It is the oldest planet of the Solar System thus it was the first to take shape out of the remains of the solar nebula. It is the biggest planet of the Solar System. Jupiter is also twice as massive as all the other planets combined. Jupiter does not have a solid surface being comprised mostly out of swirling gases and liquids such as 90% hydrogen, 10% helium – very similar to the sun. Jupiter currently has only 79 known satellites. Jupiter has 3 ring systems though they are fainter and smaller than Saturn’s. They are mostly made up of dust and small rocky pieces.

• Saturn is the sixth planet from the Sun, with the largest planetary rings in the Solar System. It is the second-largest planet after Jupiter. The rings of Saturn are the most extensive of any other planet. They cannot be seen without an unaided eye. The low density is attributed to its composition. Saturn has the lowest density of all the planets. The planet is largely made up of gases such as hydrogen and helium. Periodic storms are present on Saturn. They are large enough to be seen from Earth and are named White Spots. Saturn now has 82 confirmed moons. The largest moon of Saturn is named Titan. It’s the second biggest moon in the Solar System after Jupiter’s moon Ganymede. Titan is even bigger than the planet Mercury.

• Uranus is the seventh planet discovered in the Solar System that also led to the discovery of the last planet, Neptune they are both referred to as ice giants. Officially recognized in 1781 after many observations in the past, it is the third-largest planet of the Solar System. It has the third-largest planetary radius and fourth-largest mass in the Solar System. It takes Uranus 84 years to complete an orbit of the Sun, the longest from all the planets in the solar system. Uranus has a similar atmosphere to Jupiter and Saturn in its primary composition of hydrogen and helium yet, it contains more “ices” such as water, ammonia, methane and traces of other hydrocarbons. Uranus has 13 known rings around it. The innermost rings are narrow and dark, and its outermost rings are brightly colored. Until now, 27 moons have been discovered orbiting Uranus.

• Neptune is the fourth largest and the farthest planet of the Solar System with the most powerful wind speeds out of all the planets. It is the smallest of the gas giants. Neptune’s color is believed to be influenced by the presence of methane in its atmosphere and also an unknown factor. As a result from its distance, it also has the longest orbital duration completing a trip around the Sun in about 165 years. This is a characteristic of the ice giants, their “rocky”, icy cores who are proportionally larger than the amount of gas they contain, unlike the gas giants. Neptune has a total of 6 known rings with some containing ring arcs or clusters of dust particles in a ring. Neptune also has 14 known moons, the largest is called Triton, and it is the seventh largest known moon of any planet, also being the only one in the solar system that orbits in retrograde or in opposition to the planet’s rotation. This means that it is a captured object by Neptune’s gravity.?

Five Dwarf Planets

• Ceres is the first dwarf planet to receive a visit from a spacecraft. It is the only dwarf planet located in the inner solar system. It does not have any moons or rings, and scientist believe that it also lacks a magnetosphere. Unlike other asteroids, Ceres is round because it is large enough for gravity to mold its shape into a sphere. The dwarf planet is made up mostly from ice and rock, with a rocky interior and an icy exterior. It follows an orbit between Mars and Jupiter, within the asteroid belt and closer to the orbit of Mars.

• Pluto is the largest known dwarf planet in the Solar System, discovered in 1930. It has five moons: Charon, Styx, Nix, Kerberos and Hydra. Charon is the largest with a diameter just over half of Pluto. It is the biggest known moon of a dwarf planet. It is made primarily out of ice and rock. It is relatively small compared to Earth’s moon being about one-sixth of the moon's mass, and one-third of its volume.

• Haumea is the fastest rotating dwarf planet with the most interesting/controversial shape. It is located beyond the orbit of Neptune. It is shaped like an egg thus making it the least spherical of all the dwarf planets. Along with its weird shape it also rotates quite fast and displays large fluctuations of brightness.?Its speedy rotation is considered to be the fastest among all the known equilibrium bodies in the Solar System. Haumea has two known moons that were named Hi’iaka and Namaka. Haumea also has a ring system. Haumea is the first trans-Neptunian object discovered to have a ring system. Its surface is believed to be made out of rock and thick coating ice. Due to its fast rotation, a day on Haumea lasts about 3.9 hours.

• Makemake is the second furthest dwarf planet from the Sun located beyond Neptune’s orbit. It was the fourth dwarf planet to be discovered. This included Pluto who was reclassified. Makemake is large enough and bright enough to be studied by high-end amateur telescopes. Like other dwarf planets, it travels through the Kuiper Belt. Large amounts of ethane, tholins and small amounts of ethylene, acetylene and high-mass alkanes like propane may be present, likely created by photolysis of methane by solar radiation.

• Eris is the most distant dwarf planet, located beyond the orbit of Neptune. It was discovered in 2005 and was originally classified as a planet. It is the second-largest dwarf planet discovered and it led to both it and Pluto’s demotion from planets to dwarf planets. It doesn’t have rings, nothing is known about its magnetosphere but it does have a moon named Dysnomia.

Types of Planets

• The planets inside the orbit of the earth are called the Inferior Planets: Mercury and Venus.
• The planets outside the orbit of the earth are called the Superior Planets: Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.
• The planets inside the asteroid belt are termed the Inner Planets (or the Terrestrial Planets): Mercury, Venus, Earth, and Mars.
• The planets outside the asteroid belt are termed the Outer Planets: Jupiter, Saturn, Uranus, Neptune, and Pluto.
• The planets sharing the gaseous structure of Jupiter are termed the Gas Giant (or Jovian) Planets: Jupiter, Saturn, Uranus, and Neptune.

Minor planets, moons and comets????????????????????????

• Less than 0.00001 Earth masses: asteroidan
• to 0.1 Earth masses: mercurian

Terrestrial planets (rocky composition)

• 0.1-0.5 Earth masses: subterran
• 0.5-2 Earth masses: terran (Earths)
• 2-10 Earth masses: superterran (super-Earths)

Orbits

An orbit is indeed a regular, repeating path that one celestial object, like a planet or a moon, takes around another, due to the gravitational pull between the two objects. A satellite, in the broadest sense, is a body in space that orbits around a larger body.

Satellites can be natural or man-made:

• Natural satellites are celestial bodies that orbit a planet or other larger body. For example, the moon is a natural satellite of the Earth.
• Artificial satellites are man-made devices that are launched into space and orbit around the Earth or another body in the solar system. They are used for various purposes, such as communication, weather monitoring, scientific research, navigation, etc. The International Space Station (ISS) is an example of a man-made satellite.

The sun, being the most massive object in our solar system, holds all the planets, comets, asteroids, and other objects in their respective orbits due to its strong gravitational force. These orbits generally lie along an imaginary plane known as the ecliptic plane. This plane is essentially the Earth's orbital plane projected outwards to the celestial sphere. It's important to note that while most orbits in the solar system are close to the ecliptic plane, they are not all perfectly aligned with it. Some orbits, like those of Pluto and many comets, are tilted or "inclined" relative to this plane.

Orbital mechanics, or the study of the motions of artificial satellites and space vehicles moving under the influence of forces such as gravity, is a critical aspect of space exploration and satellite deployment. Understanding these principles allows scientists and engineers to predict the movements of these objects and carry out successful space missions.

The shape of an orbit can vary greatly depending on the velocity and angle at which an object enters its orbit.

Most of the planets in our solar system have nearly circular orbits, meaning their ellipses have low eccentricity. Eccentricity is a measure of how much the orbit deviates from being a perfect circle. An eccentricity of 0 would be a perfect circle, while an eccentricity close to 1 would be a very elongated ellipse.

Comets, on the other hand, typically have highly eccentric orbits. This means that their distance from the Sun can vary greatly during their orbit. When they are closest to the Sun (at perihelion), they can be within the inner solar system, but at their farthest point (at aphelion), they can be many times the distance of the outer planets from the Sun.

When a satellite is closest to Earth, it's at its perigee. When it's farthest from Earth, it's at its apogee. The Moon's perigee and apogee, for example, can vary by several thousand kilometers. This is why we sometimes have "super-moons" where the Moon appears slightly larger in the sky - this happens when the Moon is full or new at the same time as it's at its perigee.

We thank ?pek Seyitoglu , the founder, researcher and content creator of LUX Space Science & Technology - LSS , who wrote this article.

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