With the last Big Bang story I completed covering large and small structures in Universe, but before we hit individual planets of our solary system - something about solar system itself needs to be said too. During this article I will mosly focus on our Solar system. As you know by know, our universe consists of large scale structures where galaxies are one of those structures. These galaxies are grouping of stars, planets, moons, comets and building materials like gas, dust and rocks. Each star may have associated planets in orbit around the star and they together make what we call solar system. Our solar system is located within one of the outer arms of Milky Way galaxy, which contains about 200 billion stars. We can say that everything in the solar system orbits or revolves around the sun. Our Sun contains around 98% of all the material in the Solar System. The larger an object is, the more gravity it has. Because the Sun is so large, its powerful gravity attracts all the other objects in the Solar System towards it. At the same time, these objects, which are moving very rapidly, try to fly away from the Sun, outward into the emptiness of outer space. The result of the planets trying to fly away, at the same time that the Sun is trying to pull them inward is that they become trapped half-way in between. Balanced between flying towards the Sun, and escaping into space, they spend eternity orbiting around their parent star. Following video describes it very well.
Scientists believe that the Solar System evolved from a giant cloud of dust and gas. This dust and gas began to collapse under the weight of its own gravity some 4.568 billion years ago. This initial cloud was likely several light-years across and probably birthed several stars. As the region that would become the Solar System, known as the pre-solar nebula, collapsed, conservation of angular momentum made it rotate faster. As it did so, much like the water in a drain moves around the center of the drain in a circle, at the center of this spinning cloud a small star began to form. This star grew larger and larger as it collected more and more of the dust and gas that collapsed into it. Further away from the center of this mass where the star was forming, there were smaller clumps of dust and gas that were also collapsing. Within 50 million years, the pressure and density of hydrogen in the centre of the protostar became great enough for it to begin thermonuclear fusion forming our Sun (or any other star), while the smaller clumps became the planets, minor planets, moons, comets, and asteroids. Once ignited, the Sun's powerful solar winds began to blow. These winds, which are made up of atomic particles being blown outward from the Sun, slowly pushed the remaining gas and dust out of the Solar System. With no more gas or dust, the planets, minor planets, moons, comets, and asteroids stopped growing. Today, when we look at the solar system components we see following picture:
The four smaller inner planets, Mercury, Venus, Earth and Mars, also called the terrestrial planets, are primarily composed of rock and metal. The four outer planets, the gas giants, are substantially more massive than the terrestrials. The two largest, Jupiter and Saturn, are composed mainly of hydrogen and helium; the two outermost planets, Uranus and Neptune, are composed largely of ices, such as water, ammonia and methane, and are often referred to separately as "ice giants". You may have noticed upon looking pictire above that the four inner planets are much smaller than the four outer planets. Why is that? Because the inner planets are much closer to the Sun, they are located where the solar winds are stronger. As a result, the dust and gas from the inner Solar System was blown away much more quickly than it was from the outer Solar System. This gave the planets of the inner Solar System less time to grow. As the outer planets grew larger, their gravity had time to accumulate massive amounts of gas, water, as well as dust.
While not visible on picture above, between Mars and Jupiter we have what we call main Asteroid Belt. It is made up of thousands of objects too small to be considered planets. Some of them no larger than a grain of dust, while others, like Eros can be more than 100 km across. A few, like Ida, even have their own moons. Within these populations, five individual objects, Ceres, Pluto, Haumea, Makemake and Eris, are recognized to be large enough to have been rounded by their own gravity, and are thus termed dwarf planets. Further out, beyond the orbit of the dwarf planet Pluto, sits another belt known as the Kuiper Belt. Like the Asteroid Belt, the Kuiper Belt is also made up of thousands, possibly even millions of objects too small to be considered planets. A few of these objects, like Pluto, are large enough that their gravity has pulled them into a sphere shape. These objects are made out of mostly frozen gas with small amounts of dust. They are often called dirty snowballs. However, you probably know them by their other name - comets. Every once in a while one of these comets will be thrown off of its orbit in the Kuiper Belt and hurled towards the inner Solar System where it slowly melts in a fantastic show of tail and light. Beyond the Kuiper Belt sits a vast area known as the Oort Cloud. Here within this jumbled disorganized cloud live millions of additional comets. These comets do not orbit the Sun in a ring or belt. Instead, each one buzzes around in a completely random direction, and at extremely high velocities.
Going back to the solar winds, Sun's solar winds continue pushing outward until they finally begin to mix into the interstellar medium, becoming lost with the winds from other stars. This creates a sort of bubble called the Heliosphere. Scientists define the boundaries of the Solar System as being the border of the Heliosphere, or at the place where the solar winds from the Sun mix with the winds from other stars. The Heliosphere extends out from the Sun to a distance of about 25 billion km, which is more than 160 times further from the Sun than is the Earth. For some more details, refer to earlier posts about IBEX (see here and here).
What did the early solar system look like? Is the order of the planets the same as we see it today? Surprisingly, we believe it is not - say hello to Nice model. In the September 2007 issue of Sky & Telescope, Mark Littmann described the fascinating finding of how celestial dynamicists collaborated to discover that an intense game of interplanetary billiards led to our current arrangement of planets and smaller bodies. The best way to figure out what is going on is the movie which you can download from here. In pictures, it looks like following:
Unless you know what is going on, above picture (and movie too) an be somewhat confusing. Important to understand is these orbits are by outter planets - no inner planet is seen here. First picture shows early configuration, before Jupiter and Saturn reached so called 2:1 resonance, second picture shows scattering of planetesimals into the inner Solar System after the orbital shift of Neptune (dark blue) and Uranus (light blue), and third picture shows setup after after ejection of planetesimals by planets. Movie shows the same except that you start to appreciate time dimension. The movie begins state shown on first picture where the giant planets (Jupiter through Neptune) are in circular orbits surrounded by an even belt of icy objects beyond. As time moves forward, the progression seems slow, even though millions of years (Myr) are passing. If you look closely, you will notice gaps appearing in the proto-Kuiper Belt, much like the Kirkwood gaps in the main asteroid belt. As the eons fly by, the orbits of the three outer planets expand outward, and the belt of planetesimals spread out a bit. But all **** breaks loose just after the chronometer reaches 878 million years. It's then, as Littmann explains in his article, that Saturn and Jupiter reach a 2:1 orbital resonance (as on second picture). The gravitational tugs from Jupiter accumulate to greatly alter Saturn's orbit. With Saturn on the move, this in turn shifts the orbits of Uranus and Neptune, which plunge into the planetesimal reservoir, scattering bodies like the police arriving at a high-school house party. At that point animation is slowed down as otherwise the transformation would be over in the blink of an eye. Afterward, the planets settle down into the orbits we have today as seen on third picture.
However, in 2011 theory with fifth planet appeared and as it seems this planet was kicked out by Jupiter. It’s pretty well established that the outer planets have moved around a bit since the solar system formed, with a possibility that Uranus and Neptune even swapped places. But the models have a hard time explaining how this could’ve happened without Jupiter totally messing up the inner solar system. The models seem to indicate the orbits of Mars and Earth would not look at all as they do today if this were the case. Using the known math of physics and gravity, the author of this new study set about trying to figure out how this might be the case, and got the idea that maybe there was a fifth big planet back then, billions of years ago, with perhaps the mass of Uranus. He found that under certain circumstances, he could show that the addition of this planet explains the way both the outer and inner solar system look today, and why we don’t see this planet any more, since it was tossed out of the solar system through interactions with Jupiter. It’s a cool idea supported by models, but we don’t know how real it is. Maybe there’s something else we don’t know: maybe there were two extra planets, or maybe Jupiter interacted with the three other giant planets in a different way. This is just a model and nothing more. We do know that rogue planets roam the galaxy, and they were almost certainly ejected in this manner, so the idea of a lost solar system planet isn’t crazy that much.
A star is a large ball of glowing gas that produces energy by fusing hydrogen and helium into heavier and heavier elements. When the entire core has been converted into iron, no more energy can be extracted and the star dies flinging massive clouds of dust and gas out into space. These large clouds of gas and dust condense and are recycled into new stars and planets in a gigantic cosmic cycle. The new stars that are formed will have a higher content of heavier elements than the previous and for each generation of star formation there are more and more of the heavy elements and metals. The planets are formed from the remnants of the clouds of gas and dust that rotate in disc around the newly formed star. In this protoplanetary disc, the elements begin to accumulate and clump together and slowly the planets are formed. In the later generations of stars with a high content of heavy elements, the rotating disc of dust and gas particles has an elemental composition that is most likely to promote the formation of gas giants like Saturn and Jupiter.
And if you find to imagine that there was some other start before Sun, check this out. The general mechanism of solar system creation is well understood - a giant cloud of gas and dust must have collapsed to form the Sun and planetsl. But, what could have triggered such a collapse? There are various clues. The most tantalising come from the study of isotopes inside meteorites. These are important because astrophysicists think these rocks were formed during the collapse and have remained untouched ever since. So their isotopic make-up is a direct reflection of the conditions that existed inside the gas cloud as it condensed. One puzzle is the amount of aluminium-26 in these rocks when they formed. Al-26 has a half-life of about 700000 years. So it doesn't take long for the ratio between Al-26 and its cousin, Al-24, to change. But the ratio is strangely high in a class of carbonaceous chondrite meteorites called CV-chondrites (the 'V' comes about because they are named after a meteorite that fell over Vigarano, Italy). Something must have been injecting freshly forged Al-26 into this gas cloud as it collapsed. The isotope measurements also time stamp the formation of these meteorites and this raises another puzzle. The measurements indicate that the meteorites must all have formed within 20000 years of each other - that's practically simultaneously on these timescales. So what could have produced this Al-26 and triggered the formation of meteorites so quickly? There are several possibilities. Various types of stars release Al-26 in the wind that streams away from them. One idea is that our Solar System formed near one of these. Most astrophysicists favour another idea, however. This is that a supernova occurred nearby, sending a shockwave of hot gases, including Al-26, careering through our progenitor cloud. The difficulty is in distinguishing between these scenarios. Matthias Gritschneder at Peking University unveiled a new computer simulation of the formation of the Solar System that clearly favours the supernova hypothesis. The new model recreates what happens when a shockwave of hot gases from a supernova passes through a progenitor cloud of cold gases. Not only does the supernova provide exactly the right amount of Al-26, the shockwave also causes our gas cloud to collapse, thereby triggering the formation of the Solar System. What's more, the entire process happens very quickly. CV-chondrites probably formed when the temperature of gas cloud dropped below about 1800 degrees C. The new model shows that this would have occurred over a timescale compatible with the 20000 years that the evidence suggests. There are certainly improvements that could be made to this model. It is a 2D simulation, rather than 3D, so some physical processes may not be simulated exactly. There are also other isotope ratios that need to be explained. But these will have to wait until more advanced simulations are possible with greater computing horsepower. In the meantime, astrophysicists will be relieved that their most cherished ideas about the formation of the Solar System are bearing up, indeed flourishing, under the scrutiny of the strongest numerical tests possible today. More details can be found here.
Establishing chronologies of past events or determining ages of objects require having clocks that tick at different paces, according to how far back one looks. Nuclear clocks, used for dating, are based on the rate of decay of an atomic nucleus expressed by a half-life, the time it takes for half of a number of nuclei to decay, a property of each nuclear species. Radiocarbon dating for example can date artifacts back to prehistoric times because the half-life of radiocarbon (carbon-14) is a few thousand years. The evaluation of ages of the history of earth or of the solar system requires extremely "slow-paced" chronometers consisting of nuclear clocks with much longer half-lives. The activity of one of these clocks, known as nucleus samarium-146 (146Sm), was examined recently by scientists. 146Sm belongs to a family of nuclear species which were "live" in our sun and its solar system when they were born. Events thereafter, and within a few hundred million years, are dated by the amount of 146Sm that was left in various mineral archives until its eventual "extinction". 146Sm has become the main tool for establishing the time evolution of the solar system over its first few hundred million years. This by itself owes to a delicate geochemical property of the element samarium, a rare element in nature. It is a sensitive probe for the separation, or differentiation, of the silicate portion of earth and of other planetary bodies. The main result of the work of the international scientists is a new determination of the half-life of 146Sm, previously adopted as 103 million years, to a much shorter value of 68 million years. The shorter half-life value, like a clock ticking faster, has the effect of shrinking the assessed chronology of events in the early solar system and in planetary differentiation into a shorter time span. The new time scale, interestingly, is now consistent with a recent and precise dating made on a lunar rock and is in better agreement with the dating obtained with other chronometers.
Although organic compounds are commonly found in meteorites and cometary samples, their origins presented a mystery. In 2012, these complex organic compounds, including many important to life on Earth, were readily produced under conditions that likely prevailed in the primordial solar system by scientists at the University of Chicago and NASA Ames Research Center.
Here is one interesting thing. Every second, lightning flashes some 50 times on Earth. Together these discharges coalesce and get stronger, creating electromagnetic waves circling around Earth, to create a beating pulse between the ground and the lower ionosphere, about 100 km up in the atmosphere. This electromagnetic signature, known as Schumann Resonance, had only been observed from Earth's surface until, in 2011, scientists discovered they could also detect it using NASA's Vector Electric Field Instrument (VEFI) aboard Communications/Navigation Outage Forecast System (C/NOFS) satellite. Recentrly researchers described how this new technique could be used to study other planets in the solar system as well, and even shed light on how the solar system formed. The frequency of Schumann Resonance depends not only on the size of the planet but on what kinds of atoms and molecules exist in the atmosphere because they change the electrical conductivity. So we could use this technique remotely, say from about 1000 km above a planet's surface, to look at how much water, methane and ammonia is there. Water, methane and ammonia are collectively referred to as "volatiles" and the fact that there are different amounts on different planets is a tantalizing clue to the way the planets formed. Determining the composition of a planet's atmosphere can be done with a handful of other techniques - techniques that are quite accurate, but can only measure specific regions. By looking at the Schumann Resonance, however, one can get information about the global density of, say, water around the entire planet. And if we can get a better sense of the abundance of these kinds of atoms in the outer planets. That way, we would know more about the abundance in the original nebula from which the solar system evolved.
Accurate descriptions of planetary atmospheres might also help shed light on how the evolution of the solar system left the outer planets with a high percentage of volatiles, but not the inner planets. Detecting Schumann Resonance from above still requires the instruments to be fairly close to the planet, so this technique couldn't be used to investigate from afar the atmospheres of planets outside our solar system. Instead, scientists imagine something much more dramatic. After a spacecraft is finished observing a planet, it could continue to detect Schumann resonance as it begins its death dive into the atmosphere. During the process of self-destruction, the spacecraft would still provide valuable scientific data until the very last minute of its existence.
For many thousands of years, humanity, with a few notable exceptions, did not recognize the existence of the Solar System. People believed the Earth to be stationary at the centre of the universe and categorically different from the divine or ethereal objects that moved through the sky. Although the Greek philosopher Aristarchus of Samos had speculated on a heliocentric reordering of the cosmos, Nicolaus Copernicus was the first to develop a mathematically predictive heliocentric system. His 17th-century successors, Galileo Galilei, Johannes Kepler and Isaac Newton, developed an understanding of physics that led to the gradual acceptance of the idea that the Earth moves around the Sun and that the planets are governed by the same physical laws that governed the Earth. Additionally, the invention of the telescope led to the discovery of further planets and moons. In more recent times, improvements in the telescope and the use of unmanned spacecraft have enabled the investigation of geological phenomena such as mountains and craters, and seasonal meteorological phenomena such as clouds, dust storms and ice caps on the other planets. Today, we are asking ourselves whether there are other solary systems out there wil life forms like ours while we launch missions to explore further planets of our solar system.
After this short introduction to solar system, I plan to continue this series with dedicated posts to planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uran and Neptune), asteroid belt, Kuiper belt and Oort cloud. Indeed, I'll won't be writing specific topic about Sun. but this is because Sun and stars in general have been coevered several times before (you can start here and here for example)
Credits: Wikipedia, Hebrew University of Jerusalem, NASA, University of Copenhagen, arXiv, Tehcnology Review, Phil Plait