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Write the complete summary of the attached articles and Critically analyze the attached articles?!!

The solar systems Look out toward Venus at twilight The solar system violent past ns tranquil, but in its early days, our cosmic neighborhood was a decidedly dangerous place. by Nola Taylor Redd from a combination of asteroid and comet impacts and volcanic activity The solar systems planets form a pretty stable family. These siblings occasionall interact from a distance but dont constantly disturb one another. But several billion years ago, the family may have been much larger, with as many as seven other worlds fighting things out and violent interactions kicking some out of the clan completely over a longer period of time. Formation doesnt guarantee survival. The first planets jostled one another as they settled into their orbits. Collisions between planets and planetary hopefuls kept everyone on their toes. The constant crashes may have birthed Earths The solar system was a much more compli- cated place early on, says David Nesvorny, an astronomer at the Southwest Research Institute (SwRI) in Colorado.What we see now is just a result of dynamic erosion, instability, scattering, and the captures of different planets as they fly through space Moon, given Venus its backward spin and Uranus its unusual tilt, and stripped Mercury of its outer layers. They also may have booted as many as half the original members out of the fold. The biggest hint that the planets could have lost some siblings came not from observations of the solar system, but from discoveries of planets out- Nesvorny models how planets could have inter side it. Early technology limitations led scientists acted with one another early in their lives. He found to search for planets unlike any in the solar system strong evidence for the existence of at least one gas_massive Jupiter-sized worlds in orbits near their giant ejected from the solar system in its first few star. These hot Jupiters appeared in droves, lead- hundred million years. And it may not have been ing to new ideas of how planets behaved in their the only world to lose its way. younger years. Planets form from a disk of gas and dust sur- rounding a young star. The gas escapes fairly quickly, so giant worlds have to wrap things up within 4 million or 5 million years. Rocky ter- restrial planets can take a bit longer, as much as 100 million years, because much of the lighter material that makes up their atmosphere and surfače (such as Earths water) may have come Hot Jupiters gave us that kick in the rear to start considering what happens when we allow planets to move all over the place, says Kevin Walsh, a senior research scientist also at SwRI. Planetary scientists believe the solar system formed within a giant accreting disk of matter warmed by the infant Sun 22 ASTRONOMY . EBRUAHY 201Gas giants like Uranus have quirks that may have resulted from the violent history of the early solar system, such as slightly more elliptical orbits than those of the inner planets and dipping up and down from the plane of dust from which they formed.MASAUPL Mercury may have been one of a number of larger bodies close to the Sun in the young solar system. NASA.HUARCENEGIENSITUTIO Walsh studies terrestrial planets, and in 2012 he led a team to develop a new theory that Jupiter and Saturn once migrated in the solar system. NASAs Kepler spacecraft also revealed a host of within the confined space. Some would get smashed to bits while new systems whose behavior may resemble the planets of the early others would wind up ejected from the system, usually crashing solar system occurred in all systems, not just those spotted by Kepler. Like sib- lings sharing a single room, the planets would constantly collide into their sun. After 100 million years, only about 10 percent of the Its difficult to prove what happened in the first few million systems they modeled remained tightly packed years around the Sun. Planets dont leave behind written records of where and how they lived. But by studying other systems, mod says. eling how planets might have interacted, and scouring the heavensIfSTIPs formed around the early Sun, it could help explain for any surviving clues, scientists have concluded that the early eral of Mercurys mysteries. While the centers of most solar system family life of the planets was nothing if not eventful. We think that they are consistent with almost all stars, Volk worlds are small in relation to their crust and mantle, Mercury has an enormous iron core containing a large percentage of its mass. The planets orbit is less circular than any other in the solar system. Models predict that, if Mercury formed in place, the planet should lie closer to the Sun and be larger than it is today. Mysterious Mercury In its youth, a larger, more social Mercury may have zipped through the solar system. The rocky world could have been one of several flying around the Sun like close-traveling cars at rush hour, dizzyingly near to one another. Like traffic, the system may have suffered high-speed collisions that sent the other worlds reeling, leaving Mercury the sole survivor of the carnage. In the solar systems first billion years or so Mercury could have crushed its competition, making it the ultimate winner in a case of deadly sibling rivalry But if the solar system started out with sv- eral inner planets zipping around the Sun inside Mercurys present orbit, some of these mysteries could be explained. A slew of colli- sions could send the other worlds reeling, vaporizing most of Mercurys natal shell and The varied systems revealed by Kepler include stars with several planets traveling within the distance of Mercurys orbit around our Sun. Known as Systems of Tightly Packed Inner Planets, or STIPs, the close neighbors make up about 10 percent of Keplers discoveries. leaving a thin veneer to shield the core. In the solar systems first billion years or so, the rocky world could have crushed its competition, making it the ultimate winner in a case of deadly sibling rivalry Secrets of the gas giants While some scientists considered STIPs intriguing outliers, The solar systems gas giants have their own particular puzzles Kathryn Volk and Brett Gladman, both at the University of British Planets forming from a disk of dust should have a nearly circulai Columbia, wondered if they might be the norm. They modeledorbit, but thats not the case for the outer planets. While Venu:s what would happen if dense clusters of planets near their suns and Earth have nearly circular orbits, the gas giants find them- selves stretched out in slightly moreelliptical paths. They dip Astronomy. In 2005, a group of scientists hammered out a model to help 24 A STRONOMYFEBRUARY 201explain the strange quirks of the gas giants, naming it after their host town of Nice, France. The Nice model explains not only how the solar systems largest planets may have moved around carly in their lifetime but also why there was a burst of collisions in the solar system between 3.8 billion and 4.1 billion years ago. As the gas giants danced, they sent rocks and ices hurling in toward the smaller rocky planets. This period is known as the Late Heavy Bombardment The four gas giants started out much closer together under the Nice model. As Neptune and Uranus moved around, they hurled some material inward to rain down on the terrestrial worlds while they tossed others far from the Sun. Eventually the ices returned as comets. They may have slowly nudged Pluto into p swapped places, sending another icy giant or two flying. Nesvorny tried to trace Neptunes steps by studying clues in the Kuiper Belt, the ring of ice and rock that starts near Pluto. When a massive object like a giant planet travels through the solar system, it can sweep up smaller material and carry it in a special orbital path known as a resonance. But when gas giants interact, their rapid jumps leave behind smaller friends. This could explain a puzzling collection known as the kernel, found in the Kuiper Belt. The kernel is a mix of materials, some of which formed in place while others migrated from elsewhere. More than a dozen objects have been detected to date, but because aninward journey. kAA its tough to spot smaller bits of ice and rock at this distance, scien tists think there may be as many as 20,000 more. The gas giant Neptune may have played a substantial role in mixing materials in the outer solar system. It also likely migrated outward after Trying to understand their orbits tells us something about how another planet could have caused it to jump, making it fly out to and why they survived, and where they came from, says SwRIs Hal its present location. If Neptune shepherded the kernels material, it Levinson. It tells us something about how the planets migrated.would have left its flock behind when it jumped. Levinson helped to develop the Nice model, and he has played a Which planet could have caused the sudden change? None that role in refining it over the years. The model calls for Neptune to e know journey deeper into the solar system before landing at its current location about 30 astronomical units from the Sun. (An astronom duced with Neptune having an encounter with Jupiter, Uranus, or ical unit, or AU, is the distance between Earth and the Sun.) By Saturn, Nesvorny says. Theres no object in the present solar sys- modeling the planets path in combination with the kernel, Nesvorny found that the planet had a smooth trip for most of its journey, which probably took about 100 mlon years. At about on that made Neptune jump could have sent the missing world 28 AU, however, a sudden wrench caused by interactions with The kind of jump that is needed for the kernel cannot be pro tem that could have produced this jump in Neptunes orbit. He proposed that a fifth gas giant caused the leap. The interac- hurtling out of the solar system. Sorting out planetary movements 40 AU 40 AU 20 AU 20AUİ 20 AU Jupiter Saturn Uranus The so-called Nice model, produced in France in 2005, could explain how migration of the giant planets in our solar system took place. After the dissipation of gas in the protoplanetary disk, the model suggests, migrations help to explain the Late Heavy Bombardment, the formation of the Oort Cloud, and the existence of the Kuiper Belt. This illustration depicts, from left to right, the solar system in its early configuration; the scattering of planetesimals into the inner solar system and the migrations of Uranus (green) and Neptune (blue; and the solar system after the ejection of planetesimals by the planets. ASTROWNRON ELI www.ASTRONOMY.COM 25unsolved mysteries involved what Walsh and others call the small Mars problem. Long before Nice came along, models of the solar system suggested that the material of the inner solar system should create a Mars five to 10 times larger than todays Red Planet. Grand Tack model Year 0 Jupiter forms This was a problem that persisted for a while, Walsh says Although he and others hoped that the Nice model could help resolve the issue, it fell short. Asteroids Uranus Neptune Comets Jupiter also plays an important role in defining these ideas. With an eye toward the hot Jupiters, in 2011 Walsh and a team of scientists presented the Grand Tack theory, involving an carly solar system with Jupiter spiraling closer to the Sun. Traveling alone, it might have been just another hot Jupiter. But the gas giant had a partner on its road trip, the ringed planet Saturn. Thanks to each others gravitational influence, the pair halted on their inward journey and reversed course, much like a sailboa Year 70,000 Jupiter migrates inard Year 100,000 Satu migrates to 3:2 resonance; formation of the terrestrial planets in the outer solar system to t But the trip wasnt smooth sailing. Along the way, the two mas- sive bodies stirred up the asteroid belt, first when they passed through en route to sunnie regions and then again when they returned to colder climes. And while the terrestrial planets may have felt the brunt of the mixing, the movement of the gas giants would have stirred up the outer solar system as well Year 300,000 Outvard migration Although the theory developed after Nice, Grand Tack took place earlier in the history of the solar system, within the first 10 million years. Once the two giants finished their inward trip, the terrestrial planets would have had plenty of time to form. Thanks to the stirring up of the rocky pieces near the Sun, less material would have been available to build the Red Planet, solving the small Mars problem Year 500,000 End of the Grand Tack Along comes Planet Nine Suggestions that the early solar system could have housed more worlds, both rocky and gaseous, became especially intriguing with the announcement that a distant planet may exist at its fringes. Two planetary scientists at the California Institute of Technology Konstantin Batygin and Mike Brown, proposed the existence of Planet Nine thanks to observed gravitational effects on objects in the outer Kuiper Belt. Estimated to hold ahout 10 times Earths mass, the missing planet only gets to within about 250 AU of the Sun nearly 10 times as far as Neptune. Year 600,000 to present After Nice scenario Sun 2 AU 4 AU 6 AU 8 AU 10 AU The so-called Grand Tack model of the solar system, developed in 2011, suggests that after its formation some 3.5 astronomical units from the Sun, Jupiter migrated inward to 1.5 AU before heading back outward to its present position 5.2 AU from the Sun. This strip showsArizona, Volk located at least four other large objects at the edge of critical events along that process, from the formation of Jupiter to 600,000 years after the solar systems formation, when the present planetary configuration existed. RCKTLLY Working with a new team in her new home at the University of the solar system that strengthened the case for Planet Nine. The enormous chunks of rock and ice never draw any closer to the Sun than 40 AU, spending most of their orbits about 1.3 times as far out as Pluto. According to Volk, they never interact with the giant planets, suggesting that another massive body may have disturbed Several models have suggested the possibility of a fifth, and even a sixth, gas giant. In 2012, a revised look at the Nice model their paths. found that the solar system worked best with five gas giants, including one with a mass similar to Uranus and Neptune. B have joined the Sun after being cast out of its own system. rewinding the clock can be difficult; its almost impossible to find Observations have shown that free-floating, sunless planets are proof for what might have been. The kernel makes a strong argu- fairly common in the Milky Way, and the researchers thought per- ment that the solar system once held more gas giants than today. haps one had wandered close enough to be picked up by the Sun Originally, Batygin and Brown suggested Planet Nine could The kernel could be evidence for the fifth planet in the outer Since then, models by other scientists have dismissed that possibil- ity claiming that the Suns weak grasp would have made it easy for another star to strip the world away. It would take between 20 and 50 objects to be captured and stripped in order for one to last long solar system, Nesvorny says. Devil in the details The Nice model helped to explain many things, but it didnt quite enough to disrupt the solar system. nail down everything. According to Walsh, while Nice was good for The idea that Planet Nine could have formed with its existing answering questions about the outer solar system, many questions orbit is also unlikely. The icy planet takes about 20,000 years to remained about the rocky terrestrial planets. One of the biggest complete a single orbit. According to Batygin, it would have made 26 ASTRONOMYFEBRUARY 201Planet Nine, shown in this artwork, was calculated to exist by Caltech research ers in 2016 but has not yet been observed. The planet could explain some intricate orbital dynamics of the outer solar system. CALTKHR HURT PMC only about 100 passes through the disk of gas and dust around the young Sun, nowhere near enough time to build a planet 10 times as massive as Earth world would have needed to take flight early on The mystery world may instead be a starter planet, a core from an ice giant that wanted to form. Raymond is working on model- ing how an object like Planet Nine could have accreted. He thinks that after Jupiter and Saturn formed, they may have carved a gap You cannot assemble a 10 Earth-mass object in a mere 100 orbits, Batygin says. Could the missing planet be the fifth gas giant whose interac the gas disk around the Sun. As other material migrated tion with Neptune made the gas giant jump? Nesvorny and other nward, it got stopped and ultimately ejected by the two young scientists say no. The slim odds of an ice giant surviving a kick to planets. The result would be five to 10 leftover objects about five the extreme edges of the solar system would mean booting 10 to ms more massive than Earth. 100 icy worlds, which would have taken more material to build than was available. His icy Neptune is likely long gone, floating faster or a little closer in, Raymond says. freely somewhere in the galaxy, Nesvorny says. They would have become ice giants if they had formed a little The early solar system should have had plenty of mass. Similar problems exist for the super-Earths that might have survived their crash course with Mercury. Most would have wound up crashing into the Sun, Volk says. Any fortunate enough to be completely ejected from the solar system would have been flying out far too late to stick around. According to Raymond, the formation of Neptune and Uranus wasnt completely efficient. Building the planets would have required starting out with nearly twice as much By understanding how other clusters of worlds behave scientists gain a better understanding of what happened in our own planetary family mass as wound up in the planets, providing enough material to build several cores. Similar objects showed up in previous Grand Tack sinm It really seems difficult to imagine that, if it exists, Planet Nine is related to late insta- bility in the solar system,Nesvorny says ulations, booted by Jupiter. Focused on re- creating the known planets, the scientists paid little attention to the extra ingredients. To be a full-sized world, Planet Nine would have had to join early in the solar system in the first million years, Walsh says. Thats because the only way to keep the planet inside the solar system would have been for another nearby star to push it back. Once it gets kicked out, you forget about it, Raymond says. Maybe we shouldnt have just ignored this stuff because it could have ended up as Planet Nine. Its hard to look back at what might have been, but its starting to look like the early solar system Stars like the Sun form in dense clusters of stars. Over the Suns 4.6 billion-year life, the cluster has, like childhood standing how other clusters of worlds behave, scientists gain a friends, slowly drifted apart. Theres no way to know today which better understanding of what happened in our own planctary stars were part of that cluster or how close they may have beenfamily. Scientists have only scratched the surface with Kepler Studying the outer solar system helps to provide some constraints. Instruments like NASAs Transiting Exoplanet Survey Satellite Tightly packed stars would have had stronger gravitational influ- (TESS), scheduled to launch in 2017, wl take a look at small ences on the outer regions of the Kuiper Belt, and even on the outer planets. Astronomer Sean Raymond of the Laboratoire dAstrophysique de Bordeaux estimated that the Suns natal cluster We keep having to change our thinking about how planets contained somewhere between 1,000 and 10,000 stars, For a nearby star to kick Planet Nine back into the solar system, the was far more active than originally thought. By under worlds lying even farther from their parent stars. These systems may provide more insight into the young solar system. form because of all the exoplanet data, Volk says. Its not good to get attached to any particular idea. » www.ASTRONOMY.CoM 27

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