My Science School

          All comets begin their lives as dirty snowballs. They are relics from the birth of the Solar System around 4.6 billion years ago, and are made up mainly of ice, gas and rock. As the comet approaches the inner Solar System, the Sun’s heat causes the ice to evaporate. It turns into gas and forms a glowing head around the nucleus.

          In order to understand what are comets made of, we need to break down the three main parts of the comet: the nucleus, coma, and tail. Comet nuclei are known to range from about 100 meters to more than 40 kilometers across. They are composed of rock, dust, ice and frozen gases such as carbon monoxide, carbon dioxide, methane, and ammonia. Sometimes called dirty snowballs, recent studies have shown that the ice of a comet is covered by a crust. Comets also contain a variety of organic compounds as well as the gases already mentioned. Some of these are methanol, hydrogen cyanide, formaldehyde, ethanol, and ethane. More complex molecules such as long-chain hydrocarbons and amino acids may also be in comets. Because of their low mass, comets cannot become spherical under their own gravity, and will thus have irregular shapes.

          The coma is the nebulous envelope around the nucleus of a comet. It is formed when the comet passes close to the Sun on a highly elliptical orbit. As the comet warms, parts of it turn from solid to gas (sublimate). Larger charged dust particles are left along the comet’s orbital path while smaller charged particles are pushed away from the Sun into the comet’s tail by solar wind. This helps astronomers distinguish comets from stars because it creates a fuzzy appearance.

          The tail is illuminated by the Sun and may become visible from Earth when a comet passes through the inner solar system, the dust reflecting sunlight directly and the gases glowing from ionization. The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet’s orbit in such a manner that it often forms a curved tail called the antitail. At the same time, the ion tail, made of gases, always points directly away from the Sun, as this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory. Parallax viewing from the Earth may sometimes mean the tails appear to point in opposite direction.

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          Although all comets seen from Earth have tails, they do not always look like this. When a comet is a long distance from the Sun, it exists purely as a lump of ice, frozen gas and rocky dust. However, as the comet’s orbit takes it closer to the Sun, the temperature rises, and the ice begins to melt. Gas and dust are released, forming a huge cloud around the comet. This cloud is blown by the solar wind to form a tail.

          A comet tail—and coma—are features visible in comets when they are illuminated by the Sun and may become visible from Earth when a comet passes through the inner Solar System. As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. Separate tails are formed of dust and gases, becoming visible through different phenomena; the dust reflects sunlight directly and the gases glow from ionization. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible to the naked eye.

          In the outer Solar System, comets remain frozen and are extremely difficult or impossible to detect from Earth due to their small size. Statistical detections of inactive comet nuclei in the Kuiper belt have been reported from the Hubble Space Telescope observations, but these detections have been questioned, and have not yet been independently confirmed. As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. The streams of dust and gas thus released form a huge, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the Sun.

          The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail called the antitail, only when it seems that it is directed towards the Sun. At the same time, the ion tail made of gases, always points along the streamlines of the solar wind as it is strongly affected by the magnetic field of the plasma of the solar wind. The ion tail follows the magnetic field lines rather than an orbital trajectory. Parallax viewing from the Earth may sometimes mean the tails appear to point in opposite directions.

          Some scientists are convinced that the orbits of Uranus and Neptune are being distorted by the gravitational pull of a planet beyond Pluto. The recent discovery of minor members beyond Pluto which could be responsible for this distortion makes the existence of a tenth planet unlikely. New planets are being discovered continually, however, not in our Solar System, but orbiting other stars.

          Pluto has been downgraded from 'ninth planet' after we found the rest of the Kuiper belt, including Eris which is more massive, though with a slightly smaller volume. Even before that, the apparent need for an extra planet to explain deviations in the orbit of Neptune no longer applied. Voyager 2 made more accurate measurements and found nothing out of line. (And found that Neptune was more massive than Uranus, despite its smaller radius and volume.)

          Anything large enough to count as a 10th Planet would have been expected to have caused disturbances in the orbits of known planets and in the Voyager and Pioneer probes as they moved outwards.

          There is a small possibility of one or more Earth-sized bodies very far out. Under current rules these would probably not count because they would probably not have cleared their orbits of similar bodies.

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          Beyond the orbit of Neptune, stretching deep into the outer Solar System, lies a belt of celestial bodies - made of rock and ice. The astronomer Gerard Kuiper first suggested the existence of this zone of comet-like objects, and so it was named the Kuiper Belt. There are at least 70,000 minor members in the Kuiper Belt with a diameter of over 100km (62 miles). The largest of these is 1992QBI, otherwise known as Smiley, which is 220km (137 miles) across.

          The Kuiper belt occasionally called the Edge worth–Kuiper belt, is a cicumstellar disc in the outer Solar System, extending from the orbit of Neptune (at 30 AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times as wide and 20 to 200 times as massive. Like the asteroid belt, it consists mainly of small bodies or remnants from when the Solar System formed. While many asteroids are composed primarily of rock and metal, most Kuiper belt objects are composed largely of frozen volatiles (termed "ices"), such as methane, ammoni and water. The Kuiper belt is home to three officially recognized dwarf planets: Pluto, Haumea and Makemake. Some of the Solar System's moons, such as Neptune's Triton and Saturn's Phoebe, may have originated in the region.

          The Kuiper belt was named after Dutch-American astronomer Gerard Kuiper, though he did not predict its existence. In 1992, Albion was discovered, the first Kuiper belt object (KBO) since Pluto and Charon. Since its discovery, the number of known KBOs has increased to thousands, and more than 100,000 KBOs over 100 km (62 mi) in diameter are thought to exist. The Kuiper belt was initially thought to be the main repository for periodic comets, those with orbits lasting less than 200 years. Studies since the mid-1990s have shown that the belt is dynamically stable and that comets' true place of origin is the scattered disc, a dynamically active zone created by the outward motion of Neptune 4.5 billion years ago; scattered disc objects such as Eris have extremely eccentric orbits that take them as far as 100 AU from the Sun.

          The Kuiper belt is distinct from the thyeoretical Oort cloud, which is a thousand times more distant and is mostly spherical. The objects within the Kuiper belt, together with the members of the scattered disc and any potential Hills cloud or Oort cloud objects are collectively referred to as trans-Neptunian objects (TNOs). Pluto is the largest and most massive member of the Kuiper belt, and the largest and the second-most-massive known TNO, surpassed only by Eris in the scattered disc. Originally considered a planet, Pluto's status as part of the Kuiper belt caused it to be reclassified as a dwarf planet in 2006. It is compositionally similar to many other objects of the Kuiper belt and its orbital period is characteristic of a class of KBOs, known as "plutons" that share the same resonance with Neptune.

          The Kuiper Belt and Neptune are noted as one of the ways to define the extent of the Solar System, along with the heliopause and the radius at which the Sun's gravitational influence is matched by other stars, estimated to be between 50000 AU to about 2 light-years.

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          The solar system does not end at Pluto but stretches outwards in all directions for billions of kilometres. Many scientists believe that the boundary of the Solar System could be an immense cloud of comets, called the Oort cloud, which surrounds the planets like a spherical cage. Scientists believe that there are over ten trillion comets in this spherical halo, stretching nearly 8 million kilometres (5 million miles) from end to end.

          With recent searches of what lies beyond Pluto, astronomers have now detected a small planet some 200 miles in diameter which lies at the incredible distance of some 7.9 billion miles from the Sun. The new object has been named 2015 TG387, a rather bland name for this most distant object. When Pluto was discovered here in Arizona back in 1930, there was much excitement that this object was thought to be at the farthest edge of the solar system. Since that time, many new objects have been discovered at much greater distances than Pluto.

          The names, Eris, Makemake, Sedna, Quaoar, Varuna and Haumea, are not part of most peoples’ vocabulary, as these are a few of the new dwarf type planets that lie within this amazing region in the solar system, known as the Kuiper Belt. Simply, the solar system is packed with so many small planets and the discoveries just keep on coming! When Pluto was first discovered in 1930, many had the belief that a true 10th planet was lurking far beyond Pluto. To this day, there are many in the scientific community who believe that “Planet X” still exists as a large body yet undiscovered.

          Astronomers know of the “Kuiper Cliff,” a region in the Kuiper Belt that seems to end at a distance from the Sun of 48 AU (1 AU is the distance of the Sun from the Earth), around 4.46 billion miles from the Sun! The vast distance of this object at 2.5 times the distance of the dwarf planet Pluto makes this a most interesting object out in a region known as the Kuiper Belt.

          This is very important, as this might signify that there is a much more massive-type object lurking well beyond this distance. Some have labeled this object “Planet 9.” It might be an object as large as the earth or larger in an orbit that takes well over 20,000 years to circle the Sun.

          Beyond the realm of the Kuiper Belt is a large cloud like formation, known as the “Oort Cloud.” This is a region of the distant solar system where comets are thought to be and there is even speculation that a large gas giant planet much like Jupiter may inhabit this region of space.

          Pluto’s only moon, Charon, is over half the diameter of Pluto, making it the biggest moon in relation to its parent planet in the Solar System. They are only 20,000km (12,430 miles) apart, and are caught in a gravitational headlock that scientists call a dual-planet system. They are so similar in size that they can be thought of as a double planet, as shown below.

           NASA/ESA Hubble Space Telescope has obtained the clearest pictures ever of our solar system's most distant and enigmatic object: the planet Pluto. The observations were made with the European Space Agency's Faint Object Camera. The ninth and last real planet known, and the only planet that has not been visited by a fly-by spacecraft, Pluto was discovered just 60 years ago by the American astronomer Clyde Tombaugh, who was searching for the source of irregularities seen in the orbits of Uranus and Neptune. It has become apparent since then that Pluto is a very peculiar object. Its orbit is tilted and is more elliptical than the orbits of any of the other planets in the solar system. Pluto also rotates upside down with its North Pole below the plane of the solar system in the opposite sense of the Earth and most of the other planets. Pluto is smaller than our own Moon and also denser than any of its neighbors in the outer solar system. But, perhaps, its most fascinating property was uncovered only 12 years ago when a huge companion "moon" called Charon was detected from ground based photographs. Subsequent investigations have shown that Charon is about half the size of Pluto making it the largest known satellite relative to its planet in the solar system. In fact, because of this, Pluto is often referred to as a double planet. The rotation period of the Pluto-Charon system is a mere 6 days. A recent Faint Object Camera image of Pluto and Charon is shown in the upper right hand frame of the accompanying photograph. This image is the first long duration HST exposure ever taken of a moving target. In order to avoid smearing of the images, ground controllers had to pre-program the HST spacecraft to track Pluto extremely accurately and compensate exactly for the "parallax" introduced by the combined motions of Pluto, the Earth and HST in their respective orbits. Pluto is currently near its closest approach to the Earth in its 249 year journey around the Sun, and is approximately four and a half billion kilometers away. 

          At nearly 5900 million km (3670 million miles) from the Sun, Pluto is a cold, dark world, even in the middle of the day. The Sun appears over 1000 times fainter from the surface of Pluto that it does from Earth — little more than a bright star. Because of this, surface temperatures average around —230°C (-382 °F). In summer, Pluto has a slight atmosphere because the surface warms up enough to turn some of the ice to gas. As Pluto moves away from the Sun, the gas freezes and becomes ice again.

          Pluto lies in an area classified as the Kuiper belt in the trans-Neptunian region of our solar system.  The Kuiper Belt is composed of thousands of icy, solid objects extending from near the orbit of Neptune to nearly 5 billion miles from the Sun.  Pluto is the largest known body in the Kuiper Belt with enough mass to exhibit a spherical shape.  Even though Pluto has enough mass to give it a spherical shape, Pluto is in fact smaller in physical size than seven of the solar system's moons, including Earth's.  Pluto rotates on its axis every 6.39 days and takes 247.8 years to revolve around the Sun.  It has five known satellites.  The satellites in order of distance from Pluto beginning with the closest are Charon, Styx, Nix, Kerberos and Hydra.  Charon is the most interesting as it is about half the size of Pluto and spherical.  Charon orbits Pluto every 6.39 days and also rotates once during this time frame, matching Pluto's rotation.  Therefore, Pluto and Charon are acting like a dumbbell in space, rotating around each other in a near perfect lock-step.  This is the closest thing to a binary-planet system that it is in our solar system.  

          NASA's New Horizons space probe flew by Pluto on July 14th, 2015, giving scientists a wealth of data previously not known about the icy body.  Pluto's very thin atmosphere extends 100 miles from its surface, about 5 times higher then earlier models had predicted.  Although the atmosphere is thin, it's possible that it is enough to give Pluto day to day weather variances.  The atmosphere primarily exists as a gas when Pluto is closest to the sun (perihelion), but then slowly freezes onto the surface as it moves further away.  New Horizons was able to observe Pluto's atmosphere as the probe was moving away, photographing the haze of the atmosphere as Pluto was backlit by the sun (shown below).  The atmosphere actually gives off a bluish haze, which is caused by sunlight being scattered by haze particles abundant in its atmosphere.  Before the New Horizons flyby, scientists had expected that much of the nitrogen encompassing Pluto's atmosphere had escaped into space.  However New Horizons showed a surprisingly thick haze, revealing that a replenishment of nitrogen has to be coming from somewhere, possibly from the planet's interior via ice volcanoes or geysers.  Indeed, New Horizons showed that Pluto's surface has plenty of diverse geography such as craters, ice dunes, mountains, plains, and erosional features such as surface troughs and ridges.   

         New Horizons gathered data from Pluto for over 5 months.  Therefore New Horizons was able to image one side of Pluto and found an extremely interesting large, heart-shaped body (shown above), primarily composed of nitrogen ice.  This feature gives rise to evidence that Pluto has a large, sub surface, salty, slushy-liquid ocean, perhaps up to 60 miles in deep.  How can a body this cold, whose surface temperature ranges from about -370 to -400 degrees F, support liquid water, even under its surface?  Given Pluto's internal pressure and heat budget, it is possible.  Furthermore, the heart-shaped feature, named Tombaugh Regio (after Pluto's discoverer Clyde Tombaugh) holds a key.  This region is nearly exactly opposite Pluto's largest moon, Charon (shown below), exhibiting a constant pushing and pulling of gravity in that region.  Astronomer's focused their attention on the western side of the heart, a region named Sputnik Planum, an area thought to have formed from an impact with a meteor.  An impact would basically blast material away from it, giving that area a "negative mass anomaly". But that is not the case with this area.  It has a positive mass anomaly.  For this to happen, simulations show that a sub-surface ocean would have had to of spread out across the planet below the surface after the impact.  Therefore it is possible that Pluto's rocky interior is surrounded by a slushy-ice ocean, which in turn is surrounded by the icy surface. 

          Like Uranus, Pluto's rotational axis is highly tilted at 122.5 degrees.  This would give one side of the planet extremely long periods of darkness or light, depending on that side's orientation to the Sun.  However it is likely that the planet's temperature is very uniform around the globe.  This is due to the fact that the sun casts such feeble light at that distance.   

          Because Pluto is so far away from Earth, it is very difficult to explore the planet using telescopes. Even the Hubble Space Telescope cannot make out the planet in very much detail. So far, no probes have visited the ninth planet.

          Pluto, once considered the ninth and most distant planet from the sun, is now the largest known dwarf planet in the solar system. It is also one of the largest known members of the Kuiper Belt, a shadowy zone beyond the orbit of Neptune thought to be populated by hundreds of thousands of rocky, icy bodies each larger than 62 miles (100 kilometers) across, along with 1 trillion or more comets. 

          In 2006, Pluto was reclassified as a dwarf planet, a change widely thought of as a demotion. The question of Pluto's planet status has attracted controversy and stirred debate in the scientific community, and among the general public, since then. In 2017, a science group (including members of the New Horizon mission) proposed a new definition of planethood based on "round objects in space smaller than stars," which would make the number of planets in our solar system expand from 8 to roughly 100.

          American astronomer Percival Lowell first caught hints of Pluto's existence in 1905 from odd deviations he observed in the orbits of Neptune and Uranus, suggesting that another world's gravity was tugging at these two planets from beyond. Lowell predicted the mystery planet's location in 1915, but died without finding it. Pluto was finally discovered in 1930 by Clyde Tombaugh at the Lowell Observatory, based on predictions by Lowell and other astronomers.

          Pluto got its name from 11-year-old Venetia Burney of Oxford, England, who suggested to her grandfather that the new world get its name from the Roman god of the underworld. Her grandfather then passed the name on to Lowell Observatory. The name also honors Percival Lowell, whose initials are the first two letters of Pluto. 

          Pluto is the most mysterious planet in the Solar System because it is the one that astronomers know least about. Many have questioned Pluto’s status as a planet, arguing that it is too small, and its orbit too elliptical, to be classified as such. Pluto may be the largest of the asteroids in the Kuiper Belt. Or it may once have been one of Neptune’s moons that broke free of its parent’s gravity. However, there are no plans to demote Pluto yet.

          Pluto’s status as a planet has once again been called into question after the head of NASA said he believed the celestial body to be a planet. Speaking at the FIRST Robotics event in Oklahoma, NASA administrator Jim Bridenstine went against convention by placing himself firmly on one side of the Pluto debate.

          “Just so you know, in my view Pluto is a planet,” he said. “You can write that the NASA administrator declared Pluto a planet once again. I’m sticking by that, it’s the way I learned it and I’m committed to it.” Pluto was first declared a planet in 1930 after it was discovered by American astronomer Clyde Tombaugh. At the time it was believed to be the ninth planet from the Sun, existing on the outer edges of the solar system in the Kuiper belt.

          Its status as a planet was called into question 62 years later after other similarly-sized objects were discovered in the same region of space. In 2005, astronomers discovered a dwarf planet called Eris that was 27 per cent larger than Pluto. A year later, the International Astronomical Union laid out its official definition for what constituted a planet. Pluto was not included.

          Since then it has been classified as a dwarf planet, though the icy object has attracted a dedicated following of people who claim Pluto should be considered a planet. In 2015 Nasa’s New Horizons mission to Pluto made several major discoveries that added fuel to the debate.

          Pluto and Charon in synchronized Orbits. This means that they keep the same side, facing each other as they turn. From one side of Pluto, Charon always appears at the same point in the sky. On the other side of the planet, the moon is never visible.

          Scientists studying Uranus and Neptune discovered that these planets were very different from Jupiter and Saturn. They are much younger than their bigger neighbours, and therefore unable to feed on the enormous clouds of hydrogen and helium that made Jupiter and Saturn so large. Uranus and Neptune have been called ice giants as opposed to gas giants because beneath their cloud tops they may have oceans of water, heated by the energy from their cores.

          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.

          Triton, one of Neptune’s moons, has been found to be the coldest world in the Solar System. Parts of its surface reach temperatures as low as —235°C (-391°F), which is only 38°C (100°F) above the coldest temperature possible — absolute zero. Voyager 2 discovered that the moon is also geologically active. In several places, pockets of nitrogen gas explode from vents, sending plumes of gas, dust and ice up to 8km (5 miles) into the air. Wind blows the plumes into long streaks that can stretch for 150km (90 miles).

         And upon our arrival, we discovered that this strange moon is, well, even stranger than we assumed. In fact, it is orbiting the wrong way, and it is the only large moon among any of the other planets in the solar system that does this.

          Triton has what is called a “retrograde orbit,” which means that its orbit is opposite to Neptune’s rotational direction, indicating that the moon did not form along with its host planet. This orbit contradicts everything we know about how moons form. If that’s not enough to convince you that it’s clearly no ordinary moon, Triton’s surface is smooth, seemingly like metal. This may be caused by eruptions on the surface, as Triton is geologically active to this day, driven by the radioactive heating from its core. And this heating means that it could have liquid water in its interior, just like Europa.

          One theory is that Triton was originally a binary object that visited our solar system from somewhere else. In this case, Triton would have once been a dwarf planet. However, it had a close encounter with Neptune that ended with the loss of the other body, making the former planet a moon. Sadly, Triton’s orbit is decaying. Scientists estimate that, in approximately 3.6 billion years, it will pass below Neptune’s Roche limit and will be torn apart…so enjoy this little world while you can.