My Science School

Uranus and Neptune are called ice giants because they are smaller and compositionally different from Jupiter and Saturn, the gas giants. Jupiter and Saturn are composed of mostly hydrogen and helium, with large mantles of metallic hydrogen (which acts like a metal, due to the pressure and temperature within these planets) and only small cores of rock and ice. This is why they are called gas giants: They are mostly gaseous, with very little rock and ice.

Uranus and Neptune are composed of some hydrogen and helium, but they also contain heavier elements such as oxygen, carbon, nitrogen, and sulfur. Beneath their relatively thin outer shells of hydrogen and helium, these planets’ mantles are largely made of compressed, slushy water and ammonia. The ice giants’ rocky, icy cores are also proportionally larger than the amount of gas they contain, unlike the gas giants. This is why Uranus and Neptune are called ice giants.

The “ice giant” terminology took hold in the 1990s when researchers realized Uranus and Neptune were compositionally different from Jupiter and Saturn. Classifying them differently better reflects the variations in the formation of the outer planets, giving astronomers a clearer picture of how our solar system and others formed.

Credit : Astronomy.com 

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Triton is the largest of Neptune's 13 moons. It is unusual because it is the only large moon in our solar system that orbits in the opposite direction of its planet's rotation?a retrograde orbit.

Scientists think Triton is a Kuiper Belt Object captured by Neptune's gravity millions of years ago. It shares many similarities with Pluto, the best known world of the Kuiper Belt.

Like our own moon, Triton is locked in synchronous rotation with Neptune?one side faces the planet at all times. But because of its unusual orbital inclination both polar regions take turns facing the Sun.

Triton has a diameter of 1,680 miles (2,700 kilometers). Spacecraft images show the moon has a sparsely cratered surface with smooth volcanic plains, mounds and round pits formed by icy lava flows. Triton consists of a crust of frozen nitrogen over an icy mantle believed to cover a core of rock and metal. Triton has a density about twice that of water. This is a higher density than that measured for almost any other satellite of an outer planet. Europa and Io have higher densities. This implies that Triton contains more rock in its interior than the icy satellites of Saturn and Uranus.

Triton's thin atmosphere is composed mainly of nitrogen with small amounts of methane. This atmosphere most likely originates from Triton's volcanic activity, which is driven by seasonal heating by the Sun. Triton, Io and Venus are the only bodies in the solar system besides Earth that are known to be volcanically active at the present time.

Triton is one of the coolest objects in our solar system. It is so cold that most of Triton's nitrogen is condensed as frost, giving its surface an icy sheen that reflects 70 percent of the sunlight that hits it.

NASA's Voyager 2?the only spacecraft to fly past Neptune and Triton?found surface temperatures of -391degrees Fahrenheit (-235 degrees Celsius). During its 1989 flyby, Voyager 2 also found Triton has active geysers, making it one of the few geologically active moons in our solar system.

Credit : NASA Science 

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Neptune has five rings: Galle, Le Verrier, Lassell, Arago, and Adams. Its rings were named after the astronomers who made an important discovery regarding the planet. The rings are composed of at least 20% dust with some of the rings containing as much as 70% dust; the rest of the material comprising the rings is small rocks. The planet’s rings are difficult to see because they are dark and vary in density and size.  Astronomers think Neptune’s rings are young compared to the age of the planet, and that they were probably formed when one of Neptune’s moons was destroyed.

The Galle ring was named after Johann Gottfried Galle, the first person to see the planet using a telescope. It is the nearest of Neptune’s rings at 41,000–43,000 km.  The La Verrier ring was named after the man who predicted Neptune’s position. Very narrow, this ring is only about 113 kilometers wide. The Lassell ring is the widest of Neptune’s rings. Named after William Lassell, it lies between 53,200 kilometers and 57,200 kilometers from Neptune, making it 4,000 kilometers wide.  The Arago ring is 57,200 kilometers from the planet and less than 100 kilometers wide.

The outer ring, Adams, was named after John Couch Adams who is credited with the co-discovery of Neptune. Although the ring is narrow at only 35 kilometers wide, it is the most famous of the five due to its arcs. Adams’ arcs are areas where the material of the rings is grouped together in a clump. Although the Adams ring has five arcs, the three most famous ones are Liberty, Equality, and Fraternity. The arcs are the brightest parts of the rings and the first to be discovered. Scientists are unable to explain the existence of these arcs because according to the laws of motion they should distribute the material uniformly throughout the rings.

The rings of Neptune are very dark, and probably made of organic compounds that have been baked in the radiation of space. This is similar to the rings of Uranus, but very different to the icy rings around Saturn. They seem to contain a large quantity of micrometer-sized dust, similar in size to the particles in the rings of Jupiter.

Credit : Universe Today 

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Neptune is the fourth largest planet in terms of diameter, making it the smallest in physical size of the gas giants. The average distance from the center of the planet to its surface is 15,299 miles (24,622 kilometers). But like most spinning bodies, Neptune's rotation causes it to bulge slightly around the equator. The resulting shape is known as an oblate spheroid. The radius at the poles is 15,125 miles (24,341 km), slightly smaller than the equatorial radius of 15,388 miles (24,764 km). The average diameter across the planet is 30,598 miles (49,244 km), almost four times the diameter of Earth.

Although Neptune comes in fourth in terms of diameter, it ranks third in terms of mass, ahead of Uranus. The gas giant weighs in at 1.02 x 1026 kilograms, or 102 trillion trillion kilograms. It is more than seventeen times as massive as Earth.

The rock, ices, and gas that make up the icy giant fill a volume of 15 trillion cubic miles (62 trillion cubic kilometers), almost 58 times the volume of Earth.

The density of Neptune is 1.638 grams per cubic centimeter. The low density indicates that, like Uranus, its atmosphere is made up of more ices than Saturn and Jupiter, causing scientists to call it an "icy giant". The distance to Neptune from the sun keep the planet's temperature low through the year, although some astronomers suspect the planet originally formed closer to the star.

Despite hosting a significantly lower mass, Neptune's surface gravity is second only to Jupiter.

Credit : Space.com

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Dark matter is a mysterious substance thought to compose perhaps about 27% of the makeup of the universe. What is it? It’s a bit easier to say what it isn’t.

It isn’t ordinary atoms, the building blocks of our own bodies and all we see around us. Atoms make up only somewhere around 5% of the universe, according to a cosmological model called the Lambda Cold Dark Matter Model (aka the Lambda-CDM model, or sometimes just the Standard Model).

Dark matter isn’t the same thing as dark energy. Dark energy makes up some 68% of the universe, according to the Standard Model.

Dark matter is invisible; it doesn’t emit, reflect or absorb light or any type of electromagnetic radiation such as X-rays or radio waves. Thus, instruments can’t detect dark matter directly, as all of our observations of the universe, besides detecting gravitational waves, involve capturing electromagnetic radiation in our telescopes.

Thirty years later, astronomer Vera Rubin provided a huge piece of evidence for the existence of dark matter. She discovered that the centers of galaxies rotate at the same speed as their extremities. They should rotate faster. Think of a vinyl LP on a record deck: its center rotates faster than its edge. That’s what logic dictates we should see in galaxies too. But we do not. The only way to explain this is if the whole galaxy is only the center of some much larger structure. Imagine it as only the label on the LP, causing the galaxy to have a consistent rotation speed from center to edge.

Vera Rubin, following Zwicky, postulated that the missing structure in galaxies is dark matter. Her ideas met much resistance from the astronomical community, but her confirmed observations now create pivotal proof of the existence of dark matter. In honor of this crucial and historic piece of detective work toward establishing the existence of dark matter, the revolutionary Large Synoptic Survey Telescope recently received the name Vera C. Rubin Observatory.

Credit : Earth Sky 

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Our Sun orbits the Supermassive black hole that is at the centre of our galaxy, the Milky Way galaxy at a distance of about 27,000 Light Years. You'll probably be surprised to hear that the Canis Major dwarf galaxy is nearer to us than the galactic centre.

Stars in an Elliptical Galaxy such as the Milky Way orbit the centre and the Milky Way is no different. The time it takes for the Sun to complete a full orbit of the Supermasssive is known as the Galactic Year. There might be an Universe Year but we'll never know probably.

The time it takes to complete a full orbit of the galaxy is about 225 - 250 million Earth years. 250 Million Years ago was the Triassic Period when the first dinosaurs were walking the Earth. If we turned the time back one Galactic Year, Earth would be much different, no Internet, no Mobiles, not even humans could be found orbiting the Earth.

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The Milky Way is the galaxy in which Earth resides. Part of it is visible on a clear night (from sufficiently dark locations) as a thick opaque band of stars and dust stretching across the sky. We can see thousands of these stars with the naked eye, and many more with a telescope. But how many stars are in the Milky Way?

"It's a surprisingly difficult question to answer," said David Kornreich, an assistant professor at Ithaca College in New York. "You can't just sit around and count stars, generally, in a galaxy." 

Even in the Andromeda galaxy — which is bright, large and at 2.3 million light-years away relatively close to Earth  — we can distinguish only the largest and brightest stars. A sun-size star would be too difficult for us to see. 

So is there any way to figure out how many stars are in the Milky Way for sure? 

According to Jos de Bruijne, a scientist at the European Space Agency (ESA) who works on the galaxy-mapping Gaia mission, the current estimate is between 100 to 400 billion stars. Getting to a definitive number, de Bruijne told Space.com, will be difficult. 

The Gaia mission, in orbit since 2013, has managed to map positions of 1.7 billion stars in the sun's neigborhood up to the distance of 326 light-years. While astronomers could extrapolate those numbers to model the entire galaxy, even Gaia struggles to see some of the faintest and smallest stars and its results are therefore not perfectly accurate.

"The fundamental problem is to measure the luminosity [distribution] for very faint red dwarfs and then extrapolate to the brown-dwarf limit," de Bruijne told Space.com.

Red dwarfs are the most common stars in the universe and also the longest-lived ones. However, because of their low luminosity they are sometimes hard to spot. Brown dwarfs are even dimmer. These are basically failed stars that didn't manage to accumulate enough material to kick-start nuclear fusion in their core. They are therefore something between a star and a planet and therefore even more difficult to spot than faint red dwarfs, especially at long distances.

"A second complication for the entire story are double stars, the frequency of which is still not perfectly characterised," de Bruijne added. 

De Bruijne expects that by the end of Gaia's mission in 2025, scientists will have a somewhat better idea about the number of stars in our galaxy but "significant uncertainties will likely remain". 

Credit : Space.com

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The planets in our solar system orbit around the sun. One orbit of the Earth takes one year. Meanwhile, our entire solar system – our sun with its family of planets, moon, asteroid and comets – orbits the center of the Milky Way galaxy. Our sun and solar system move at about about 500,000 miles an hour (800,000 km/hr) in this huge orbit. So in 90 seconds, for example, we all move some 12,500 miles (20,000 km) in orbit around the galaxy’s center.

Our Milky Way galaxy is a big place. Even at this blazing speed, it takes the sun approximately 225-250 million years to complete one journey around the galaxy’s center.

What about the Milky Way galaxy? Yes, the whole galaxy could be said to rotate, but like our sun, the galaxy is spinning at different rates as you move outward from its center. At our sun’s distance from the center of the Milky Way, it’s rotating once about every 225-250 million years – defined by the length of time the sun takes to orbit the center of the galaxy.

The planets in our solar system orbit (revolve) around the sun, and the sun orbits (revolves) around the center of the Milky Way galaxy. We take about 225-250 million years to revolve once around the galaxy’s center. This length of time is called a cosmic year.

Credit : Earth Sky 

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