GYÖRGY KOVÁCS

ON THE ORIGIN OF EARTHQUAKES

Ad Librum

Budapest, 2025

TABLE OF CONTENTS

1. The Solar System

2. The Earth

2.1. Drifting Continents

2.2. The Structure of the Earth

2.3 The Structural Composition of the Earth’s Crust

2.4 Effects of the Drifting of the Earth’s Crustal Plates

3. Tectonic Phenomena Observed on the Earth’s Crustal Plates

3.1 Steam vents

3.2 Mud Pots and Mud Volcanoes

3.3 Thermal Springs and Water Eruptions

3.4 Volcanic Eruptions

3.5 Earthquakes

4. The Underlying Cause of Earthquakes

5. The Phenomenon of Explosions Triggering Earthquakes

6. Earthquake Prediction

7. Can Earthquakes Be Prevented?

8. Earthquake-Like Events Triggered By Extraterrestrial Explosions

Bibliography

“Should enemy come your way,

seven men must hold at bay.

One who starts his day anew,

one who works to pay his due,

One who teaches kids for free,

one who’s blind but tries to see,

One who grows like seeds of oaks,

one defended by his folks.

All their might and tricks in vain,

The seventh one be you, remain!”

(Attila József: The Seventh, second stanza
– translated from Hungarian)

1. THE SOLAR SYSTEM

Our home, the Earth, orbits a star known as the Sun, and is the third planet of our Solar System. The Sun itself, along with approximately 100 billion other stars, makes up a galaxy called the Milky Way. On the other hand, this galaxy is only one of the billions of galaxies comprising the Universe. The circumstances of how the Universe was born remain shrouded in the mists of the distant past; current analyses offer only theoretical assumptions about this early period. The most widely accepted explanation for the origin of the Universe is the so-called Big Bang, which is believed to have occurred about 13.7 billion years ago. According to analyses, this theory is based on the fact that the galaxies, observed in all directions at distances of at least 10 billion light-years around us, are moving away from each other. The Big Bang theory is also supported by the cosmic background radiation detectable with microwave radiometers. It is believed that in the first few minutes following the Big Bang, only the most elementary particles known today – quarks and photons – were present in space. A few minutes later, protons, neutrons, and electrons began to form from these particles. Initially, lighter elements such as hydrogen and helium came into being, followed later by heavier ones.

Over time, gas and dust clouds formed from these particles – an interval which, according to analyses, spanned from several hundred thousand years to about one billion years after the Big Bang. Due to gravitational effects these gas and dust clouds began to condense and merged into distinct clouds, from which – during a period estimated to range from about one billion to about four billion years after the Big Bang – the so-called protogalaxies emerged. Within these protogalaxies, gases and dust clumped together into dense accumulations, forming protosolar disks. Once the density and mass of these disks reached a critical threshold, they ignited to form stars – celestial bodies capable of emitting their own light and heat. The stars observable in the surrounding galaxies are the result of these processes.

Our Solar System is located on one of the spiral arms of the galaxy called the Milky Way, moving through space at an estimated velocity of 215 km/s. Regarding its formation, a number of scientific theories have been developed seeking to explain how this complex system came into being.

The Laplacian theory – which was presented by Laplace himself in one of his popular works – envisioned the entire Solar System as initially being a massive, rotating, high-temperature body. As it cooled and contracted, its rotation accelerated, and upon reaching a critical value, a ring – similar to Saturn’s ring – formed at its edge. From this ring, the planet corresponding to that orbital path emerged. This phenomenon was thought to have repeated until all the planets were formed. […]

According to Kant’s concept, the material of the Solar System originally consisted of a cloud of meteorites. Some of the particles in this cloud gradually moved toward the center of gravity, forming the mass of the Sun, while others fell into one another and formed rings around the central sphere – similarly to the Laplacian idea – and these rings themselves eventually condensed into planets. […]

The first representative of the collision theories was the concept proposed by Buffon. According to Buffon, the Solar System formed as a result of a foreign star colliding with the Sun, and the material ejected by the collision gave rise to the individual planets. […]

A partly modernised version of Buffon’s Collision Theory is the one developed by Jeans. […] A particularly ingenious version of the collision theories suggests that the Sun was originally part of a binary star system. The companion star had a total mass equal to the current mass of the planets. A third star collided with this companion, causing it to explode, and from the scattered fragments, the planets were eventually formed. […]

The Planetesimal Theory sought to explain the origin of the planets by suggesting that, for some reason (such as the close passage of a star), tiny meteor-like particles were torn from the Sun, which then cooled and solidified. These planetesimals orbited the Sun and gradually accumulated into planets, thus forming the Solar System. […]

According to the Schmidt Theory the material of the planets does not originate from the Sun, but was instead captured by the Sun as it passed through a galactic dark nebula. The Schmidt theory presents in detail how the planets could have formed from a swarm of meteorites orbiting the Sun. It illustrates how circular orbits and synchronous rotation emerged, and how the distances between the planets developed, and so on. […] (Egyed László: A Föld fizikája [The Physics of the Earth], pp. 348–349)

In our Solar System, eight planets orbit our star, the Sun — starting with Mercury as the closest, followed by Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

The Sun is a gaseous star with a diameter of approximately 1.4 million kilometres. It consists of roughly 73% hydrogen, 25% helium, and about 2% other elements. The Sun’s age is estimated to be around 4.6 billion years, and analyses suggest that its density is approximately 1.41 g/cm³, its surface temperature is about 5,500 °C, while the temperature at its core may range between 10 and 15 million °C. The Sun’s rotation period is about 25 days near the equator and around 30 days at the poles. As a result of the nuclear fusion processes taking place inside the Sun, roughly 600 million tons of hydrogen are converted into helium every second, and about 4 × 10²³ kW of energy is emitted per second. Like other stars, the Sun was formed through the gravitational contraction of interstellar matter clouds. When the resulting temperature and pressure reached a critical value, the fusion of hydrogen nuclei began, forming helium nuclei while generating light and heat energy. An equilibrium state was established – one which, according to analyses, may last about 15 billion years in the case of the Sun – and it continues as long as the Sun contains enough hydrogen mass to sustain the fusion process. Once the hydrogen in the Sun’s core is no longer sufficient to maintain fusion, it will expand in volume, swell up, and become a red giant star. At this point, it will enter another state of equilibrium, sustained by the fusion of helium nuclei into carbon nuclei. If the helium is also depleted, the Sun – depending on its remaining mass – will either explode or collapse into a high-density white dwarf.

Mercury has a diameter of approximately 38% that of Earth (4,880 km), an average density of 5.43 g/cm³, and orbits the Sun in an elongated elliptical path at an average distance of 58 million kilometres. Its orbital period is 88 days, and its rotational period is 176 days. According to analyses, the planet’s internal structure consists of an iron core that makes up about three-quarters of its diameter, a silicate mantle, and a rocky silicate crust. The surface is covered with craters caused by meteor impacts. In addition, several hundred-kilometre-long linear features called wrinkle ridges can be observed, some reaching heights of around 3 km. These are likely of tectonic origin, and must have been formed as the planet cooled. Mercury does not have an atmosphere in the sense the term applies to Earth; however, a thin accumulation of gases around the planet can be detected through measurements and consists of helium, sodium, and oxygen, and traces of hydrogen. Due to its virtually non-existent atmosphere, the surface temperature can reach 450 °C during the day and drop to –180 °C at night. Mercury has no moons.

Venus has a diameter approximately 95% that of Earth (12,100 km), an average density of 5.29 g/cm³, and orbits the Sun in an elliptical path at an average distance of 108 million kilometres. Its orbital period is 224 days, and its rotation period is 243 days. Venus rotates in a retrograde direction, that is opposite to its orbital motion. The planet’s internal structure includes an iron-nickel core, a silicate mantle, and a rocky silicate crust. In addition to meteorite craters, the surface of Venus features vast plateaus – similar in size to Earth’s continents – such as Aphrodite Terra and Ishtar Terra, as well as mountain ranges rising from these plateaus, like the Maxwell Montes, which reach a height of about 11 kilometres. The surface is also marked by volcanic craters, suggesting tectonic formation processes. The meteorite craters are large – some exceeding 300 km in diameter – which is due to the fact that smaller meteors burn up in the planet’s dense atmosphere. The lower layer of Venus’s atmosphere is composed almost entirely of carbon dioxide, with clouds of sulphuric acid with a thickness of 50–70 km moving above them at such high speeds that they circle the planet in just around t four days. The average surface temperature exceeds 460 °C, and the atmospheric pressure is about 90 times that of Earth’s. Venus has no moons either.

Earth is a planet with a diameter of 12,756 km and an average density of approximately 5.52 g/cm³. It orbits the Sun in an elliptical path at an average distance of 149.6 million kilometres. Its orbital period is 365 days, 5 hours, 48 minutes and 46 seconds (a tropical year), while its axial rotational period is 23 hours, 56 minutes and 4.09 seconds (a sidereal day). Earth’s axis of rotation is tilted by approximately 23.5° from the perpendicular to the plane of its orbit. The structure of the Earth consists of a metallic core composed of iron and nickel with a radius of about 3,450 km, and a molten, liquid silicate mantle approximately 2,900 km thick. Only the outer crust, measuring 20 to 40 km in thickness, is solid. This solid crust is divided into six major and many smaller tectonic plates, which float on the molten rock of the mantle and shift relative to one another. The diversity of geological surface formations of Earth is due to this tectonic movement. The surface of the Earth covers an area of approximately 510 million km², about 71% of which is covered by water in the form of seas and oceans, and the continents rise from these. The highest points of the continents reach nearly 9 km above sea level, while the deepest parts of the oceans and seas reach depths of up to 11 km below average sea level. The continents are typically composed of vast lowlands, hills, and mountainous regions. Fold mountains – such as the Himalayas – formed along subduction zones, while volcanic mountains were created by molten magma from the mantle erupting through the solid crust. Meteorite craters are rare on Earth and tend to be large, as smaller meteorites burn up in the atmosphere. According to analyses about 97.2% of Earth’s water supply (roughly 1,300 million km³) is found in the global ocean, 2.15% (around 28.5 million km³) in the polar ice sheets and mountain glaciers, 0.64% (around 8.3 million km³) in freshwater lakes, rivers, and groundwater, and about 0.001% (approximately 0.013 million km³) as water vapour in the atmosphere. The total amount of water accounts for approximately 0.025% of Earth’s total mass. The Earth is surrounded by an atmosphere roughly 85 km thick, composed of 78% nitrogen, 21% oxygen, and 1% other gases. As a result of solar radiation, water vapour forms on the Earth’s surface, rises as vapour, and creates clouds at altitudes exceeding 2 km. Some of these clouds continue to rise into higher layers of the atmosphere, where, due to lower temperatures, the water vapour cools below the dew point and condenses. As a result, precipitation – in the form of rain, snow, or hail – falls back to the surface. This water cycle involves the movement of approximately 425,000 km³ of water annually. A 12 km extent of the lowest layer of the atmosphere, where cloud and precipitation formation takes place, is called the troposphere. The region between 12 and 50 km, which contains no water vapour and where no clouds form, is called the stratosphere. The layer from 50 to 85 km is known as the mesosphere, whose lower parts are characterised by high ozone concentration. Above 85 km lies the thermosphere, which gradually transitions into interplanetary space at an altitude of around 10,000 km. The temperature of the Earth’s surface is shaped by heat generated within the planet and heat received from the Sun. The amount of solar heat depends on the time of day, the season, and the latitude of the area exposed to radiation. Extreme surface temperatures on Earth can be as high as almost 60 °C and as low as –90 °C. Beyond the polar circles, the annual mean temperature ranges from –30 °C to 10 °C; between the polar circles and the tropics, from –10 °C to 30 °C; and between the tropics and the equator, from 20 °C to 40 °C. (Water freezes at 0 °C and boils at 100 °C.)

A celestial body of significant size relative to Earth, the Moon orbits our planet. With a diameter of 3,476 km, it is approximately one-quarter the diameter of Earth. It follows an elliptical orbit at an average distance of about 384,400 km from Earth. Its average density is 3.36 g/cm³, and its gravitational acceleration is only one-sixth that of Earth’s. The Moon orbits in the same direction as Earth’s rotation and has synchronous rotation – that is, it rotates on its axis (in approximately 27 days and 7 hours) in the same amount of time it takes to orbit the Earth, so it always presents the same face toward us. The Moon’s axis of rotation is tilted by 6.7° from the perpendicular to its orbital plane. The plane of the Moon’s orbit is inclined at an angle of about 5° to the plane of Earth’s orbit, which results in the occurrence of spectacular solar eclipses visible from limited areas of Earth approximately once a year. This is due to the fact that the apparent diameter of the Moon is nearly the same as that of the Sun, so when the Moon covers the Sun, it causes almost total darkness on Earth. Lunar eclipses, which occur two or three times a year and are visible from much larger parts of Earth, give the Moon a reddish hue as it passes through Earth’s shadow. The internal structure of the Moon consists of an inner core about 150–160 km thick, a surrounding outer core made up of 300–350 km of molten rock, and a mantle approximately 1,200 km thick, the inside of which may be partially molten. Above this lies the solid crust, about 20 km thick on the side facing Earth and about 60 km on the far side. Around 100 moonquakes are triggered every month in the lower part of the upper mantle. Space probes have identified the presence of water compounds in material from northern lunar craters, with analyses estimating their volume at around 600 million cubic metres. The Moon does not have an atmosphere in the sense the term applies to Earth. The lunar surface is covered by vast plains, referred to as “seas”, as well as circular depressions, ring-shaped mountain ranges, and craters. Most of these features were formed by meteorite impacts, while a smaller proportion resulted from volcanic activity. In addition to chasms deeper than 1 km, mountain peaks on the Moon can reach elevations of up to 6 km. Surface temperatures on the Moon range between 130 °C and –150 °C. The Moon’s gravitational pull is responsible for the tidal phenomenon seen on Earth, which causes the periodic rise and fall of sea and ocean levels. The same effect also leads to a slight slowing of Earth’s rotation over time.

Mars has a diameter approximately 53% that of Earth (6,794 km), with an average density of 3.94 g/cm³, and orbits the Sun in an elliptical path at an average distance of 228 million kilometres. Its orbital period is about 687 days, while its sidereal day lasts 24 hours and 37 minutes. The planet’s axis of rotation is tilted by about 25.2° relative to the perpendicular to the plane of its orbit. Based on the presence of several volcanic craters Mars is believed to have had a relatively small molten metallic core and a molten silicate mantle at the time of its formation – a structure that, based on recent reports of mars-quakes, still persist today. (“InSight is the first probe to detect seismic activity on Mars…” [see pp. 194–196]). The planet’s rocky, solid crust is covered by dust-filled formations reminiscent of deserts, their reddish hue resulting from the presence of iron oxide. In addition to numerous impact craters caused by meteorites, the surface of Mars features rift valleys and fractures extending for thousands of kilometres attributed to tectonic plate movements, as well as thousands of basins and channels originating from an earlier period when water was present on the planet, and volcanic peaks formed by past volcanic activity. Particularly notable among these formations is Olympus Mons – a volcanic cone rising over 21 km and spanning nearly 600 km at its base. The atmosphere of Mars consists mainly of carbon dioxide, with a surface pressure of just 0.6–0.7% that of Earth’s atmosphere. At the equator, daytime temperatures can reach 15–20 °C, but at night they can drop to –70 °C. In the polar regions, nighttime temperatures may fall below –130 °C. Mars has two moons, Phobos and Deimos, relatively small compared to Mars, whose irregular shapes suggest that they are asteroids captured by the planet’s gravity and forced into orbit around it. Phobos has approximate dimensions of 28 × 23 × 20 km and orbits the planet at a distance of 9,270 km, completing a revolution in 7 hours and 39 minutes. Deimos measures approximately 16 × 12 × 10 km and orbits at a distance of 23,400 km, with an orbital period of 30 hours and 21 minutes. The surfaces of both moons are marked by a significant number of meteorite impacts.

Between Mars and the next planet of the Solar System, Jupiter, lies the inner asteroid belt, located at an average distance of approximately 2.9 astronomical units (AU) from the Sun. Interestingly, based on the Titius–Bode law, astronomers once assumed that a planet of similar size to the other planets of the Solar System would exist at this location. Instead, thousands of small celestial bodies – asteroids – were discovered within this zone. Most of their orbits lie between 2.2 and 4.5 AU, but some of them have orbits that reach inside that of Mars, or even inside Earth’s orbit, while others extend beyond the orbit of Saturn. The diameters of the asteroids in the inner belt range from a few hundred kilometres to just a few kilometres. The largest among them is Ceres, discovered by Giuseppe Piazzi in 1801 as the first asteroid; its diameter is about 930 km. The total number of asteroids is estimated to be in the hundreds of thousands and, according to analyses, their combined mass may be comparable to that of a planet the size of Mars. Contrary to the earlier popular theory whereby asteroids are fragments of a destroyed large planet, current analyses suggest that the inner asteroid belt formed because the gravitational disturbance caused by Jupiter prevented the smaller, condensed matter clusters in this region from coalescing into a larger planet during the early stages of the Solar System’s formation. This theory may also provide an explanation for the so-called Trojan asteroids, which orbit along Jupiter’s path and have accumulated at positions approximately 60 degrees ahead of and behind the planet.

Jupiter is the largest planet in the Solar System, with a diameter about 11 times that of Earth (142,796 km). Its average density is 1.34 g/cm³, and it orbits the Sun at a distance of 5.2 AU. Its orbital period is 11.9 years, and its rotation lasts 9 hours and 50 minutes. The planet’s axis of rotation is tilted by about 3.8° relative to the axis perpendicular to its orbital plane. Jupiter is noticeably flattened at the poles, with an oblateness ratio of approximately 1:16. According to analyses, Jupiter’s internal structure includes a relatively small core – possibly composed of metals and rocks – surrounded by solid, electrically conductive metallic hydrogen, liquid hydrogen and helium, and finally a gaseous atmosphere composed of hydrogen (about 90%) and helium (about 10%) extending approximately 1,000 km in thickness. The most distinctive feature of Jupiter’s cloud layer is its banded structure parallel to the equator, consisting of brighter zones and darker belts. These bands move at different speeds and, in some cases, in opposite directions. Wind speeds in the atmosphere can exceed 500 km/h. The most prominent feature in the atmosphere is the Great Red Spot, an anticyclonic oval formation with a major axis exceeding 40,000 km. According to measurements by space probes, Jupiter emits roughly twice as much energy as it receives from the Sun, indicating significant internal heat production, which also results in higher temperatures at the poles due to the planet’s oblateness. At the upper level of the cloud layer, temperatures remain around –140 °C regardless of the time of day, but increase at lower depths. Jupiter possesses a powerful and extensive magnetic field, oriented in the opposite direction to that of Earth, which extends sunward up to 70 times the diameter of Jupiter and stretches beyond Saturn’s orbit in the opposite direction. Jupiter has sixteen moons and a thin ring system. Its four largest moons were discovered by Galileo as early as 1610. Among them, Io has a diameter of 3,630 km and a density of 3.5 g/cm³, orbiting Jupiter at a distance of 422,000 km with an orbital period of 1.8 days. A remarkable feature of Io is its volcanic activity, observed by the Voyager space probes, which suggests that Jupiter’s gravitational influence keeps its internal structure in a molten state. Beneath the molten interior lies a several-kilometre-deep sulphur ocean, overlaid by a thin, solid crust of frozen sulphur. Volcanic eruptions beneath the sulphur ocean squeeze liquid sulphur through the surface of the frozen crust. Europa, one of Jupiter’s moons, has a diameter of 3,038 km. It orbits the planet at a distance of 671,000 km and completes a revolution every 3.6 days. Its surface is covered by a solid ice crust marked by massive cracks. According to analyses, a subsurface ocean of approximately 60–80 km deep lies beneath a 10–15 km thick ice shell. From the fractures in the ice crust, geysers occasionally erupt. Ganymede, another moon boasting a diameter of 5,262 km – making it even larger than Mercury – orbits Jupiter at a distance of 1,070,000 km with a period of 7.2 days. Its surface features vast dark regions partially covered with impact craters, as well as grooved, lighter-toned bands, both indicative of internal activity. Callisto, another moon of Jupiter, has a diameter of 4,808 km and a density of 1.8 g/cm³. It orbits the planet at a distance of 1,883,000 km, taking 16.7 days to complete one orbit. Its dark surface is densely cratered due to meteoritic impacts. Jupiter’s other moons are smaller than the Galilean moons.

Saturn is the second-largest planet in the Solar System, with a diameter more than nine times that of Earth (120,660 km). Its average density is 0.69 g/cm³, and it orbits the Sun at a distance 9.6 times greater than that between the Earth and the Sun. Saturn’s orbital period is 29.46 years, and it rotates on its axis in 10 hours and 14 minutes. The planet’s rotational axis is tilted at about 26.8° relative to the perpendicular of its orbital plane. Saturn is significantly flattened at the poles, with a flattening ratio of approximately 0.1. According to analyses, Saturn’s structure comprises a small core of rock, relative to its size, surrounded by layers of solid metallic hydrogen (electrically conductive and exhibiting metallic properties), liquid hydrogen and helium, and finally an atmosphere composed of gaseous hydrogen (approx. 96%) and helium (approx. 4%). Its cloud cover exhibits a banded structure parallel to the equator, consisting of alternating light and dark zones. These atmospheric zones move in the same direction but at varying speeds, with maximum wind speeds in the central regions of each band reaching up to 1,800 km/h. Like Jupiter, Saturn emits more energy than it receives from the Sun, indicating the presence of internal heat sources. At the upper levels of its cloud cover, the temperature is around –185 °C. Saturn is known to have more than thirty moons. The largest of them is Titan, with a diameter of 5,150 km and an orbital period of 16 days. It is the only one of Saturn’s moons to possess a substantial atmosphere, composed primarily of nitrogen and methane. The Enceladus moon has shown evidence of volcanic activity caused by the melting of ice, as detected by space probes. Some of Saturn’s smaller moons share orbital paths with larger ones. The average density of Saturn’s moons does not exceed 1 g/cm³, suggesting that their structure consists of ice mixed with rock. Saturn’s most spectacular feature is its ring system, which spans about 420,000 km in width but is only a few hundred metres thick. The rings are divided into segments of varying widths and brightness, separated by gaps. They are composed of ice and rock fragments ranging in size from about 10 metres to a few micrometres.

Uranus is the third-largest planet in the Solar System and was discovered in 1781 by the British astronomer William Herschel. Its diameter is about four times that of Earth (51,120 km). Its average density is 1.32 g/cm³, and it orbits the Sun at a mean distance 19.1 times greater than the Earth-Sun distance. Its orbital period is approximately 84 years, and its retrograde axial rotation takes 17 hours and 14 minutes. The planet’s axis of rotation is tilted by about 98° relative to the perpendicular to the plane of its orbit, meaning it is nearly aligned with the plane itself. This results in 42 years of darkness and 42 years of daylight at the poles. According to analyses, Uranus is composed of a rocky inner core, a mantle of water ice, methane and ammonia, and an atmosphere consisting of gaseous methane (2.3%), helium (15.2%), and hydrogen (approx. 82.5%). Winds in the atmosphere blow in the same direction as the planet’s rotation. The average atmospheric temperature is around −215 °C. Uranus has eleven thin rings and twenty-one moons, all of which orbit within the plane of the planet’s highly inclined equator. The rings are composed of rocky debris less than 1 metre in diameter. Temperature measurements indicate that the hemisphere facing the Sun is not warmer than the equatorial regions with low solar elevation, suggesting the presence of internal heat production. Uranus is surrounded by a strong magnetic field. Its greenish colour may be attributed to methane clouds.

Neptune is the fourth-largest planet in the Solar System and was discovered in 1846 by J. G. Galle, based on calculations by U. J. Le Verrier and the theoretical predictions of F. W. Bessel, who inferred the existence of another planet beyond Uranus due to irregularities in Uranus’s orbit. Neptune’s diameter is about 3.9 times that of Earth (49,520 km). Its average density is 1.64 g/cm³ and it orbits the Sun at a mean distance 30.2 times greater than that between Earth and the Sun. Its orbital period is about 165 years, and its retrograde axial rotation lasts 18 hours and 7 minutes. The planet’s rotational axis is tilted by about 28.8° relative to the perpendicular to the plane of its orbit. According to analyses, Neptune has a molten rocky core, a mantle of water ice, methane and ammonia, and an atmosphere composed of gaseous hydrogen (80%), helium (19%), and methane (1%), which gives the planet its bluish colour. The average temperature is around −215 °C. Neptune has at least four rings and eight moons and is surrounded by a magnetic field similar in strength to that of Earth. On its moon Triton, which has a diameter of 2,720 km, geyser-like features have been observed, suggesting volcanic activity.

Beyond Neptune’s orbit lies the outer asteroid belt known as the Kuiper Belt, whose existence was hypothesised in 1951 by Gerard P. Kuiper. According to analyses, this belt spans a region from approximately 30 to 50 times the Earth-Sun distance and is currently known to contain around 1,000 objects. The largest known object in the Kuiper Belt is Pluto, which was long considered the ninth planet of the Solar System, but was reclassified as a dwarf planet nearly seventy years after its discovery. Pluto was discovered by Clyde W. Tombaugh in 1930, after the discovery of Neptune did not fully account for the orbital irregularities observed in Uranus, prompting a search for another planet in the early 20th century. Pluto’s diameter is about 18% that of Earth (2,280 km), its mass is approximately 0.012 times Earth’s mass, and its average density is around 2 g/cm³. Pluto completes an orbit around the Sun in 248 years at an average distance 39.5 times greater than that between Earth and the Sun. Its orbit is highly eccentric and tilted by 17° relative to the ecliptic. Pluto’s axial rotation lasts 6 days and 10 hours. It is generally believed to consist of a rocky core, a mantle of water ice, and a crust of frozen methane. Methane has also been detected in its atmosphere, and surface temperatures are estimated to be around −220 °C. Pluto has one moon, Charon, which has a diameter of about 1,200 km and orbits at a distance of 19,600 km.

“It was only recently discovered that Pluto and Charon are mutually tidally locked, meaning they always show the same face to one another. This configuration is unique in the entire Solar System, which is why the term “binary planet” is often considered appropriate in their case.” (Henrik Hargitai, PhD student in Geography at the Faculty of Science, Eötvös Loránd University, https://planetologia.elte.hu/1plu- toch.html)

The Oort Cloud, a cometary zone named after Jan H. Oort, is located at the edge of the Solar System, in a region extending approximately 50,000 times the average Earth–Sun distance. (Certain analyses estimate the outer boundary of the Oort Cloud to extend as far as 100,000 or even 200,000 times that distance.) The outermost limit of the Oort Cloud marks the gravitational boundary of the Solar System. According to analyses, the Oort Cloud can be divided into an inner Oort Cloud, which is thought to have a torus-like (doughnut-shaped) structure, and an outer Oort Cloud, which is presumed to be spherical in shape. The number of objects in the Oort Cloud is estimated in the billions, and a significant portion of them are comets composed of frozen gases (such as water, ammonia, and methane ice) and rocky fragments. The Oort Cloud is only weakly bound gravitationally to the Solar System, which means the gravitational influence of the Milky Way or of nearby passing stars may dislodge some comets from their orbits within the cloud, redirecting them toward the Solar System – or toward the stars themselves. According to one group of theories, the objects of the Oort Cloud originally formed closer to the Sun and were later scattered into this distant region by the gravitational influence of Solar System planets. Other analyses suggest that many of these objects were captured by the Solar System from other stars. Short-period comets, some of which are believed to originate from the inner Oort Cloud, follow orbits that pass close to the Sun and remain near the plane of the ecliptic. In contrast, long-period comets, which come from the outer Oort Cloud, may arrive from any direction in space. As a comet approaches the Sun, the heat causes gases and dust to escape from the comet’s nucleus, forming a spectacular tail that always points away from the Sun. The most famous example is Halley’s Comet, whose orbit was the first to be calculated (by Halley in 1705). Some comets eventually collide with the Sun or the planets. A notable example is Comet Shoemaker–Levy 9, which broke apart to several pieces and impacted Jupiter in 1994. Cosmic objects that enter Earth’s atmosphere are called meteors. Some of these originate from cometary nuclei. Most meteors burn up in the atmosphere, producing bright streaks of light known as shooting stars. However, larger meteors may survive entry and cause natural disasters, such as the Tunguska event in 1908, when a meteor exploded in the atmosphere over Siberia, flattening trees within a zone with a radius of several tens of kilometres. Meteors that reach the surface of the Earth are called meteorites, which are typically composed of iron, or a mixture of iron and stone. Upon impact, large meteorites can create craters several kilometres in diameter. Some 120 meteorite craters are currently known on Earth. The largest of these reach diameters of up to 100 kilometres (e.g. the Amirante Basin in the Indian Ocean, 300 km; Sudbury, Ontario, 140 km; Popigai, Russia, 100 km; Vredefort, South Africa, 100 km; Manicouagan, Canada, 100 km; and Sierra Madera, Texas, 100 km). The destructive power of impacting meteorites is illustrated by the Barringer Crater (Arizona), which is about 1.2 kilometres in diameter and 175 metres deep. According to analyses, it was caused by the impact of a nickel–iron meteorite measuring approximately 40 metres in diameter.