{"id":58,"date":"2018-01-04T23:48:09","date_gmt":"2018-01-05T04:48:09","guid":{"rendered":"https:\/\/opentextbc.ca\/physicalgeologyh5p\/chapter\/2-3-how-to-build-a-solar-system-2\/"},"modified":"2023-07-04T12:43:20","modified_gmt":"2023-07-04T16:43:20","slug":"how-to-build-a-solar-system","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/physicalgeologyh5p\/chapter\/how-to-build-a-solar-system\/","title":{"raw":"2.3 How to Build a Solar System","rendered":"2.3 How to Build a Solar System"},"content":{"raw":"A solar system<\/strong> consists of a collection of objects orbiting one or more central stars. All solar systems start out the same way. They begin in a cloud of gas and dust called a nebula<\/strong>. Nebulae are some of the most beautiful objects that have been photographed in space. They have vibrant colours from the gases and dust they contain, and brilliant twinkling from the many stars that have formed within them (Figure 2.6). The gas consists largely of hydrogen and helium, and the dust consists of tiny mineral grains, ice crystals, and organic particles.\r\n\r\n[caption id=\"attachment_52\" align=\"aligncenter\" width=\"600\"]\"\" Figure 2.6<\/strong> The Pillars of Creation within the Eagle Nebula viewed in visible light (left) and near infrared light (right). Near infrared light captures heat from stars and allows us to view stars that would otherwise be hidden by dust. This is why the picture on the right appears to have more stars than the picture on the left. Source: NASA, ESA, and the Hubble Heritage Team (STScI\/AURA) (2015) Public Domain.[\/caption]\r\n

Step 1: Collapse a Nebula<\/h1>\r\nA solar system begins to form when a small patch within a nebula (small by the standards of the universe, that is) begins to collapse upon itself. Exactly how this starts isn\u2019t clear, although it might be triggered by the violent behaviour of nearby stars as they progress through their life cycles. Energy and matter released by these stars might compress the gas and dust in nearby neighbourhoods within the nebula.\r\n\r\nOnce triggered, the collapse of gas and dust within that patch continues for two reasons. One reason is that gravitational force pulls gas molecules and dust particles together. But early in the process, those particles are very small, so the gravitational force between them isn\u2019t strong. So how do they come together? The answer is that dust first accumulates in loose clumps for the same reason dust bunnies form under a bed: static electricity. Given the role of dust bunnies in the early history of the solar system, one might speculate that an accumulation of dust bunnies poses a substantial risk to one\u2019s home (Figure 2.7). In practice, however, this is rarely the case.\r\n\r\n[caption id=\"attachment_53\" align=\"aligncenter\" width=\"650\"]\"Image<\/a> Figure 2.7<\/strong> Dust bunnies have mixed reviews as pets, owing largely to people not knowing how to manage them once they start forming planets and stars. Source: Karla Panchuk (2021) CC BY 4.0. Click for more attributions.[\/caption]\r\n

Step 2: Make a Disk with a Star at Its Centre<\/h1>\r\nAs the small patch within a nebula condenses, a star begins to form from material drawn into the centre of the patch. The remaining dust and gas settle into a protoplanetary disk<\/strong> that rotates around the star. The disk is where planets will eventually form. Figure 2.8 (top left) is an artist\u2019s concept of a protoplanetary disk, and Figure 2.8 (top right) is an actual protoplanetary disk surrounding the star HL Tauri. Notice the dark rings within the HL Tauri protoplanetary disk. These are gaps formed by the collection of dust and debris by incipient planets, called protoplanets<\/strong>, as they orbit the star. There is an analogy for this in our own solar system, because the dark rings are akin to the gaps in the rings of Saturn (Figure 2.8, bottom), where moons can be found.\r\n\r\n[caption id=\"attachment_54\" align=\"aligncenter\" width=\"650\"]\"\"<\/a> Figure 2.8<\/strong> Protoplanetary disks and Saturn\u2019s rings. Top left: Artist's concept of a protoplanetary disk containing gas and dust, surrounding a new star. Top right: A photograph of the protoplanetary disk surrounding HL Tauri. The dark rings within the disk are thought to be gaps where newly forming planets are sweeping up dust and gas. Bottom left: A photograph of Saturn showing similar gaps within its rings. The bright spot at the bottom is an aurora, similar to the northern lights on Earth. Bottom right: a close-up view of a gap in Saturn\u2019s rings showing a moon as a white dot. Source: Karla Panchuk (2021) CC BY 4.0. Click for more attributions.[\/caption]\r\n

Step 3: Build Some Planets<\/h1>\r\n

Types of Planets<\/h2>\r\nBroadly speaking, planets can be classified into three categories based on their composition (Figure 2.9). Terrestrial planets<\/strong> are planets like Earth, Mercury, Venus, and Mars, which have a metal core surrounded by rock. Jovian planets <\/strong>(also called gas giants<\/strong>) are planets like Jupiter and Saturn that are mostly hydrogen and helium. Ice giants<\/strong> are planets like Uranus and Neptune with rocky cores enveloped by water ice, methane (CH4<\/sub>) ice, and ammonia (NH3<\/sub>) ice.\r\n\r\n[caption id=\"attachment_55\" align=\"aligncenter\" width=\"650\"]\"\"<\/a> Figure 2.9<\/strong> Three types of planets. Jovian (or gas giant) planets such as Jupiter consist mostly of hydrogen and helium. They are the largest of the three types. Ice giant planets such as Uranus are the next largest. They contain water, ammonia, and methane ice, and have rocky cores. Terrestrial planets such as Earth are the smallest, and they have metal cores covered by rocky mantles. Source: Karla Panchuk (2021) CC BY-NC-SA 4.0. Click for more attributions.[\/caption]\r\n\r\n
\r\n\r\nWhat were the earliest planets like?<\/strong>\r\n
[h5p id=\"26\"]<\/div>\r\n
\r\n\r\nAstronomers looking for some of the earliest stars in the universe were surprised to find a planetary system called HIP 11952, which existed 12.8 billion years ago. This was very early in the universe\u2019s history, when stars still consisted largely of hydrogen and helium.\r\n\r\nDid HIP 11952 have terrestrial planets?<\/strong>\r\n\r\nAnswer: No. The planetary system had two Jupiter-sized gas giant planets. Gas giant planets contain large amounts of hydrogen, which was plentiful in the early universe. In contrast, terrestrial planets have heavier elements, especially silica, iron, magnesium, and nickel, that had yet to be manufactured by stars in sufficient abundance to form terrestrial planets.\r\n\r\n<\/div>\r\n<\/div>\r\n

Arrangement of Solar System Objects<\/h2>\r\nThe three types of planets are not mixed together randomly within our solar system. Instead they occur in a systematic way, with terrestrial planets closest to the sun, followed by the Jovian planets and then the ice giants (Figure 2.10).\r\n\r\nSmaller solar system objects follow this arrangement as well. The asteroid belt<\/strong> contains bodies of rock and metal. Bodies ranging from metres to hundreds of metres in diameter are classified as asteroids<\/strong>, and smaller bodies are referred to as meteoroids<\/strong>. In contrast, the Kuiper belt <\/strong>(Kuiper<\/em> rhymes with piper<\/em>), and the Oort cloud<\/strong> (Oort<\/em> rhymes with sort<\/em>), which are at the outer edge of the solar system, contain bodies composed of large amounts of ice in addition to rocky fragments and dust.\r\n\r\n[caption id=\"attachment_56\" align=\"aligncenter\" width=\"864\"]\"\"<\/a> Figure 2.10<\/strong> Our solar system. Top: Solar system objects with distances to scale. Distances are in astronomical units (AU), where 1 AU is the average distance from Earth to the sun. The edge of the Kuiper belt extends to 50 AU (7.5 billion km), but this distance is minuscule compared to the size of the solar system as a whole, which extends to the edge of the Oort cloud, thought to be 15 trillion km away. Bottom: Solar system with the sun and planets to scale. The gas giants are the largest planets, followed by the ice giants, and then the terrestrial planets. Note that the planets in this diagram likely do not reflect the entire population of planets in our solar system because evidence suggests that large planets are present beyond the Kuiper belt. Source: Karla Panchuk (2018) CC BY-NC-SA 4.0. Click for more attributions.[\/caption]\r\n\r\nThis arrangement is related to the temperatures surrounding the young sun. The frost line<\/strong> (or snow line) marks the division between the inner part of the protoplanetary disk closer to the sun, where it was too hot to permit anything but silicate minerals and metal to crystalize, and the outer part of the disk farther from the sun, where it was cool enough to allow ice to form. As a result, the objects that formed in the inner part of the protoplanetary disk consist largely of rock and metal, while the objects that formed in the outer part consist largely of gas and ice.\r\n
\r\n\r\nConcept check: Frost line<\/strong>\r\n\r\n
Tau Ceti is a solar analog, meaning that it's very similar to our sun in terms of size, temperature, and colour. It's not exactly like the sun, however\u2014it's slightly smaller, cooler, and older.\"\"\r\n\r\nHow does Tau Ceti's frost line compare to that of the sun? Choose one:\r\n