12. Formation and Nature of Planetary Systems

Transcript: The direct detection of Earths or even Jupiters is extremely difficult. As seen from afar, a small planet reflects a tiny fraction of the sunlight from the nearby star, and as seen through the Earth’s atmosphere, the light reflected from the planet blurs into the wings of the image of the star. But currently, techniques are being developed using interferometry and adaptive optics that allow images of much greater sharpness to be obtained. This will allow for the first time the direct detection by imaging techniques of Jupiters and even potentially Earth-like planets. With this as a possibility and the use of very large aperture telescopes, enough photons can be gathered from the planet to disperse into a spectrum and look at the atmospheric composition. This prospect is the next stage in the search for life in the universe. Having found that sites for life, planets, are ubiquitous beyond the Earth and the solar system, we now have the possibility of looking at the atmospheric chemistry of extrasolar planets. If we should detect oxygen or ozone in the spectra of these planets it would be good evidence for metabolism at work and good evidence for life.

Transcript: Knowing what the Sun is made of does not tell us how it gets it energy. This was the subject of active debate throughout the nineteenth century. Around the middle of the nineteenth century, the only known energy source for the Sun was chemical energy, such as is obtained by burning fuel such as coal, or natural gas, or petroleum. Unfortunately it’s easy to show that this energy source is insufficient to explain the Sun’s radiation. We know how far away the Sun is and we know how much energy it emits. We know its size, and so we know how much matter it contains. If the Sun were composed of a chemical energy source, it could only last about ten thousand years at the energy rate that it’s emitting. We suspect and strongly knew even in the early nineteenth century that the Sun was substantially older than ten thousand years; therefore, simple chemical energy processes cannot power the sun.

Transcript: The spectrum of the Sun tells us important things about the atmosphere of the Sun. Two of Kirchhoff’s laws are involved. First, a sufficiently hot gas will emit a thermal spectrum whose radiation peaks in the visible part of the spectrum. This is what we see for sunlight, and the wavelength of the peak of the emission is a clue that the temperature of the atmosphere, or edge of the Sun, is about 5,700 degrees Kelvin. The fact that the Sun’s spectrum is crossed by narrow absorption lines means, by Kirchhoff’s law, that the interior gas must have a cooler outer layer. Thus, the 5,700 degree gas at the outer edge of the Sun lies outside gas that must be hotter yet. The Sun gets hotter as we go deeper inside it.

Transcript: In 1868 French astronomer Pierre Janssen and English astronomer Norman Lockyer independently discovered spectra lines that corresponded to no known element on Earth. They named the element helium from the Greek word helios for Sun. This was the first element to be discovered in space, not Earth, and it raised the uncomfortable question of weather space was unusual enough that we might never understand it. Eventually, in 1895 Lockyer detected helium on Earth. Helium is extremely rare on Earth because it mostly escaped into space early in the Earth’s history. However, the larger question was answered in a reassuring way. Since then there has been no element discovered in space that has not got a component on Earth that we can measure in the laboratory.

Transcript: Isaac Newton was the first to take a prism and disperse the Sun’s light and show that it was composed of a smooth spectrum of radiation from blue to red wavelengths. We also know that the Sun emits invisible electromagnetic waves at infrared and ultraviolet wavelengths. The smooth, continuous radiation of the Sun is a thermal spectrum with a peak wavelength that Wien’s law tells is associated with a temperature of about 5,700 Kelvin. In the early nineteenth century the German physicist Joseph Fraunhofer used higher dispersion to show that the Sun’s spectrum was crossed by a series of dark absorption lines. He compared the positions of wavelengths of these absorption lines to the spectrum of hydrogen measured in the laboratory. The wavelengths matched exactly showing a chemical fingerprint in the Sun of the element hydrogen and that the Sun was mostly made of hydrogen.

Transcript: The properties of extrasolar planets leave us with a puzzle. Our solar system has gas giants that are 5 astronomical units or further from the Sun. Almost all the extrasolar planet systems have giant planets much less than this distance, in most cases less than 1 astronomical unit. Is our solar system atypical? We think we understand the formation process of our solar system, so how did these extrasolar planets form? There are many theories, and we don’t know for sure. But there are certainly dynamical processes by which gas giant planets could migrate from larger distances to smaller distances. However, many of these theories imply that we should be catching these planets at a particular and maybe short-lived phase of their evolution. A large amount of study will be needed for us to understand their formation.

Transcript: The detection of extrasolar planets is exciting, but planets the mass and size of Jupiter are very unlikely to be able to harbor life either in their atmospheres or on their surfaces. So astronomers are still interested in pushing the detection techniques towards the detection of Earth-like objects. The detection of Earths, by the technique of the Doppler Effect, is hundreds of times more difficult than a detection of Jupiters and is beyond the limits of current technology from ground-based telescopes. However, projects are under way to build interferometers in space that would have the stability, the baselines, and a light gathering power to detect the Doppler wobble of Earth-like planets around solar stars. This would be the most direct evidence we could have that there might be suitable sites for life beyond the solar system.

Transcript: From ancient times Chinese and Indian astronomers noticed and recorded sunspots, blemishes or dark spots on the surface of the Sun. This work improved in the 1600s with the invention of the telescope which allowed the counting and tracking of sunspots. Galileo used such observations to prove that the Sun was not a perfect sphere, a decisive break in the tradition of Greek ideas. Unfortunately, through his long and careless observations of the Sun, Galileo ended his life blind. Four hundred years of sunspot observations have allowed us to show the way that the Sun rotates. The rotation period of the Sun is 25.4 days at the equator relative to the stars, 27.3 days relative to the Earth because the Earth moves around the Sun while the sun rotates. The Sun rotates differentially, meaning that the rotation of 25 days at the equator is faster than the rotation of the poles which takes about 33 days. This is proof that the Sun is gaseous and not solid.

Transcript: The Sun is the source of all life on Earth. Radiation from the Sun reaches us in eight minutes. We are bathed in light and radiation from this glowing ball of gas, a hundred times the Earth’s size. At a distance of 150 million kilometers, or 98 million miles, the Sun is 300 thousand times nearer then the next nearest star. As a result, we have learned about it in great detail with implications for the way all the other stars in our galaxy and beyond work.

Transcript: The reflex motion of stars caused by planets that orbit them has the effect of creating a slight wobble, but it also has a second important consequence, a Doppler effect. Jupiter, for example, causes the Sun to wobble as Jupiter moves in its orbit in a twelve year period. The distance that Jupiter moves the Sun in twelve years can be converted into a speed or Doppler shift of the Sun as it wobbles; it’s 13 meters per second, about the speed of a car. For a smaller planet further away the leverage is less, and the speed is slower. Uranus on its own would create a Doppler shift of 0.3 meters per second in the Sun. The Earth is closer to the Sun but a much smaller mass, so its leverage is even less. The Doppler effect caused by the Earth on the Sun is only 0.09 meters per second, about the walking pace of an ant. The Doppler effect cannot always be detected because geometry comes into play, as with eclipses or transits. In general, the Doppler effect will not show unless the motion of the planet is in the plane of the observation. For example, if we were staring down on the orbits of the solar system all of the motions would be perpendicular to our line of sight, and we’d see no Doppler effect. On average, astronomers will see some fraction of the full size of the Doppler shift.

Transcript: In 1995 years of painstaking work with the Doppler technique began to bear fruit. Discoveries were announced by a Swiss team of Mayor and Queloz and an American team led by Marcy. A steady increase in the number of extrasolar planets has occurred. By 2002, over 100 were known and 8 to 10 new ones are discovered every year, but there are surprises. Among the first twenty extrasolar planets to be discovered they’re almost all Jupiter or super-Jupiter sizes, 1 to 10 Jupiter masses, the smallest about 40 percent the mass of Jupiter. Among those first twenty, 14 are at closer distances from their stars than the Earth is in its orbit of the Sun, less than 1 AU, and all are less than 30 AU. This manifestation of planetary systems, large, massive, gas giant planets in tight, inner solar system orbits was a complete surprise to astronomers who viewed the data.

Transcript: Measurements of extrasolar planets are difficult and uncertain, but enough have been found to give a sense of their statistical properties. They are neither rare nor ubiquitous. Around Sun-like stars they occur in about 10 to 20 percent of the cases. Almost all the masses are in the range one to ten times the mass of Jupiter. Roughly half have orbits that are very tight around their stars, less then an astronomical unit, and with orbital periods of less than a year. Several have been found with multiple planets indicating that solar systems are not unique, and techniques of lensing and eclipses have been used to measure the sizes in several cases showing that they are indeed gas giant planets like those in our solar system.

Transcript: Astronomers have been hoping and expecting to find planets around Sun-like stars, so it was a great surprise when the first extrasolar planets were detected around a pulsar. PSR 1257+12 is a dead star, yet it has two Earth-like objects moving in tight orbits around it. The detection of these planets was aided by the high precision radio timing measurements that are possible for a radio emitting pulsar. Pulsars form from the death of a massive star, a supernova, and it is very unlikely that a planetary system could survive such an explosion. So this system did not form in the standard way that our planetary system did. Perhaps these planets formed from the aggregation of debris left over from the supernova. Either way, this exotic and bizarre system is unlikely to tell us much about the formation process of our own solar system.

Transcript: When an unseen planet orbits a star it makes a slight wobble in the star as the star moves around its center of gravity. This is called a reflex motion. The reflex motion is very small and very subtle because planets are so much less massive then stars. By contrast two equal mass stars in orbit around each other, a binary system, is usually easy to see the motions of the stars on the sky. The situation of the Sun and Jupiter gives a typical example. Jupiter is 0.1 percent the mass of the Sun, and so the center of gravity must be one-thousandth the distance of Jupiter from the Sun. This places the center of gravity around the edge of the Sun. So when Jupiter orbits the Sun as the largest mass in the solar system, the sun essentially pirouettes, or wobbles, about its edge. The angular motion caused by this wobble seen at the distance of the nearby stars, would be only a hundredth of an arcsecond, like looking at the wobble of a hula hoop ten thousand miles away, impossible to detect with current technology.

Transcript: The indirect Doppler technique is the most promising way to detect extrasolar planets. We can see what the size and the signature of the effect should be. If a Jupiter were orbiting a Sun-like star, the signature of Jupiter would be a periodic sinusoidal variation in the Doppler shift of the star with an amplitude of 13 meters per second and a period of 12 years. This is the data variation that would be observed to detect the planet. The amplitude would be less if the orientation of the orbit were not exactly parallel to the line of sight. It would be less if the planet were smaller or less massive, and it would be less if the planet were further from the star. The requirement, therefore, is extremely high precision and signal-to-noise spectroscopy over a period of many years, which is one of the reasons that it took so long to detect extrasolar planets.

Transcript: A clever way to detect extrasolar planets is to look for transits, the situation where the dark planet passes in front of the bright star. A giant planet in principle might cover about 1 percent of a star. However, in practice the situation is not this good because the giant planet has a diffuse atmosphere that doesn’t block out light very well, so really the drop in light intensity from the star would only be about a tenth of a percent or even less. So we’d be looking at a star varying in its brightness momentarily by less than a percent. In actual fact the situation is not even this good because we can only see the situations or geometries where the planet and the star were lined up so that the transit could occur. Geometry shows that only 1 in 500, or 0.2 percent, of all the situations will have this favorable orientation. Finally, the transit does not occur for very long. The time it would take a Jupiter to pass in front of a Sun as seen from afar is only 0.03 percent of its orbit, one day in a 12 year orbit.

Transcript: In Newton’s law of gravity, the gravity force works equally in both directions, so when two stars orbit each other, they each exert gravity on the other. Two starts of about equal mass orbit a common center called the center of gravity. In the situation of a planet and a star, the center of gravity moves closer to the star, and in the situation of a small planet and a massive star the center of the gravity can be inside the star itself. It’s analogous to the situation of beam balancing on a fulcrum or a see-saw. Two equal weights at the ends of the beam will balance out the fulcrum at the middle, but if one of the weights becomes much larger, it must move closer to the fulcrum to create a balance. This is the same as the situation in gravity.

Transcript: The most obvious way to detect an extrasolar planet is direct by imaging. However, some very simple numbers show that this is a very difficult experiment. As seen from afar Jupiter reflects some of the Sun’s light, but it’s very little, only two-billionths. You can work this out from the inverse square law and the size of Jupiter relative to its distance from the Sun. It’s actually worse than that because we only see half of the reflected light, think of the phases of Venus, and Jupiter is not a perfect mirror, reflecting only 30 percent of the photons that fall on it. So the total fraction of light reflected by Jupiter from the Sun is 3 times 10-10, a very small fraction. What about a smaller planet? For the Earth the situation is actually worse. Although Earth is five times closer to the Sun than Jupiter and intercepts twenty-five times more light, from the inverse square law, it’s ten times smaller, and so its cross-sectional area is a hundred times less. The product of those two means a four times more difficult experiment. Also, as seen from afar, the distance of a nearby star, the angular separation of the Sun and Jupiter is only a couple of arcseconds, and the angular separation of the Sun and the Earth would be less than an arcsecond. Thus, it’s like trying to see a candle flame in close proximity to a football stadium arc light, an extremely difficult experiment that has not yet succeeded.

Transcript: Direct detection of planets is very difficult, but the situation can be improved by moving to infrared wavelengths. The Sun emits the peak of its radiation, by Wiens’s law, in visible light. By infrared wavelengths the energy distribution is falling off. Planets however are cooler. and the peak of their radiation, their intrinsic thermal radiation, is at infrared wavelengths. So by moving to infrared wavelengths the contrast of the planet with respect to the star, the Sun, is increased by as much as a factor of a thousand. This means that the visible light situation, where Jupiter reflects only a few billionths of the Sun’s light, is improved to a situation of reflecting a few millionths of the Sun’s light, a difficult experiment but not impossible.

Transcript: The Sun is a star like other stars. This raises a question: if the sun has planets, do other stars have planets orbiting around them, and are planets a natural byproduct of star formation? In this case we would expect to find planets throughout the Milky Way galaxy surrounding many of the billions of stars contained within our galaxy. For centuries astronomers could do no more then speculate about the answer to this question. In 1995 success was achieved for the first time with the discovery of an extrasolar planet, a planet beyond the solar system. This young subject is now maturing and there are many extrasolar planets to study giving us indications that planets form frequently throughout the cosmos and leading to the speculation of whether life might not form frequently also.

Transcript: What caused the solar nebula to collapse into the Sun and the planets? In the 1970s, studies of carbonaceous meteorites gave some clues. They revealed inclusions of minerals containing aluminum-rich compounds that had condensed at high temperature and in addition, a significant number of heavy radioactive substances with relatively short decay times. For example, Zenon-129 was found which decays from Iodine-129 with a half-life of only 17 million years. Since Zenon-129 is inert, it can’t form mineral grains, so it must have been trapped in the meteoric material in the form of iodine with its short half-life. This is good evidence that the solar system formed, or was triggered, over a relatively short period of time, less then 20 million years. But where did the iodine come from? Radioactive iodine and other heavy elements are produced in supernova explosions caused by the death of ancient nearby stars. Thus, there is good indirect evidence that the formation of the Sun and the solar system, the birth of a star, was triggered by the death of a nearby star. In this way the cycle of birth and death of stars takes place in the nearby universe.

Transcript: Satellites or moons in the solar system have diverse properties and several different types of origin. The most important process is similar to the accretion that formed the planets themselves. Each of the four giant planets in the outer solar system was massive enough to attract gas from the solar nebula and formed miniature version of the solar nebula centered on the planet. Accretion within that gas gradually built moons of ices and carbonaceous material moving on circular prograde orbits. This explains the properties of most if the interior moons to the giant planets. For exterior moons capture is another possibility. Jupiter’s eight outermost moons show four that are on prograde and four that are on retrograde orbits, indicating that they were captured from surrounding planetesimal debris. The third process is the impact formation theory which can emplane a situation with nearly similar sized planets and moons like the Earth and its moon.

Transcript: Comets are icy planetesimals from the outer solar system. They were flung by close encounters with giant planets, primarily Jupiter, far away into the Oort cloud. Remember that even a tiny bit of Jupiter’s kinetic energy transferred by gravity to a tiny comet can fling it into deep space. NASA used this technique, called the gravitational sling-shot mechanism, with the Voyager spacecraft to send them into the outer solar system. Beyond the orbit of Neptune, the icy planetesimals had no giant planet to interact with, so they remain there as the Kuiper Belt.

Transcript: The largest objects in the asteroid belt build by accretion to sizes of several hundred kilometers. These objects were like the Earth, differentiated with iron-nickel cores and rocky mantels. They had their energy increased by the gravity of nearby Jupiter. This energy added to the orbit pumped up the speeds and caused many collisions. The shattered fragments of asteroids worked their way both in and out of the solar system from the asteroid belt. Some fraction of these asteroid fragments were on Earth-crossing orbits, and they fall to Earth as meteorites.

Transcript: Asteroids are planetesimals that never made it all the way up to planet status. Why are there so many chunks of rocks stranded between the orbits of Mars and Jupiter? It seems that the accretion process was underway and never got completed. Ceres, the largest asteroid, is about a thousand kilometers across, and accretion must have stopped at this point. The reason is almost certainly the proximity of Jupiter, the most massive planet in the solar system. Although we expect a planet at the distance of the asteroid belt from the geometric spacing implied by Bode’s rule, the presence of Jupiter adds a gravitational effect to the orbits in the asteroid belt pumping up the velocities of the smaller pieces of rock and causing them to collide. This smashing together of smaller pieces of rock essentially truncates the process of accretion and never leads to the formation of a full-sized planet.

Transcript: The outer planets form in a similar way to the inner planets by the rapid accretion of material of planetesimals by collisions and gravity. In the outer solar system more material was available, so the cores of the outer gas giant planets were about ten to fifteen times the size of the Earth. At this size the gravity of these planets could attract gas left over in the solar nebula. It was cool enough and plentiful enough to be attracted to form giant atmospheres around these rocky cores. Thus, the gas giant planets have two components: rocky cores similar in composition to terrestrial planets, and giant gaseous envelopes similar in composition to the Sun, primarily hydrogen and helium. Note that we do not yet have direct evidence for the solid cores of the gas giant planets.

Transcript: The process of accretion swept up material to form Mercury to Earth-sized objects in the inner solar system, thus explaining the terrestrial planets. The content of these planets is material that can condense at the high temperatures at the inner solar nebula, thus it is mostly metallic and silicate material familiar to us in everyday rocks. These planets were relatively small. All leftover material was blown out of the inner solar system by the intense radiation field of the young Sun.

Transcript: The formation of planets in the solar system by the process of accretion occurs efficiently and quickly. The trapping of radioactive isotopes within meteoric material indicates that the entire accretion process from small-sized objects to planet-sized objects took only a couple of million years, a tiny fraction of the four and a half billion year life of the solar system. Dynamical studies indicate that accretion tends to produce planets that move in prograde orbits with rotation times of five to twenty hours as observed. More importantly the accretion process tends to create single large objects which dominate zones of their region of orbit. This spaces the planets out. Basically accretion occurs in donut shaped zones, each one of which is one and a half to two times further away from the Sun than the one before. This explains Bode’s rule and the fact that the planets have a nearly geometric spacing. Dynamical studies have explained most of the systematic properties of the solar system in terms of a formation process that had significant random components.

Transcript: Accretion is the mechanism by which planets are built. Remember, we have to do more then make a mountain out of a molehill. We have to make an entire world out of motes of dust. In the 1960s Soviet scientist Victor Safronov worked out the theory of accretion in the solar nebula. He in particular looked at the orbits of particles in elliptical trajectories in the solar nebula and asked at what rate would they collide, and what would happen when they collided. It turned out that the tiny microscopic particles clustered into snowflake type clusters. Held by inter-atomic, forces these delicate clusters of particles eventually grew to the size that gravity could take over. Once gravity takes, over the process accelerates and becomes non-linear, so while you might think that it would take a very long time to build a planet out of microscopic particles, once gravity accelerates the process, sweeping up of material in the orbits of the solar system very quickly creates a few large bodies with a relatively small amount of debris left over.

Transcript: The collapse of the solar nebula creates a young star with a surrounding disk of gas and dust. Condensation takes us from molecules and groups of molecules to dust grains about a millimeter or so across. These grains are rocky in the inner solar system and primarily icy in the outer solar system. Most of the solar nebula at this point is gas. Ninety-eight percent of the composition is hydrogen and helium, the same as the composition of the Sun, so only about 2 percent of the material is in the form of heavy elements that can form dust grains. But there’s an important piece of the story missing because even the largest dust grain is trillions of times smaller than the smallest planet.

Transcript: If the solar nebula had cooled uniformly throughout, there would be no composition variations in the solar system. Instead, the young Sun remained hot, and the inner regions of the solar nebulae were hotter than the outer regions. As a result, the condensation sequence can be used to explain the composition differences of the inner and outer solar system. At the distances of the inner planets, several astronomical units, the temperatures are high enough that only iron, nickel, and silicates could condense out, forming the rocky basis for the terrestrial planets. Around the distance of the middle of the asteroid belt the temperature was low enough for black carbonaceous compounds to condense out, forming the soot line. At the edge of the asteroid belt ices begin to condense out at the temperature of a few hundred Kelvin; this is the frost line. And in the entire outer solar system ices can condense. These ices remain in a solid form even in the presence of direct sunlight. Thus, the entire outer solar system is filled with icy material.

Transcript: If the solar nebula had cooled uniformly throughout, there would be no composition variations in the solar system. Instead, the young Sun remained hot, and the inner regions of the solar nebulae were hotter than the outer regions. As a result, the condensation sequence can be used to explain the composition differences of the inner and outer solar system. At the distances of the inner planets, several astronomical units, the temperatures are high enough that only iron, nickel, and silicates could condense out, forming the rocky basis for the terrestrial planets. Around the distance of the middle of the asteroid belt the temperature was low enough for black carbonaceous compounds to condense out, forming the soot line. At the edge of the asteroid belt ices begin to condense out at the temperature of a few hundred Kelvin; this is the frost line. And in the entire outer solar system ices can condense. These ices remain in a solid form even in the presence of direct sunlight. Thus, the entire outer solar system is filled with icy material.

Transcript: The gravitational collapse of an initially large and diffuse gas cloud is slowed by the increasing pressure force as the density rises. Eventually, a stable configuration is reached with a disk-like distribution of gas and dust around the forming Sun. This is called the solar nebula. Within the disk-like distribution of gas and dust the orbits are nearly circular because any non-circular obits would lead to collisions among the particles which would damp out the non-circular motions.

Transcript: If the original gas cloud that formed the solar system had not been rotating at all it would have indulged in a spherical gravitational collapse towards a point. However, in general any large gas cloud in space will have rotation. Thus it has angular momentum, a measure of the rotary motion of any object which is equal to the product of the mass times the circular velocity times the distance of the mass from the center of rotation. In the original solar system, the small amount of initial rotation is amplified during the process of collapse because the original gas cloud can collapse more easily down the axis of rotation, or the pole of the distribution, then along the equator where the motion resists the collapse. Thus, by the product of two forces, rotation and collapse, a large, slightly rotating gas cloud turns into a much denser gas cloud with a cylindrical or disk-like shape.

Transcript: At the birth of the solar system the gas cloud that originally is very diffuse begins to collapse. This is a runaway process of gravitational collapse, but as the cloud contracts a new force comes into play. When the cloud is big and diffuse the density is so low that collisions between particles rarely occur, but as it becomes smaller the rate of collisions increase, and the cloud begins to act like a gas. As with any gas, when the density becomes higher, the pressure also increases due to particle collisions, and this pressure force acts to resist gravity. German physicist Hermann von Helmholtz produced the theory that explained how a gas cloud collapses in the presence of pressure resisting gravity. The collapse halts when the cloud reaches a temperature of a few thousand degrees Kelvin.

Transcript: Consider a defuse cloud of gas and dust sitting in space, the situation before the solar system formed. A particle at the center of this distribution will feel roughly equal gravitational pull in every direction, but a particle near the edge of this distribution will feel more tugged towards the center of the cloud because that is where more material is. Thus, particles at the edge will begin to move towards the center. As they do so the cloud contracts. As the density rises in the smaller cloud the distance between particles decreases, and the gravity force between them increases. Thus, as the cloud shrinks the gravity increases, and this becomes a runaway process. Astronomers call this gravitational collapse.

Transcript: Any theory of the formation of the solar system must explain a specific set of facts. The Sun occupies most of the mass of the solar system with the planets barely 0.2 percent. The Sun and the planets orbit in a single plane, and the Sun also rotates in that same plane. The planets and the sun all have prograde rotation, in the same direction, from west to east, and are in nearly circular orbits. The planets differ in their composition with the planets nearer the Sun being dense, small, and rocky and those further from the Sun being large and gaseous with hydrogen-helium atmospheres. The distribution of planet distances from the Sun is roughly geometric in spacing, and planets and their satellites act like solar systems in miniature. Cratering is common in the solar system with most of it occurring early in the first ten percent of the age of the solar system, and planets have more angular momentum, that is their coupled distribution of speed, size, and mass, then the Sun itself.

Transcript: We tend to think of asteroids in space as a potential threat to the Earth, if they ever impacted the Earth, but there’s also an opportunity here. In 1999 NASA sent the Near Earth Asteroid Rendezvous Mission, NEAR, which orbited 433 Eros, our first close-up look at an asteroid. Many asteroids are metal rich, containing iron-nickel compounds or large concentrations of the platinum group metals. Potentially mining such asteroids using free solar energy to do the work could alleviate some of the shortages of such rare metals on the Earth and provide an opportunity for commercial ventures in space.

From time to time the newspapers and magazines and even TV shows are full of the threat from interplanetary space, the possibility that a huge impact will catastrophically disrupt life on Earth as happened 65 million years ago. In truth, the timing of such events is very occasional, once every few hundred million years, and there’s no way to predict the next incident because the incident rate is random. However, scientists have a three phase strategy that protect us from even the possibility of having to worry about such an occurrence. First, astronomers use telescopes to make catalogs of all the potentially Earth-crossing asteroids down to about 100 meters in size. This project is under way using a number of telescopes scattered around the world. Only if an object appears to be on an Earth crossing orbit and a sufficiently good orbit exists do we even have to think of a threat. Given the orbit, there would be a number of years to prepare for the possibility. Most objects that appear to be on Earth-crossing trajectories would not in fact cross the Earth or hit it. The Earth is a tiny object in the vastness of space, so again the odds are very small that any action would have to be taken. But with years to plan, it is indeed possible to arrange our nuclear arsenals to deflect the asteroid in space and cause it to miss the Earth in its orbit. In this way we really need never worry about the threat from interplanetary space.

Transcript: There have been a number of mass extinctions in the history of life on Earth. The cause of many of these is uncertain, but there’s good evidence that one, occurring at the junction of the Cretaceous and the Tertiary periods 65 million years ago, when 60 percent of all species became extinct, was caused by an impact from space. A number of strands of evidence combine to point to this conclusion. First, the extinctions themselves, of all types of species occurring within a short span. Second, major global changes in the level of the flood plane. Third, a soot layer indicative of fires raging across the Earth. Fourth, a high concentration of iridium in a narrow geological zone, indicating extraterrestrial material with this rare element. And fifth, high concentrations of tektites and micro-tektites which are glassy spheres that occur when rock is melted, as in volcanic or explosive activity, and rains back down to Earth. This all occurred within a span of geologically short time, although geology can not say that it was instant. The smoking gun was found in the 1980s by the father-son team of Walter and Luis Alvarez. They located good evidence for a crater off the Yucatan Peninsula whose radioactive date was 65 million years ago. This is the crater of the 10 kilometer or so asteroid that hit the Earth and caused extinction of the dinosaurs and many other species.