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...

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...

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...

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...

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...

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...

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...

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. ...

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...

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...

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...

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...

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...

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...

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...

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...

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,...

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...

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...

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...

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...

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...

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...

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...