Click to advance the presentation Why do we have two types of planets? Jovians ‣ Ice could also form small particles outside the frost line. ‣ Larger planetesimals and planets were able to form. ‣ Gravity of these larger planets was able to draw in surrounding hydrogen and helium gases. Jovian protoplanets grew big enough to capture gas from the solar nebula Lunar and Planetary Institute According to the solar nebula theory, the Jovian planets began growing by the same processes that built the terrestrial planets. The outer solar nebula not only contained solid bits of metals and silicates—it also included abundant ices. The jovian protoplanets grew large enough (10-15 Earth masses) that their gravity was strong enough to draw in large amounts of gas from the solar nebula. The bigger protoplanets were able to attract even more gaseous material unto themselves. Because of it's position in the solar nebula, the proto-Jupiter, was able to draw an enormous amount of gas unto itself, and become the biggest of the planets. 17
Click to advance the presentation The solar wind drives out gas ‣ After ~10 million years, Sun begins nuclear reactions ‣ Solar wind becomes strong enough to blow away remaining gas ‣ Jovian planets stop accreting H/He gas ‣ Young planets and leftover planetesimals remain in disk Solar wind speeds measured by the Ulysses spacecraft ESA Astronomers calculate that after ~10 million years the sun became hot and luminous enough to blow away the gas remaining in the solar nebula. Thus, the Jovian planets must have grown to their present size in about 10 million years. The terrestrial planets grew from solids and not from the gas. So, they continued to grow by accretion from solid debris left behind when the gas was blown away. 18
Click to advance the presentation The solar nebula hypothesis: in a nutshell 1. Solar nebula contracts due to its own gravity • Spins faster as it contracts - conserve angular momentum 2. Protosun forms at center, • Nebula flattens into a disk 3. Dust grains condense out of gas; grains accrete into planetesimals 4. Large planetesimals accrete smaller ones, grow to become planets 1. 2. 3. 4. Figure by MIT OpenCourseWare The figure above summarizes our current theory of solar system formation. The sun forms from a gravitationally collapsing ball of gas and dust that flattens to form a spinning disk. The disk reaches an equilibrium size while the protosun continues to collapse in the center. Dust grains collect into clumps and the clumps collect into planetesimals. About this time nuclear reactions begin in the sun and the radiation pressure and solar wind push the remaining gases out of the solar system leaving the large planetesimals. Over time the planetesimals either merge or are ejected from the solar system, leaving the few planets in nearly circular orbits.
- Spring '11
- Solar System, Planet, solar nebula