The Origin of the Solar System

1. Its Structure and Evolution The Origin of the Solar System

2. How it all Began • Aristarchus of Samos was the first to speculate a heliocentric model of the cosmos. • Mikołaj Kopernik was the first to give it a mathematical background. • In the 17th century, Galileo, Newton and Kepler were responsible for bringing in a physical understanding of the situation and firmly establishing a heliocentric model.

3. The Initial Theories • René Descartes first proposed that the universe was filled with vortices of swirling particles and the solar system had originated from a particularly large vortex due to collapse and condensation • This was before Newton’s theory of gravitation was developed and though it explained the motion of the planets and was right with condensation, it was inaccurate as we know matter doesn’t behave this way. It also had problems explaining the distribution of angular momentum in the solar system.

4. The nebular hypothesis was first proposed in 1734 by Emanuel Swedenborg and later elaborated and expanded upon by Immanuel Kant in 1755. A similar theory was independently formulated by Pierre-Simon Laplace in 1796. • It postulates that the solar system was formed out of a cloud/ nebula due to condensation. This too fails to explain the “anomalous” angular momenta of the bodies in the system.

5. In 1749, Georges-Louis Leclerc, Comte de Buffon conceived the idea that the planets were formed when a comet collided with the Sun, sending matter out to form the planets. • However, Laplace refuted this idea in 1796, showing that any planets formed in such a way would eventually crash into the Sun. • Laplace felt that the near-circular orbits of the planets were a necessary consequence of their formation. •  Today, comets are known to be far too small to have created the Solar System in this way.

6. Attempts to resolve the angular momentum problem led to the temporary abandonment of the nebular hypothesis in favour of a return to "two-body" theories. • For several decades, many astronomers preferred the tidal or near-collision hypothesis put forward by James Jeans in 1917, in which the planets were considered to have been formed due to the approach of some other star to the Sun. • This near-miss would have drawn large amounts of matter out of the Sun and the other star by their mutual tidal forces, which could have then condensed into planets. • However, in 1929 astronomer Harold Jeffreys countered that such a near-collision was massively unlikely. Objections to the hypothesis were also raised by the American astronomer Henry Norris Russell, who showed that it ran into problems with angular momentum for the outer planets, with the planets struggling to avoid being reabsorbed by the Sun.

7. For some time people also believed that the solar system could have formed by the destruction of a companion star of the sun. • The companion could have gone nova or could have collided with another body (possibly a star) and the debris left behind could have formed the planets. • This solves the angular momentum problem but it cannot explain the terrestrial planets’ existence as they are too small to have formed in this scenario.

8. Some theories also involve the sun passing through a nebulous region in space and the subsequent capture of dust. • This dust could have then condensed into planets. • The problem with this model is that the regions of dust in space are far too scarce for enough matter to have been collected by the sun. • Moreover, once captured, the dust would remain distributed in a spherically symmetric fashion, not forming the planets.

9.  W. H. McCrea proposed the protoplanet theory, in which the Sun and planets individually coalesced from matter within the same cloud, with the smaller planets later captured by the Sun's larger gravity. • It includes fission in a protoplanetary nebula. • The inner protoplanets were Venus-Mercury and Earth-Mars. • The moons of the greater planets were formed from "droplets" in the neck connecting the 2 portions of the dividing protoplanet and these droplets could account for some of the asteroids. • It predicts certain observations such as the similar angular velocity of Mars and Earth with similar rotation periods and axial tilts.

10. Re-emergence of the Nebular Hypothesis • In 1978, astronomer A. J. R. Prentice revived the nebular model in his theory by suggesting that the angular momentum problem could be resolved by drag created by dust grains in the original disc which slowed down the rotation in the centre. • One possible explanation suggested by HannesAlfvén was that angular momentum was shed by the solar wind during its T Tauri phase. The momentum is probably transported to the outer parts of the disk, but the precise mechanism of this transport is not well understood. • Another possible process for shedding angular momentum is magnetic braking, where the spin of the star is transferred into the surrounding disk via that star's magnetic field.

12. Problems that Still persist in the Nebular Hypothesis • The formation of planetesimals is the biggest unsolved problem in the Nebular Disk Model. How 1 cm sized particles coalesce into 1 km planetesimals is a mystery. • The formation of giant planets is another unsolved problem. Current theories are unable to explain how their cores can form fast enough to accumulate significant amounts of gas from the quickly disappearing protoplanetary disk. The mean lifetime of the disks, which are less than 107 years, appears to be shorter than the time necessary for the core formation.

13. Another problem of giant planet formation is their migration. Some calculations show that interaction with the disk can cause rapid inward migration, which, if not stopped, results in the planet reaching the central regions still as a “premature Jupiter”.

14. Protostars • A protostar is a large mass that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium. • The protostellar phase is an early stage in the process of star formation. For a one solar-mass star it lasts about 100,000 years. • It starts with a core of increased density in a molecular cloud and ends with the formation of a T Tauri star. • When fully developed it releases T Tauri wind, a type of super solar wind that marks the change from the star accreting mass into radiating energy.

15. T Tauri Stars • T Tauri stars (TTS) are a class of variable stars named after their prototype – T Tauri. They are found near molecular clouds. • They are bright but non-massive stars. (Bright because of their large radius). • They are too cool for hydrogen fusion. Instead they are powered by gravitational collapse. • This stage of star formation lasts for about 100 million years. • Many have extremely powerful stellar winds. 

16. The Structure of the Solar System • One Sun • Four terrestrial planets (and a couple of moons) (Random types of rocks in the middle) • Four Gas Giants (a lot more moons) (More rocks, ice and other stuff!)

17. The Sun (Sol) • A G2 type star. • Peak emission at green. • Surface temperature ~ 5800K • Completes one rotation in ~30 days. • Accounts for 99.86% of solar system mass. • Rotates faster at the equator than the poles.

18. The Planets and the Moons • A planet is an astronomical object orbiting a star or stellar remnant that is • massive enough to be rounded by its own gravity • is not massive enough to cause thermonuclear fusion • and has cleared its neighbouring region of planetesimals.

19. Mercury • When viewed through a telescope, it shows phases like the moon. • Has a rather eccentric orbit that precesses quite noticeably. • No volcanic activity in the present. • Has an atmosphere!! • Max to min solar intensity ~ 2:1. • Tilt of axis ~ 20w.r.t plane of revolution.

20. Venus • Hottest planet in the solar system (~9000 C on surface). • Third brightest object in sky. • Retrograde rotation • “Day” longer than “year”. • No shadows on Venus. • Clouds on top layer of atmosphere complete on revolution in ~ 100 hrs.

21. Earth • Has the largest moon (relative to planet radius). • Largest of the terrestrials. • Only planet with ice-candies, lemonades and hot saunas. • Has a magnetic field. The poles switch irregularly. • The axis of the earth precesses. (the pole star hasn’t been the same forever!)

23. The Moon • Tidally locked to earth. • We can see 59% of the moon’s surface from Earth! This is due to libration. • Is responsible for tides on Earth. • Totally eclipsed moon appears red! • The Earth-Moon system probably formed as a double planet.

24. Mars • Red due to large content of iron in soil. • Has polar ice caps that melt and reform cyclically. • Is believed to once have had liquid H2O on surface. • Wind speed reach 120 kmph near the surface. • Houses the largest volcanic mountain in the solar system.

25. Jupiter • Emits more energy than it receives from the sun. • Has 67 confirmed moons. • Rotates about its axis in around 10 hours. This makes it rather ellipsoidal in shape. • The great red spot has been around as long as we have been observing Jupiter.

26. The Galilean Moons

27. Ganymede is the largest of Jupiter’s moons. Larger than Mercury. • Shallow craters imply icy crust. • Callisto is next in size. • Covered in dark material and some of its craters have concentric rings. • May have an underground ocean. It is believed to be so because it has bright areas that look like water frozen after having “bled” through craters. • Has a magnetic field. Possibly because of the salty underground ocean.

28. Io is the third in size. • Volcanically active due to tidal forces. • Has a thin atmosphere as a result. • Europa has a crust of ice and possibly a mantle of water. • Most promising body (after earth :P) in the solar system for life. • Smallest among the four. • High albedo (proportion of light it reflects).

29. Saturn • Rings!!! Huge and Brilliant!!! • Lightning in the atmosphere all the time. • Least dense among all planets. • Rotation period ~9 hrs. • A regular hexagonal cloud formation exists on the north pole. This is believed to be due to a steep latitudinal gradient of wind speeds.

30. Titan • Only moon with a significant atmosphere. So thick that it hides surface features from view. • Mainly composed of nitrogen and methane. • It is possible that it has weather systems with methane being the substance cycled through the phases.

31. Enceladus • Reflects almost all light that falls on it. • Exhibits “cryovolcanic activity”. That is, it shoots geysers of water vapour, other volatiles and crystals (like NaCl) into space. • Is in orbital resonance with Dione, the fourth largest Saturnian moon.

32. Uranus • The Tilted One! • Rings. Predicted by William Herschel. Discovered by chance by Elliot, Dunham and Mink while studying occultation of a star by Uranus. • The magnetic field is lopsided and hence you can see auroras near the equator.

33. Neptune • The only planet whose existence was predicted mathematically. • Windiest planet in the solar system. • Has a system of storms called scooters that races across the planet. • The “Great Dark Spot” forms and dissipates every few years. • Has rings. Not so remarkable.

34. Triton • Retrograde orbit ! • Chilliest surface in the solar system. ~ -2350 C. • More rocky than expected. • Greatly tilted plane or revolution. • Possibly a captured object.

35. Pluto & Charon • Not planets. • Tidally locked completely. • Is in orbital resonance with Neptune. • Highly eccentric orbit.