12. Formation and Nature of Planetary Systems

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Imaging Earths

2011-07-2201:21

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.

Energy Source of the Sun

2011-07-2201:03

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.

Solar Atmosphere

2011-07-2200:53

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.

Discovery of Helium

2011-07-2200:55

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.

Spectrum of the Sun

2011-07-2201:01

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.

Theories of Extrasolar Planets

2011-07-2200:58

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.

Detecting Earths

2011-07-2200:57

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.

Rotation of the Sun

2011-07-2201:10

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.

The Sun

2011-07-2200:35

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.

Doppler Detection

2011-07-2201:34

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.

Discovery of Extrasolar Planets

2011-07-2201:05

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.

Properties of Extrasolar Planets

2011-07-2200:52

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.

Pulsar Planets

2011-07-2201:01

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.

Reflex Motion

2011-07-2201:12

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.

Signature of Extrasolar Planets

2011-07-2201:00

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.

Detection by Transits

2011-07-2201:11

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.

Center of Gravity

2011-07-2200:55

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.

Direct Detection

2011-07-2201:33

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.

Infrared Detection

2011-07-2200:54

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.

Extrasolar Planets

2011-07-2200:56

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.

#box-pro-ellipsis-176034354993269{-webkit-line-clamp:2;}12. Formation and Nature of Planetary Systems

12. Formation and Nature of Planetary Systems