Astro 1 Section 1 Professor Brandt Monday April 26th 1999 Announcements: Lecture notes: Lecture 43. We have some basic facts about the solar system we want to explain. Disk like shape - orbits in nearly same plane. Generally common direction of rotation and revolution. Two types of planets. Terrestrial inner planets - high density. Jovian outer planets - low density. Lots of debris throughout solar system - asteroids comets meteors. Common ages for Earth Moon Mars meteors Sun. We want a formation theory that naturally explains these facts. Also another relevant fact. Some nearby stars are observed to have disks of gas and dust surrounding them. These basic facts have led us to a model for the solar system. We believe star formation and planet formation are closely linked. <a href="/keyword/solar-nebula/" >solar nebula</a> theory. Formation of solar system was a gradual event. Not a catastrophic one. As stars form in contracting clouds they remain surrounded by cocoons of dust and gas - rotation of this cocoon leads to its collapse into a disk. The planets formed inside this disk sort of as a subsidiary effect of star formation. Took 100 million years. Then Sun fully ignited and blew the rest of the disk away just leaving the planets - a strong wind and radiation pressure. Planet building in the young <a href="/keyword/solar-nebula/" >solar nebula</a> . Objects in the <a href="/keyword/solar-nebula/" >solar nebula</a> grew by 2 basic processes. The sticking together of solid bits of matter. Capturing gas directly from the <a href="/keyword/solar-nebula/" >solar nebula</a> - only worked for objects more than about 15 Earth masses. The terrestrial planets grew by the first process. The Jovians grew the same way at first but then by the second process. The inner parts of the <a href="/keyword/solar-nebula/" >solar nebula</a> were hotter than the outer parts since they were closer to the Sun and this has an important effect on planet formation. It was too hot in the central part for certain materials to condense into solids that could be used to form planets. For example ice chunks could only form quite far away from the Sun. In the very central part only metals and <a href="/keyword/metal-oxide/" >metal oxide</a> s could condense since they have high melting points. Further out metals and <a href="/keyword/metal-oxide/" >metal oxide</a> s and rocky material (silicates) could condense. Still further out 100K metals and <a href="/keyword/metal-oxide/" >metal oxide</a> s rocky material and water ice methane ice ammonia ice. So planets at different distances from Sun ended up being made of different materials. Small &quot;grains&quot; grew to planetesimals which grew to protoplanets. The outer regions had the most solid material since all different types of materials could condense there. Metals and <a href="/keyword/metal-oxide/" >metal oxide</a> s rocky materials ices. So the planets out there grew faster and were able to reach the 15 Earth mass phase when they had strong enough gravity to swallow gas directly. So we can now go back to the observed facts and see if our model can explain them. The disk like shape of the orbits of the planets is due to the disk like shape of the young <a href="/keyword/solar-nebula/" >solar nebula</a> . The two types of planets are due to the fact that the inner <a href="/keyword/solar-nebula/" >solar nebula</a> was hotter than the outer <a href="/keyword/solar-nebula/" >solar nebula</a> and this determined the type of planet that could form. The solar system debris is largely planeteesimals left over from the early solar system that did not go into planets. The common ages of Earth Moon Mars meteors Sun are due to the fact that all the planets and Sun were forming simultaneously in the <a href="/keyword/solar-nebula/" >solar nebula</a> . The final fact that we see gas and dust disks around other stars suggests that planets should be fairly common since they seem to be a &quot;side effect&quot; of star formation. In fact just recently we have found planets around sun like stars. Not Earth like but planets are giant Jovian types. We can't make pictures to see these planets but we rather detect them by the gravitational pull of the planet on its star - causes star to move slightly towards and away from us and we see Doppler shifts. Since October 1995 we have found at least 20 planets orbiting sun-like stars. These planets are quite strange - many are <a href="/keyword/giant-planets/" >giant planets</a> but are very close to the star - comparatively within orbit of Mercury. Unclear how a giant planet got there since we just learned that <a href="/keyword/giant-planets/" >giant planets</a> should form far away. Some of the others have quite elliptical orbits - slingshot ejection of a planet leaves another elliptical. Have not found Earthlike planets yet since their tugs on their stars are not strong enough to detect. Hope to find them in the future. NASA is planning missions to do this using interferometry in space.