- 3671 - SUN - how its planets formed? Our Sun is the source of life on Earth. Its calm glow across billions of years has allowed life to evolve and flourish on our world. This does not mean our Sun doesn’t have an active side. Do we need to worry?
--------------------- 3671 - SUN - how its planets formed?
- We have observed massive solar flares, such as the 1859 Carrington event, which produced northern lights as far south as the Caribbean, and drove electrical currents in telegraph lines. If such a flare occurred in Earth’s direction today, it would devastate our electrical infrastructure. But fortunately for us, the Sun is mostly calm. Unusually calm when compared to other stars.
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- Astronomers have only recently studied the activity of the Sun. The oldest study, undertaken since the 1600s, follows counts the number of spots seen on the Sun’s surface. It has shown us that the Sun goes through cycles of active and quiet periods. A four-century study is long in human terms, but is barely a moment of cosmic time.
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- Longer studies have looked at isotopes of carbon and other elements in ice cores and tree rings. When the Sun is particularly active, high-energy protons can strike atoms in the upper atmosphere, converting them into radioactive isotopes. They can then become trapped in ice and wood. This gives us an idea of solar activity across nearly ten thousand years.
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- That is still only a fraction of the Sun’s lifetime of 14 billion years. Is the past few thousand years a good sample of solar activity? What if the Sun just happens to be going through an unusually calm period, and is usually far more active?
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- Using data from the “Gaia” spacecraft astronomers found stars of similar mass, age, and surface temperature. From these they chose stars that not only had a similar metallicity, but also a similar rotational speed. They were left with 369 stars that are nearly twins of our Sun.
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- They then compared the Sun’s variation in activity over four years to the activity of these other stars. They found that the Sun’s activity is much lower than the others. The variability of other stars is five times stronger than our Sun. Solar flares such as the Carrington event are much more common on other stars.
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- This could mean that our Sun has been usually calm during the span of human civilization. If that’s the case, it could become more active in the future, which could have serious consequences for our civilization. It is also possible that there is some unknown factor that keeps our Sun so calm.
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- At the moment, there is no indication that the Sun might enter a hyperactive period. For now and for the foreseeable future we can continue to enjoy the calm of the Sun.
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- Astronomers have managed to link the properties of the inner planets of our solar system with our cosmic history using the emergence of ring structures in the swirling disk of gas and dust in which these planets were formed. The rings are associated with basic physical properties such as the transition from an outer region where ice can form where water can only exist as water vapor.
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- Around a young star, a "protoplanetary disk" of gas and dust forms, and inside that disk grow ever-larger small bodies, eventually reaching diameters of thousands of kilometers and becoming planets.
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- The first image taken by the “ALMA” observation after its completion in 2014. The image showed the protoplanetary disk around the young star “HL Tauri” in unprecedented detail, and the most stunning details amounted to a nested structure of clearly visible rings and gaps in that disk.
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- Researchers involved in simulating protoplanetary disk structures took in these new observations and it became clear that such rings and gaps are commonly associated with "pressure bumps," where the local pressure is somewhat lower than in the surrounding regions. Those localized changes are typically associated with changes in disk composition, mostly in the size of dust grains.
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- There are pressure bumps associated with particularly important transitions in the disk that can be linked directly to fundamental physics. Very close to the star, at temperatures higher than 1,400 Kelvin, silicate compounds ( "sand grains") are gaseous. It is simply too hot for them to exist in any other state.
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- That means that planets cannot form in such a hot region. Below that temperature, silicate compounds "sublimate," that is, any silicate gases directly transition to a solid state. This pressure bump defines an overall inner border for planet formation.
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- Farther out from the star, at 170 Kelvin, there is a transition between water vapor on the one hand and water ice on the other hand, known as the “water snowline“. The reason that temperature is so much lower than the standard 0 degrees Celsius where water freezes on Earth is the much lower pressure, compared to Earth's atmosphere. At even lower temperatures, 30 Kelvin is the CO snowline; below that temperature, carbon monoxide forms a solid ice.
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- Numerous earlier simulations had already shown how such pressure bumps facilitate the formation of planetesimals, the small objects, between 10 and 100 kilometers in diameter, that are believed to be the building blocks for planets.
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- The formation process starts much, much smaller, namely with dust grains. Those dust grains tend to collect in the low-pressure region of a pressure bump, as grains of a certain size drift inwards until they are stopped by the higher pressure at the inner boundary of the bump.
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- As the grain concentration at the pressure bump increases, and in particular the ratio of solid material to gas which tends to push grains apart increases, it becomes easier for those grains to form pebbles, and for those pebbles to aggregate into larger objects. Pebbles are what astronomers call solid aggregates with sizes between a few millimeters and a few centimeters.
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- Our own Solar system has the characteristic distribution of rocky, terrestrial inner planets and outer gaseous planets. Astronomers constructed a model of a gas disk, with three pressure bumps at the silicates-become-gaseous boundary and the water and CO snow lines.
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- They then simulated the way that dust grains grow and fragment in the gas disk, the formation of planetesimals, the growth from planetesimals to planetary embryos from 100 km in diameter to 2000 km near the location of our Earth. "1 astronomical unit" distance from the sun, the growth of planetary embryos to planets for the terrestrial planets, and the accumulation of planetesimals in a newly-formed asteroid belt.
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- In our own solar system, the asteroid belt between the orbits of Mars and Jupiter is home to hundreds of smaller bodies, which are believed to be remnants or collision fragments of planetesimals in that region that never grew to form planetary embryos.
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- The results suggest a direct link between the appearance of our solar system and the ring structure of its protoplanetary disk. It was a complete surprise how well models were able to capture the development of a planetary system like our own right down to the slightly different masses and chemical compositions of Venus, Earth and Mars.
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- In those models, planetesimals in those simulations formed naturally near the pressure bumps, as a "cosmic traffic jam" for pebbles drifting inwards, which would then be stopped by the higher pressure at the inner boundary of the pressure bump.
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- The simulations even go so far as to explain the slightly different chemical compositions of Mars on the one hand, Earth and Venus on the other: In the models, Earth and Venus indeed collect most of the material that will form their bulk from regions closer to the sun than the Earth's current orbit (one astronomical unit). The Mars-analogs in the simulations, in contrast, were built mostly from material from regions a bit farther away from the sun.
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- Beyond the orbit of Mars, the simulations yielded a region that started out as sparsely populated with or, in some cases, even completely empty of planetesimals, the precursor of the present-day asteroid belt of our solar systems. However, some planetesimals from the zones inside of or directly beyond would later stray into the asteroid belt region and become trapped.
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- As those planetesimals collided, the resulting smaller pieces would form what we today observe as asteroids. The simulations are even able to explain the different asteroid populations: What astronomers call S-types asteroids, bodies that are made mostly of silica, would be the remnants of stray objects originating in the region around Mars, while C-type asteroids, which predominantly contain Carbon, would be the remnants of stray objects from the region directly outside the asteroid belt.
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- In that outer region, just outside the pressure bump that marks the inner limit for the presence of water ice, the simulations show the beginning of the formations of giant planets The planetesimals near that boundary typically have a total mass of between 40 and 100 times the mass of the Earth, consistent with estimates of the total mass of the cores of the giant planets in our solar system: Jupiter, Saturn, Uranus and Neptune.
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- Kuiper-belt objects, which formed outside the outermost pressure bump, marks the inner boundary for the existence of carbon monoxide ice. It even can explain the slight differences in composition between known Kuiper-belt objects.
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- The spread of simulations led to two basic outcomes: Either a pressure bump at the water-ice snowline formed very early; in that case, the inner and outer regions of the planetary system went their separate ways rather early on within the first hundred thousand years. This led to the formation of low-mass terrestrial planets in the inner parts of the system, similar to what happened in our own solar system.
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- Alternatively, if the water-ice pressure bump forms later than that or is not as pronounced, more mass can drift into the inner region, leading instead to the formation of super-Earths or mini-Neptunes in the inner planetary systems. Evidence from the observations of those exoplanetary systems astronomers have found so far shows that case is by far the more probable, and our own Solar system a comparatively rare outcome of planet formation.
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- For the inner solar system, at least, we now know that key properties of Earth and its nearest neighboring planet can be traced to some rather basic physics: the boundary between frozen water and water vapor and its associated pressure bump in the swirling disk of gas and dust that surrounded the young sun.
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September 1, 2022 SUN - how its planets formed? 3671
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