My Science School

According to the International Astronomical Union (IAU), a dwarf planet is a celestial body that is in orbit around a star: massive enough to be rounded by its own gravity: but has not cleared the neighbourhood around its orbit, and is not natural satellite.

The key difference between a planet and a dwarf planet is that a dwarf planet has not become gravitationally dominant enough to clear the neighbourhood around its orbit. In other words, it shares its orbital space with other celestial bodies of similar size. There are five recognised dwarf planets in our solar system - Pluto, Ceres, Haumea, Makemake and Eris. Pluto was earlier classified as a planet, but it was stripped of its status in 2006, when the IAU formalised the definition of a planet and a dwarf planet. Pluto orbits in a disc-like zone beyond the orbit of Neptune called the Kuiper Belt, a region populated with frozen bodies left over from the solar system's formation.

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PLANETS

Hurtling around the Sun are eight planets. Those closest to the Sun - Mercury, Venus, our home planet Earth, and Mars - are made of rock. The vast outer planets - Jupiter, Saturn, Uranus, and Neptune - are called “gas planets” because all we see of them is their gas. All eight travel in the same direction around the Sun. The time taken to make one circuit, or orbit, increases with distance. Mercury takes just 88 Earth days to orbit, while Neptune’s longer journey takes 164.8 Earth years.

1. JUPITER: The largest and most massive planet, Jupiter is also the fastest spinner, rotating once on its own axis in less than 10 hours. This giant world is made mainly of hydrogen and helium, with a central rocky core. A thin faint ring encircles Jupiter, which also has a large family of moons.
2. SATURN: Sixth from the Sun, and second largest, is pale yellow Saturn. Its distinctive feature is its ring system, which is made of billions of pieces of dirty water ice. Saturn is mainly hydrogen and helium with a rocky core. It has a large family of moons.
3. URANUS: Nineteen times the distance of Earth from the Sun, Uranus is a cold, turquoise world bounded by a layer of haze. A sparse ring system encircles the planet’s equator. Uranus is tilted on its side, so that its rings and moons seem to orbit it from top to bottom.
4. MERCURY: Mercury is a dry ball of rock, covered by millions of impact craters. It is the smallest planet, the closest to the Sun, and has the widest temperature range of any planet. During the day it is baking hot, but at night it is freezing cold.
5. VENUS: Second from the Sun, Venus is the hottest planet. This rock world is permanently covered by thick cloud that traps heat and makes it a gloomy planet.
6. NEPTUNE: Neptune is the most distant, coldest, and windiest of all eight planets. Like Uranus, it is made mainly of water-, methane-, and ammonia ices with an atmosphere of hydrogen- rich gas. It is encircled by a thin ring system and has a family of moons.
7. MARS: Sometimes called the “red planet”, Mars is the outermost of the rocky planets and a cold, dry world. It has polar ice caps, giant volcanoes, frozen desert, and deep canyons, formed in the distant past. Mars has also two small moons.
8. EARTH: The only place known to have life is Earth, the largest of the rocky planets and third from the Sun. It is also the only planet with liquid water. Movements in Earth’s crust are constantly changing its surface. Earth has one moon.
9. DWARF PLANETS: The Solar System has five known dwarf planets - small, roundish objects that orbit the Sun amongst other objects. Ceres orbits between Mars and Jupiter within a belt of rocky asteroids, while Pluto, Haumea, Makemake, and Eris are icy worlds that orbit beyond Neptune in a region called the Kuiper Belt.

PLANET SCALES Jupiter, fifth planet from the Sun, is much larger than all the other planets. It measures 142,984 km (88,846 miles) across and is made of about two and a half times as much material as all the other planets put, together. The seven other planets and the dwarf planets are shown here roughly to scale.

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MOONS

The Solar System has more than 190 moons orbiting six of the planets — only Mercury and Venus are moonless. They range in size from Ganymede, a satellite of Jupiter, which is larger than Mercury, to S/2009 S1, a 300-metre moonlet orbiting within Saturn’s rings. All are made of rock, or rock and ice, and many have surfaces littered with impact craters, formed when the moons were bombarded by asteroids in the past. Nineteen Solar-System moons are more than 400 km (250 miles) wide. These large moons are round but the more numerous smaller moons are irregular in shape.

1. THE MOON: The Moon is Earth’s only natural satellite. It is about a quarter the size of Earth and the fifth largest of all moons. The surface of this dry ball of rock is covered in impact craters.
2. IO: Colourful lo is the most volcanic moon in the Solar System. Its surface is constantly being renewed as molten rock erupts through its thin silicate-rock crust, and fast-moving columns of cold gas and frost grains shoot up from surface cracks.
3. EUROPA: The smallest of Jupiter’s four major moons, Europa has an icy surface criss-crossed with networks of brownish grooves. The crust slowly drifts around on top of a deep ocean of liquid water that might be home to alien life.
4. GANYMEDE: At 5,262 km (3,267 miles) across, Ganymede is the largest moon in the Solar System and belongs to the largest family of moons - the moons of Jupiter. Astronomers know of 79, but the number is likely to rise as smaller moons are detected. Ganymede is made of rock and ice with an icy crust.
5. TITAN: Titan is the largest of Saturn’s 62 moons. On its surface are bright highlands, dark plains, and methane lakes and seas. It is the only moon with a substantial atmosphere, which is rich in nitrogen and extends out for hundreds of kilometres.
6. TITANIA: Titania is the largest of the 27 moons orbiting Uranus. Titania and the planet’s other major moons, Oberon, Umbriel, Ariel, and Miranda, are named after characters in English literature. Impact craters and large cracks are seen on its grey, icy surface.
7. IAPETUS: Iapetus is a moon of contrasts. Most of its crater-covered terrain is bright and icy, but the rest appears to be coated by a dark material. It is one of Saturn’s seven major moons, along with Titan, Rhea, Dione, Tethys, Enceladus, and Mimas.
8. TRITON: Triton is by far the largest of Neptune’s 14 moons, a rock-and-ice ball with a young, icy surface. It is nicknamed the cantaloupe as its linear grooves, ridges, and depressions resemble a melon’s skin.
9. SMALL MOONS Most moons are less than 400 km (250 miles) across and irregular in shape, like Saturn’s Epimetheus and Hyperion. Many of these smaller moons, like Mars’s two moons Phobos and Deimos, may have started off as asteroids or comets.

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UNIVERSE

The Universe is everything that exists, from the smallest particle on Earth to the vast galaxies of deep space. Every part of it, including space and time, came into existence in the Big Bang - a huge explosion that occurred about 13.8 billion years ago. At that time, the Universe looked nothing like it does today, and it has been expanding, cooling, and changing ever since. The hydrogen and helium of the very young Universe formed stars, which in turn produced all the other elements in today’s Universe, including those that make Earth and everything on it, including you.

COSMIC BACKGROUND By looking at the heat left over from the Big Bang, known as the cosmic microwave background radiation, scientists are able to build up a picture of the early Universe. This image is a heat map of the Universe 380,000 years after the Big Bang. It shows that matter was not evenly distributed - the hotter areas (red) are more densely packed regions, where galaxies will form.

THE BIG BANG In the beginning, the Universe was unimaginably small, dense, and incredibly hot. Within a trillionth of a second it ballooned from being smaller than an atom to bigger than a galaxy. It was made of tiny particles of energy that turned to particles of matter. Within three minutes, the Universe was made almost entirely of the nuclei of hydrogen and helium atoms.

FORMATION OF GALAXIES Over millions of years, hydrogen and helium clumped together to form vast clouds. These broke into fragments, which collapsed under gravity and became stars. About one billion years after the Big Bang, a Universe of dwarf galaxies had formed. These collided, merged, and changed shape to become spiral and elliptical galaxies.

BIRTH OF THE SOLAR SYSTEM The Solar System formed from a cloud of gas and dust within the disc of the Milky Way galaxy. The spinning cloud, known as the solar nebula, collapsed in on itself under the force of gravity. It first formed a central sphere - the young Sun - and then the unused material surrounding the Sun formed the planets, moons, asteroids, and comets.

YOUNG MILKY WAY The Milky Way galaxy, the galaxy we live in, formed at the same time as the other galaxies. Not all of today’s Milky Way stars existed at that time. Since its beginning, the galaxy has produced stars that shine brightly for millions or billions of years, but that dies eventually. Their remains produce a new generation of stars.

FORMATION OF THE MOON Earth formed as ever-larger lumps of unused material collided and joined together. Young Earth was hit by a Mars-sized asteroid. Molten rock from the collision splashed into space. This formed a ring of rubble around Earth which clumped to form a large sphere – Earth’s Moon.

HOME PLANET Earth, the third rock planet from the Sun, is the only place in the Universe where life is known to exist. Life started in its oceans at least 3.7 billion years ago. Bacteria-like cells evolved into sea creatures, then land-based plants, and animals. Humans first walked on Earth about 1 million years ago.

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GALAXIES

A galaxy is a vast group of stars held together by gravity — it is thought that there could be some 2 trillion in our Universe. They are not scattered randomly but exist in clusters, vast distances apart. All the galaxies together take up just two millionths of space.

1. SIZE Galaxies are huge. The largest are more than a million light-years across (one light-year is the distance that light travels in a year). The smallest, called dwarf galaxies are a few thousand light-years wide. Andromeda measures 220,000 light-years from side to side.
2. SHAPE A single galaxy is made of billions or trillions of stars arranged in one of four basic shapes: spiral, barred spiral, elliptical, or irregular. Spirals and barred spirals are disc-shaped with arms of stars. In a spiral, such as Andromeda, the arms wind out from a central bulge, while in a barred spiral, they flow from the ends of a central bar of stars. Elliptical galaxies are ball-shaped. Irregular galaxies have no clear shape.
3. ORBITING STARS Galaxies do not behave like solid objects. Each star follows its own orbit around the centre of the galaxy. Stars in a spiral galaxy typically take a few hundred million years to make an orbit. Those further away take longer than those closest to the core.
4. SPIRAL ARMS Stars exist throughout a spiral galaxy’s disc. The arms simply stand out because they are full of very bright young stars.
5. CORE The core of a spiral galaxy typically consists of old red and yellow stars, with a supermassive black hole in its centre. Andromeda’s black hole is as massive as 30 million Suns.
6. DUST LANES Dense clouds and lanes of dust within the galaxy’s disc hide stars from view.
7. DWARF GALAXY M110 is one of the dwarf elliptical galaxies that orbit Andromeda. It is held in its orbit by Andromeda’s gravity.

ANDROMEDA GALAXY Andromeda is one of the closest galaxies to our own, the Milky Way. It is a spiral galaxy 2.5 million light-years away from us - the most distant object that can be seen by the naked eye from Earth.

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Watching the full Moon rise over the horizon can be a spectacular experience. The Moon often looks humongous during this time, making people pull out their smartphones and capture the scene. As the night passes, the same Moon begins to look smaller higher up in the sky. But is there actually a difference in the size?

It's all in our head

The Moon never really changes in size. It remains the same whether you see it at the horizon or higher up in the sky. However, to our eyes the Moon looks different in size due to illusion. This illusion is popularly termed Moon Illusion.

Why do we see it?

Like several other mysteries that science is yet to find an answer for, there is no proven scientific explanation for Moon Illusion. However, many theories have been proposed.

Most theories on Moon Illusion revolve around how we visually perceive the world. Our brain perceives the size of objects nearer and farther away differently. Due to this, it is thought that the brain doesn't realise that the Moon's size doesn't change no matter at what point we see it in the night sky on a particular day.

Another theory is that the position and size of the objects in the foreground also plays a role. Trees, mountains, buildings or other objects in the foreground can trick the brain into thinking the Moon is closer and bigger than it actually is. Ponzo illusion is the term used to describe this effect. However, this is also not considered a perfect explanation since astronauts in orbit also see the Moon Illusion and they have no objects to act as distance cues.

Argue it out with the brain           

There are a couple of ways to prove that what we are seeing is an illusion One of the simple ways is to hold up your outstretched index finger next to the Moon. You will notice that your fingernail and the size of the Moon is the same, no matter where you spot it.

Another way to check the size is through photographs. Take a photo of the Moon when it is near the horizon and another when it is higher up. Keep the camera zoom settings the same while taking both the photos. You will notice that the Moon's width is the same side-to-side. Note that the Moon might appear a little squashed in the vertical direction when it is near the horizon because the atmosphere acts like a weak lens.

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Mercury is the Roman god of commerce, communication, travel, and thievery. Venus is the god of love and beauty in Roman mythology. Mars gets its name from the Roman god of War. Jupiter, meanwhile, is the King of Gods in Roman mythology, making the name of a fitting choice for the largest planet in our solar system. Saturn, the farthest planet that can be seen by the unaided human eye, gets its name from the Roman god of wealth and agriculture. Uranus is named after the ancient Greek deity of the skies and the heavens. Neptune, the farthest planet from the sun, is named after the Roman god of the Sea.

No Greco-Roman connect

In case you read the above paragraph, and read it again to see if you had missed out Earth in it, you can be assured that Earth isn’t mentioned in it. This is because Earth is the only planet in the solar system whose English name does not derive from Roman or Greek mythology.

The English name of the planet, Earth, comes from Old English and Germanic. It is derived from the Old English words eor(th)e, ertha, and eorpe, all of which correspond to both “ground, soil, dirt, country” and “the abode of man, the material world”. The Proto-Germanic words erde and erp-o also have similar meanings.

Common theme

While this much is known, there is no clear answer as to how exactly Earth got this name. The planet we call home, however, has many other names in many other languages. Despite the vast amount of languages spoken in the world and their very different histories, the name used for Earth in most languages is a form of a word that is connected to ‘ground’ or ‘soil’. What is Earth called in your native language?

 

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Venus, Earth‘s planetary neighbour, is our home planet‘s closest analogue in terms of mass, radius, and density in the solar system. It is, however, distinctive in its own right. While the lack of life is obvious, it is also popular due to its retrograde rotation.

243 earth days

It is well-documented that Venus has the longest day in the solar system. While it is known that Venus takes 243 Earth days to complete one rotation, scientists have been able to calculate the duration of Venusian day recently with more precision. The result? The average day on Venus based on this study is 243.0226 Earth days, give or take 0.0013 Earth days.

In order to make the measurement, researchers attempted to observe Venus on 121 instances from 2006-2020, and were successful on 21 occasions. On these 21 instances, scientists transmitted radio waves towards Venus from NASA’s Goldstone Antenna in the Mojave Desert of California, and the echo received was studied.

“A giant disco ball”

In the words of University of California, Los Angeles (UCLA) planetary astronomer professor Jean-Luc Margot, who led the study that was published in the journal Nature Astronomy in April, “each individual measurement was obtained by treating Venus as a giant disco ball”.

Based on this data, the length of the day of Venus – whose year is shorter as it completes an orbit around the sun into 25 Earth days - was calculated with greater accuracy. It was also used to measure Venus’ spin axis rotation, spin precession rate, and moment of inertia. All this, using Earth-based observations of our neighbouring planet.

 

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