The Formation of the Solar System

Model Requirements Each planet is relatively isolated in space. The orbits of the planets are nearly circular. The orbits of the planets all lie in nearly the same plane. The direction in which the planets orbit the sun (counterclockwise as viewed from above Earth’s north pole) is the same as the direction which the sun rotates on its axis. The direction in which most planets rotate on their axis is roughly the same as the direction in which the sun rotates on its axis (not Venus, Uranus, and Pluto).

Model Requirements The direction in which most of the known moons revolve about their parent planet is the same as the direction in which the planet rotates on its axis. Our planetary system is highly differentiated (terrestrial and jovian planets). Asteroids are very old and exhibit a range of properties not characteristic of either the terrestrial or the jovian planets or their moons. The Kuiper belt is a collection of asteroid-sized icy bodies orbiting beyond Neptune. The Oort cloud comets are primitive, icy fragments that do not orbit in the plane of the ecliptic and reside primarily at large distances from the sun.

Nebular Contraction Cloud of interstellar dust and gas - a nebula, begins to contract (for whatever reason) under its own gravity. As it contracts, it becomes denser and hotter, eventually forming a star at its center. As it contracts, the cloud spins faster and faster forming a flattened pancake-shaped disk (due to angular momentum).

Nebular Contraction The flattened-pancake is usually referred to as the solar nebula since it will form our solar system. The idea that planets form from such a disk is called the “nebular theory.” We have seen such disks formed in other systems. The old nebular theory is wrong as we now know clumps of matter would not form from the gas as they would have dispersed and not formed planets.

Newborn Solar System?

Condensation Theory The current theory, condensation theory, is built on the nebular theory. Key ingredient - interstellar dust in the solar nebula. The dust acts as condensation nuclei (microscopic platforms to which other atoms can attach) and helps the cloud cool enough for condensation to occur in the first place.

Planet Formation According to condensation theory, the planets formed in 3 distinct stages. First 2 apply to all planets, 3rd applies only to the Jovian worlds. Stage one –Dust grains in the solar nebula formed condensation nuclei. These clumps then stick to other clumps, causing the clumps to grow in size rapidly. –The process of accretion (gradual growth of objects by collision and sticking) created objects a few hundred km across. –At the end of the first stage, solar system consisted of hydrogen and helium gas and millions of planetesimals (objects size of small moons having gravitational fields just strong enough to affect their neighbors).

Planet Formation Stage two –Gravitational forces between planetesimals caused them to collide and merge, forming larger and larger objects. –Because larger objects have stronger gravitational pulls, eventually almost all of the planetesimal material was swept up into a few large “protoplanets” (accumulations of matter that would eventually evolve into the planets we know today). –The asteroids and comets originated as collisions between planetesimals and protoplanets sent out small chunks of material that escaped capture.

Planet Formation After 100 million years, we have –Nine protoplanets. –Dozens of protomoons. –A glowing protosun at the center. Roughly a billion years were required to “sweep” the system clear of interplanetary “trash.” This is a period of intense meteoritic bombardment whose effects on the moon and elsewhere are still evident today.

Making the Jovian Planets There are two conflicting views on how the Jovian planets formed. View one –4 largest protoplanets became massive enough to enter a 3rd stage of evolution - sweeping up large amounts of gas directly from the solar nebula. View two –Giant planets formed through instabilities in the cool outer regions of the solar nebula - mimicking on small scales the collapse of the initial interstellar cloud.

Making the Jovian Planets Many of the Jovian moons probably also formed by accretion. Some of the smaller moons may be captured planetesimals. Eventually, the sun blew away any remaining gas between the planets, which is why we don’t see it today (the outer planets must have formed before the nebular gas dispersed!).

The Differentiation of the Solar System The closer to the protosun, the hotter the temperature. The temperature determines what could form where and when. (Note that as the solar nebula contracted due to gravity, it heated up as it flattened into a disk.) In the innermost regions (Mercury), only metallic grains could form due to the high temperature. At 1 AU, rocky, silicate grains could form. Beyond 3 or 4 AU, water ice could exist, and so on. More and more matter could condense out at greater and greater distances from the sun. Further out, water vapor, ammonia, and methane could condense into solid form, creating the cores of the Jovian planets.

Lecture Tutorial: Temperature and Formation of our Solar System (p. 103) Work with a partner! Read the instructions and questions carefully. Discuss the concepts and your answers with one another. Take time to understand it now!!!! Come to a consensus answer you both agree on. If you get stuck or are not sure of your answer, ask me or another group.

Asteroids and Comets Planetesimals beyond the orbit of Mars failed to accumulate into a protoplanet due to the large gravitational field of Jupiter constantly disturbing their motion. These are in the asteroid belt and also include the Trojan asteroids. Planetesimals further out were “kicked” into outer orbits and form the Oort cloud. Most planetesimals formed beyond Neptune are still there and make up the Kuiper belt. The condensation theory could not account for the water and other volatile gases found on Earth and elsewhere. Comets, containing both water and other volatile gases, bombarded the inner planets after they were formed. Thus, the water on Earth originated in comets.

Random Encounters in the Solar Nebula Random collision of planetesimals and other bodies are allowed within the current condensation theory. These random collisions can be used to explain everything from Venus’ slow retrograde motion (due to two protoplanets of comparable mass colliding nearly head-on) to the formation of Earth’s moon.

Detecting Extrasolar Planets More than 170 extrasolar planets have been found in more than 145 separate systems. We generally can’t observe any of the extrasolar planets directly. As a planet orbits a star, gravitationally pulling one way and then the other, the star “wobbles” slightly - we can measure this wobble and determine the mass of the planet.

Detecting Extrasolar Planets More than 170 extrasolar planets have been found in more than 145 separate systems. We generally can’t observe any of the extrasolar planets directly. As a planet orbits a star, gravitationally pulling one way and then the other, the star “wobbles” slightly - we can measure this wobble and determine the mass of the planet.

Detecting Extrasolar Planets

Lecture Tutorial: Motion of Extrasolar Planets (p. 117) Work with a partner! Read the instructions and questions carefully. Discuss the concepts and your answers with one another. Take time to understand it now!!!! Come to a consensus answer you both agree on. If you get stuck or are not sure of your answer, ask me or another group.

Planetary Properties 5 % of nearby stars surveyed so far show signs of extrasolar planets. About a dozen of these systems contain more than one “observed” planet. These planets are usually Jupiter-sized and nearby their parent stars - called “hot Jupiters.” These planets often appear to be the sole large body in their system. The above two facts are due to a selection effect - the effects of smaller planets and those located at larger distances from their parent stars are difficult to detect.

Is Our Solar System Unusual? Planetary systems are quite common. Those systems discovered so far do not look like our own. We have explanations for how Jupiter-like planets can wind up close to the parent star (compared to our system), but we don’t yet know if that is the norm (or whether it’s more common for Jupiter-like planets to be farther out as in our solar system).