1445 Introductory Astronomy I Chapter 5a Planetary Systems R. S. Rubins Fall, 2010
Five Essential Things to do in Space 1 • In an article published in Scientific American in 2007, George Musser lists the following goals for NASA. • 1. Monitor Earth’s Climate (from Space). • 2. Prepare an Asteroid Defense. • 3. Seek Out New Life. • 4. Explain the Genesis of the Planets. • 5. Break out of the Solar System • He asks whether NASA is about understanding the Earth, the space shuttle and station, human exploration, exploring the solar system, exploring the outer universe and space, or science in general?
Five Essential Things to do in Space 2 • 1. Monitor Earth’s Climate Program has been underfunded for over a decade. New orbiting measuring instruments are needed to be in place before the older satellites die. • 2. Prepare an Asteroid Defense Asteroids 10 km across (dinosaur killers) hit about every 100 million years. Asteroids 50 meters across (city destroyers) hit about once per millenium. To deflect an asteroid by one Earth radius, its velocity should be reduced by 1 mm/s, a decade in advance. This program is also underfunded.
Five Essential Things to do in Space 3 • 3. Seek Out New Life Here NASA is taking a “follow the water approach”, by studying Mars, Jupiter’s moon, Europa, and Saturn’s moons, Titan andEnceladus. Many experiments are being made on Mars, but there is a need to dig at least 2 meters below its toxic surface, and also to bring samples back for study. • 4. Explain the Genesis of the Planets There is a special need to understand Jupiter, the first-born and largest planet, which probably influenced the formation of the rest. Comets, which were the collectors of the early solar system material, are also in need of much more detailed study.
Five Essential Things to do in Space 4 • 5. Break out of the Solar System This has been achieved by the Voyager 1and2spacecraft, launched in 1977, which are now over 100 AU from the Earth, having crossed a major boundary of the solar system – the termination shock – roughly 8 billion miles from the Sun, in 2004 and 2006 respectively. There are now European and American proposals for much faster and more efficient systems using ion drive propulsion, rather than rocket propulsion of earlier space vehicles..
Solar System: Significant Data 1 A theory of the solar system should explain the following. • The planets orbit in planes close to the ecliptic. • The planets revolve about the Sun in the same direction as the Sun’s rotation. • With the exceptions of Venus, Uranus and Pluto, the planets rotate in the same direction as their orbits about the Sun 4. With the exceptions of Mercury and Pluto, the planetary orbits are almost circular. 5. The smaller rocky planets (the terrestrial planets) are nearer to the Sun, and the larger gaseous planets (the gas giants or Jovian planets) are further from it. • Gas giants have ring systems.
The Solar System: Significant Data 2 7.The spacings between neighboring planets between Venus and Neptune increase with distance from the Sun. • Planets with solid surfaces (terrestrial planets) show evidence of craters. • The terrestrial planets contain less than 0.2% of the lightest elements, hydrogen and helium, which constitute over 99% of the Sun. • The gas giants are primarily composed of volatile gases, particularly H and He. 11. The existence of asteroids, meteoroids and comets. 12. Data from the Sun and planets are consistent with the solar system having been formed about 4.5 billion years ago.
Radioactive Dating • Radioactive dating measures the time at which rocks solidified, since radioactive products prior to that time would have escaped. • Such measurements give an age of close to 4.5 billion years for the oldest rocks on the Earth (the Jack Hills Zircons in Australia), as well as for samples obtained from the Moon and meteorites. • In the example shown above, the isotope 40K (potassium) decays into Ca and Ar with a half-life of 1.28 billion years.
Origin of the Solar System 1 • Stars and planetary systems are formed from gigantic gas clouds, containing about 74% H and 25% He by weight, with the remaining 1% consisting of heavier elements. • Our solar system probably evolved from a gigantic rotating cloud of gas and dust, perhaps several light years (10-20 trillion miles) across. • The elements between Li (lithium) and (iron) were created in the thermonuclear fusion process by which stars produce their energy, while elements heavier than Fe were created in massive supernovae explosions. • The fact that heavier elements are present in abundance on the Earth means that the solar system was not among the very early generations of stars.
Origin of the Solar System 2 A gas cloud collapses if it is at i. a sufficiently low temperature, so that the outward thermal pressure is small; ii. a sufficiently high density, so that the gravitational pressure causes the cloud to collapse inwards. The cloud collapses into a disk if it is rotating, because the rotation creates an effective outward force, balancing gravity.
Origin of the Solar System 3 • The heavier elements in the solar system were created in processes occurring in earlier generations of stars. These elements, and their compounds, make up the dust contained in the gas cloud. • Just as an ice-skater spins faster as she brings her arms inwards, so does a rotating mass spin faster as it collapses inwards. • The star is formed at the center of the disk, while planets condense throughout the disk, at regions where the matter is densest.
Origin of the Solar System 4 • Iron and silicates vapors condense to form dust particles when the temperature drops below 1200 K. • The condensation of easily vaporized compounds, such as water and methane, occurs only in the outer region of the disk.
Origin of the Solar System 5 • The heavier iron and silicate dust particles adhere together in the inner region of the disk, forming planetesimals, which are objects from millimeters to kilometers in size. • Lighter, ice-rich planetesimals are produced in the outer region of the disk.
Origin of the Solar System 6 • Ultimately, gravitational attractions cause the clumping of the planetesimals in both regions of the solar disk. • Because H2 and He are by far the most abundant gases in the solar disk, the cooler outer planets become surrounded by huge envelopes of these gases.
The Solar System The solar system consists of the Sun and the objects orbiting it, which are the planets, their moons, and asteroids, meteoroids and comets, all of which shine in reflected sunlight. The planet Jupiter is more than twice as massive as all of the other satellites added together, but has a mass of only about (1/1000) MSun. The inner or terrestrial planets are small rocky planets. Their order from the Sun is Mercury, Venus, Earth and Mars. The Jovian planets or gas giants are 15 to 320 more massive than the Earth, and do not have distinct surfaces. Their order from the Sun is Jupiter, Saturn, Uranus and Neptune.
Comments on Diameter, Mass and Density • Planet sizes and masses fall into four groups as follows: I (smallest):Mercury and Mars (5000 – 7000 km diameter); II : Venus and Earth (12,000 – 13,000 km diameter); III: Uranus and Neptune (50, 000 – 51,000 km diameter); IV (largest):Jupiter and Saturn (120,000 – 140,000 km diameter). [For comparison, Pluto has a diameter of roughly 2400 km.] • The average density ρ = mass/volume. • The smaller terrestrial planets have higher densities, the Earth being the densest (5520 kg/m3). • The larger gas giants have the lowest densities, Saturn (690 kg/m3) being less dense than water (1000 kg/m3).
Kepler’s Third Law of Planetary Motion • The square of the sidereal periodof a planetis proportional to its (Mean distance from the Sun)3 ; i.e.P2 = a3.
Titius-Bode Law • How the Titius-Bode Law,dn = (3n + 4)/10 AU, fits the data.
Comments on Orbits and Rotations • The Titius-Bode law is an empirical law, which fits the data well, except for Neptune, probably because nearby Pluto. • It is not a fundamental law, which can be derived simply from Newton’s laws, although computer simulations using Newton’s laws show that some agreement with it. • The period of revolution (orbital period) of a planet about the Sun is given by Kepler’s 3rd Law,P2 = a3. • The rotational periods of the planets (about 24 hours for the Earth) are not connected to their distances from the Sun. • With the exceptions of Venus and Uranus (probably due to early glancing collisions with asteroids), all the planets rotate in the same as their orbital motion, so that the Sun appears to rise in the East.
Computer Simulation of Planet Formation • A computer simulation show how a solar system, originally containing 100 planetesimals, ultimately produces four planets through accretion after a period of about 400 million years. • Time zero 30 million years 440 million years 100 planetesimals 22 planetesimals 4 planets
A Note of Caution • The proposed origin of the solar system given here appears logical, but studies of exoplanets associated with other stars have cast doubts on its correctness. • The observations of numerous Hot Jupiters, which are very large planets, much closer to their star than Mercury is to the Sun, suggests that these giant planets might have been formed far from their star, but have moved much closer to it. • The inward migrations of Hot Jupiters may pull icy objects into smaller rocky planets, giving rise to oceans on the latter. • If this happened to Jupiter, it might have flicked lesser planets out of its way as it moved to its present position.