- 3545 - PLANETS - how did ours get here? No other place has been found in the Universe that could support life as we know it. But, that has not stopped us from looking. Astronomers have found more than 5,000 planets in other solar systems.
--------------------- 3545 - PLANETS - how did ours get here?
- No other place has been found in the Universe that could support life as we know it. But, that has not stopped us from looking. Astronomers have found more than 5,000 planets in other solar systems.
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- How they formed and how they contain such wide diversity is a new mystery. We thought we had a design for planet formation that matched our Solar System. In contrast, other solar systems are so diverse and supposedly formed out of chaos.
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- The popular theory is that the Sun’s accretion disk rotating around as tiny dust grains. These dust grains clumped together to form solid nuggets of rock. As clumps grew bigger gravity helped to draw more gas and material into a spherical planet. Heavier nearer the Sun formed the rocky planets and lighter material further from the Sun formed the giant gaseous planets.
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- There is a large gap between the terrestrial planets and the gaseous planets called the “ Snow Line”. It lies between 2 and 4 Astronomical Units. The radius of the Earth’s orbit is 1 Astronomical Unit.
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- Our Milky Way Galaxy contains some 100,000,000,000 stars threaded together with clouds of gas and dust. The dust is the ashes of long-dead stars that exploded as supernovae. The dust is microscopic bits of water ice, iron, carbon, other elements and molecules.
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- When a star collapses under the force of gravity it is surrounded by a rotating disk of left over material. Close in to the hydrogen-helium star the heat is so intense as to vaporizes the grains of dust. In the cooler outer regions the dust particles survive and grow.
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- As these dust grains clump together they become the embryos of new born planets. When the clumps of dust get larger than one millimeter they experience a drag force from the intersolar gas. Their orbit slows and they begin to spiral in towards the Sun. However, things are different at the Snow Line. Here the temperatures have formed ice crystals on the grains of dust and they readily coagulate into larger bodies.
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- The larger ice grains have water on their surfaces because they intercept the stars radiation, absorb the energy and re-radiate in infrared light. They get warm. Inside the Snow Line the volatiles have already evaporated. Beyond the Snow Line the ice crystals boil off creating an expanding gas that speeds up their orbits. A tailwind that halts their inward migration.
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- The grains clumps begin to pack themselves into kilometer size bodies. These “planetesimals” begin to sweep up the gas and dust within their orbits. Between 1 and 10 million years these planetary embryos germinate. During this time there are chaotic collisions.
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- Look at all the craters on Mercury, Mars, the Moon to see evidence of these violent times. Collisions and fragmentations created the meteorites. Later the elliptical orbits of all the bodies tended to become circular and larger masses started to clean out their orbits of all the smaller bodies.
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- Jupiter somehow accumulated some 300 Earth masses of gas. Once it had grown to this size its gravity helped collect the material for Saturn, Neptune and Uranus to form. The Snow Line created Jupiter, but Jupiter created the rest.
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- Technically Uranus and Neptune are gas giants but ice giants. As all of these bodies became spaced apart in circular orbits that achieved some immunity to one another’s gravitation influence.
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- The gravity from the giant planets hurled planetesimals into the Kuiper Belt of comets and the “Oort Cloud” of comets. Some still linger or enter the inner Solar System. These meteorites are not just space relics they are space fossils. They are fragments of the original planetesimals that formed the planets.
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- Radiometric dating of asteroids put them @ 4,000,000 years after the Sun’s formation.
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-------------------------- of Mars @ 10,000,000 years
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------------------------- of Earth @ 50,000,000 years.
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----------------------- of Moon @ 100,000,000 years.
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- The Moon was created by a Mars size planetesimals colliding with Earth and splashing Earth’s crust into orbit to eventually form the Moon.
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-------------- 100,000 years the gas compresses to where nuclear fusion first starts and the Sun is born.
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-------------- 2,000,000 years the dust grains form into planetesimals. The gas giants began clearing out the first generation of asteroids.
-------------- 10,000,000 years the first gas giant, Jupiter, triggers the formation of the other giant planets.
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-------------- 800,000,000 years the planets continue to rearrange their orbits and spacing.
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--------------1,000,000,000 years the planets have settled into today’s Solar System.
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- Computer models for planet formation have not yet successfully recreated Jupiter. Jupiter’s core is small to nonexistent. It is far below the critical mass needed to allow the in falling gas to cool. Some other cooling mechanism must exist or some other mechanism eroded Jupiter’s original core. Another problem is that Jupiter should have migrated closer to the Sun. Some mechanism slowed its migration down.
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- Space dust is much more dense than astronomers had thought. Interstellar dust is obscuring approximately half of all the light the Universe is generating. After cataloguing some 10,000 galaxies using a computer model for the precise fraction of starlight blocked by dust they got total agreement between the amount of energy absorbed by dust to that re-emitted by the glowing dust. With these new calculations astronomers can determine the absolute fraction of light that escapes in each direction from a galaxy.
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- Now the models are attempting to match the data on 5,000 planets that have been formed in other solar systems. What they already know is that there is a vast diversity out there. Our Solar System is no grand design. It is simply one of many. For example:
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----------- HD13189 -------------- 4.5 solar masses
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---------- GJ317 ------------------- 0.24 solar masses
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---------- OGLE-TR-56b --------- a planet orbiting 2.25% of an AU
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---------- PSR-B-1620-26b ------ a planet orbiting 23 AU
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--------- NGC-4349- No-127b ------ a planet with 19.8 solar masses.
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--------- PSR-1257+12b ------------- a planet with 2% solar masses.
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- We have 8 planets in our Solar System. Our Sun is only one star and there are billions of stars in our own galaxy. How many other stars have planets? The answer today is 5,000 planets have been found around other suns , and, we are adding new planets continuously.
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- Astronomers are using 2 different methods to find these planets. About 16% were found by the “Transit Method”. Most of the rest were found by the “Radial Velocity Method”.
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- With both of these methods we have to be lucky and happen to be viewing the planet orbits edge-on. If we are viewing the orbits face-on we can never discover planets with these methods. Here is how they work:
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- The “Transit Method” relies on the orbit alignments just right so the planet passes in front of the star.. When this happens the star’s brightness dims slightly as the planet blocks some of the star light during its transit. The amount of dimness and the duration of dimness can tell astronomers the diameter of the planet and its mass.
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- Knowing the diameter astronomers can calculate the volume of the planet. With volume and mass the density of the planet can be calculated. Once the density and diameter are known calculations can be made for the size of the core and the size of the atmosphere and the temperature of the planet.
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- The “Radial Velocity” method works because the star and its planets orbit around a common center of gravity. This results in the star having a slight wobble as the planet orbits around it. Even though we can not see the planet we know it is there and we can calculate its mass and period of orbit from this wobble.
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- Astronomers measure a light spectrum from the star that has distinct emission lines for the elements in the star. For example the element iron has distinct emission lines in their spectrum and a known frequency or wavelength for its emission lines. When the star wobbles towards us the lines shift in wavelength to smaller wavelengths, the blue end of the spectrum. When the star wobbles away from us the wavelength gets wider shifting to the red end of the spectrum.
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- The repetition of these blues shifts and red shifts is the “period” of the wobble of the star and thus the period of the orbit of the planet. If we can calculate the mass of the star we can use the planet period of orbit to calculate the mass of the planet.
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- There are a couple of reasons astronomers use iron element emission lines. First, iron lines are easy to detect. In contrast, oxygen emissions lines are very difficult to detect. Secondly, iron rich stars are the most likely to have planets orbiting them. The reason for this gets into the chemistry of planet formation.
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- The chemistry of the four terrestrial planets:
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------------------ Mercury is 70%metal, 27% silicates, or Si02, or in other words, sand. Mercury is almost all core without an outer crust or mantel. This suggests Mercury was involved in a giant collisions in its early formation that exploded all the mantel in to outer space.
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------------------ Venus and Earth are almost exactly the same chemistry.
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------------------------------ Their mantel is composed of these elements:
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------------------ 46% silicate, SiO2, or sand
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------------------ 38% Magnesium oxide
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----------------- 8% FeO2 , or Ferrous Oxide, or rust.
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----------------- 4% Al2O3, or Aluminum Oxide.
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------------------------------------ Their core is composed of these elements:
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----------------- 80% Fe, or iron.
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----------------- 10% Oxygen or Sulfur, we do not know which it is?
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---------------- 4% Nickel
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------------------------------------- Mars is about the same as Earth-Venus except we do know its core has 14% sulfur, not oxygen.
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----------------- Mar’s core is composed of these elements:
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----------------- 80% Fe, or iron.
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----------------- 14% Sulfur
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---------------- 4% Nickel
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- Jupiter and Saturn are giant gas planets. Uranus and Neptune are giant ice planets. The giant planets have a core and an atmosphere of gas or ice. They do not have a crust or mantel like the terrestrial planets. Jupiter’s core is only 3% of its mass but the core is 50% water ice.
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------------------ Jupiter’s core: ------------ 50% H2O, water
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--------------------------------------------------- 31% rock and metal
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--------------------------------------------------- 1% volatiles
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--------------------------------------------------- 12% refractory CHON
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---------------------------------------------------- 6% NH3, ammonia
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Jupiter is considered a wet planet with that much water. Earth is considered a dry planet. Add up all the water vapor, oceans and rivers and it comes to only 0.02% of Earth mass. Refractory CHON are compounds of carbon, hydrogen, oxygen, and nitrogen.
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----------------- Jupiter’s atmosphere -------- is all hydrogen and helium, with about the same proportions as that of the Sun. Jupiter has a relatively small core at 3%.
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- Uranus and Neptune have mostly core at 90%. Uranus and Neptune have larger cores because they have longer orbits and slower orbital velocities so it took them much longer to form into planets.
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---------- Ratio of core to atmosphere mass ----------- Jupiter -------- 3%
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---------------------------------------------------------------- Saturn -------- 9 to 23% ?
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---------------------------------------------------------------- Uranus ---------90%
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---------------------------------------------------------------- Neptune --------90%
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- Jupiter is the biggest planet and mostly liquid hydrogen and gas atmosphere because its distance from the Sun is right at the “Snow Line” at 5 AU. 5AU is 5 astronomical units which is 5 times the Sun-Earth distance. That spot in the planets orbits is where water freezes into ice.
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- As you move away from the Sun the density of the accretion disk steadily decreases until you reach where water freezes. Then, the density increase by a large amount, from 7 grams / cm^2 to 14 grams / cm^2. That is right where Jupiter formed and it had the densest dust bunnies and snowballs to form with.
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- Jupiter’s core quickly grew to greater than 10* Earth-mass. Once it became this massive its immense gravity quickly attracted a lot of gas into its atmosphere. In less than 1,000 years Jupiter became 320 times more massive than Earth. Saturn is 75 times more massive than Earth.
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- That is the chemistry of the planets, now let’s look at how it all came together. The System started over 5 billion years ago. It started as a massive cloud of gas and dust. Slowly gravity collected and concentrated the cloud into denser material. When dust became denser it started collecting together due to static electricity, like dust bunnies on your kitchen floor.
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- The gas collected into snowballs. As these grew larger they became “planetesimals” in the 1 to 100 kilogram range. Gravity continued to pull the material towards the center. The center became a proto-Sun. As the material gravitated toward the center it was not perfectly symmetrical so it started to rotate. As it condensed further it rotated faster like when an ice skater with arms extended pulls in her arms. As the material rotated faster it flattened out into a disk like a spinning pizza doe.
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- The proto-Sun got massive enough for gravity to start nuclear fusion of hydrogen into helium. A Sun was born. The dust bunnies and snowballs collected together under their own self gravity. Once they became greater than 100 kilograms they became “proto-planets“. Big collisions occurred at a rapid rate. Gravity between proto-planets pulled them into elliptical orbits causing even more collisions. Mercury experienced a collision that knocked away its outer crust. Venus experienced a collision that reversed its rotation on its own axis. Earth experienced a collision that splashed its crust into orbit and formed the Moon.
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- Jupiter became the biggest because it grew right at the “snow line”. In less than 1,000 years it collected all its gas atmosphere at a runaway rate. The planets formed in the rotating disk in 2 to 3 million years. After 5 million years the disk disappears and planets can no longer form.
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- Astronomers do not know what happens to the Accretion Disk. One theory is that after the dispersion of centripetal force flattens and thins out the rotating gas and dust the ultraviolet radiation from the Sun heats up the hydrogen molecules to an escape velocity that sends them out of the solar system and into outer space.
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- Planet formation happens fast. 3 million years out of the 4,500 million year age of the Solar System is a very short time.
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- The raw ingredients needed to form the planets is iron, magnesium, silicon, carbon, and oxygen. All of these elements came from earlier supernovae explosions. Only hydrogen and helium was created in the Big Bang.
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- The Universe had to wait for the first stars to form in order for supernovae to form the heavier elements. So it is these second and third generation stars that have the heavier elements in their accretion disks to form the planets.
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- The element oxygen actually contributes the most mass to the planets. Oxygen is in many forms of silicates, silicon oxides, carbon dioxide and water. The heavier elements are key for planet formation so astronomers look for planets around iron-rich stars.
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- So far, 25% of the iron-rich stars explored have been found to have planets, 322 so far and counting. We are so lucky to have evolved on this particularly nice one.
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April 16, 2022 PLANETS - how did ours get here? 928 965 3545
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--------------------- --- Saturday, April 16, 2022 ---------------------------
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