5)  Earth in space and time. The student understands the solar nebular accretionary disk model. The student is expected to:

(d)  explore the historical and current hypotheses for the origin of the Moon, including the collision of Earth with a Mars-sized planetesimal;

• Thea planetesimal collision creates Earth’s Moon

(f)  compare extra-solar planets with planets in our solar system and describe how such planets are detected.

• Observation with telescopes of other star systems based on the stars radial velocity caused by the gravitational force between the planets and the star (Wiggle Effect) • Transit Method: Photometric method

This theory states that the Moon formed in orbit around the Earth. Theories state that physical structures and compositions of the planets depend on their distance from the Sun.

• Mercury, which is closer to the Sun than we are, is considerably richer in dense materials• Mars, which is further from the Sun, is considerably richer in less dense materials.

There have been many explanations as to the origin of Earth’s one natural satellite, the Moon.

The Sister Theory

We discount the sister theory now, because the Moon has a density like that of Mars, and considerably lower than that of the Earth. It would be much easier to understand this low density if the Moon were formed near the orbit of Mars.

This theory states that the Moon was captured from somewhere else.

When an object comes by a planet, it can either run into it, or pass by it, and if close enough, get captured by it.

The Capture Theory

The capture theory states that Earth was sufficiently large enough to “capture” the migrating moon in its gravitational field.

We discount the capture theory today, because while the moon is less dense than the Earth, it would be made of materials that are MUCH different than Earth if it had formed elsewhere, and it is not.

Four of Jupiter’s largest natural satellites were thought to have been “captured”, as they are large enough to be considered dwarf planets, were it not for the fact that they orbit Jupiter.

Remember, fission is what occurs when objects “split”.

The fission theory states that Earth's Moon probably formed out of material splashed into orbit by the impact of a large body into the early Earth.

The Moon's density is substantially less than that of Earth, due to its lack of a large iron core.

What could be the answer to this dilemma?

There is a problem with the fission theory, however. If the moon was a piece of the Earth it should be the same in make-up, with equal portions of iron, nickel, and silicates.

.

We now embrace a Big Crunch, or Big Collision theory in which an object about the size of Mars runs into the Earth, knocking off a part of its mantle. This incorporates a part of the “fission theory”. The pieces blasted out into space orbit the Earth, and form into the Moon.     

     

This theory is attractive because it solves all the problems of previous theories.

• The low density of the Moon is explained by its being made up mostly of Earth mantle material. •The similarity with Earth rocks is explained by the Moon having been part of the Earth.

Differentiation on Earth had probably already separated many lighter materials toward the surface, so that the impact removed a disproportionate amount of silicate material from Earth, (lighter) and left the majority of the dense metal behind.

1. Describe the “Sister Theory” as to the origin of the Moon.

2. What’s wrong with the “Sister Theory”?

3. Describe the “Capture Theory” as to the origin of the Moon.

4. What’s wrong with the “Capture Theory”?

5. Describe the “Fission Theory” as to the origin of the Moon.

6. Why do scientists like the new “Big Crunch” fission theory?

Instead, astronomers have generally had to resort to indirect methods to detect extrasolar planets. At the present time, several different indirect methods have yielded success.

Any planet is an extremely faint light source compared to its parent star.

In addition to the incredible difficulty of detecting such a faint light source, the light from the parent star causes a glare that washes it out. For those reasons, only a very few extrasolar planets have been observed directly.

A star with a planet will move in its own small orbit in response to the planet's gravity. This leads to variations in the speed with which the star moves toward or away from Earth. I.e. the variations are in the radial velocity of the star with respect to Earth

This has been by far the most productive technique used by planet hunters. It is also known as Doppler spectroscopy. It is generally only used for relatively nearby stars out to about 160 light-years from Earth. It easily finds massive planets that are close to stars, but detection of those orbiting at great distances requires many years of observation. One of the main disadvantages of the radial-velocity method is that it can only estimate a planet's minimum mass.

A pulsar is produced by a neutron star: the small, ultra-dense remnant of a star that has exploded as a supernova. Pulsars emit radio waves extremely regularly as they rotate. Because the rotation of a pulsar is so regular, slight changes in the timing of its observed radio pulses can be used to track the pulsar's motion. Like an ordinary star, a pulsar will move in its own small orbit if it has a planet. Calculations based on pulse-timing observations can then reveal the size and shape of that orbit, and the probable planet.

The main drawback of the pulsar-timing method is that pulsars are relatively rare, so it is unlikely that a large number of planets will be found this way. Also, and perhaps more importantly, life as we know it could not survive on planets orbiting pulsars since high-energy radiation there is extremely intense. https://youtu.be/t2xTlv_I6ac

When a planet crosses in front of a star, then the star’s brightness dims by a small amount.  This is the detection method used by space telescopes that were launched in the last decade. One limitation of this method is that it is only possible to detect these crossings, also known as transits, when the planet and the star are perfectly aligned with the line of sight of the detection instrument.

This method, also called the photometric method can determine the radius of a planet.

The amount the star dims depends on the relative sizes of the star and the planet.

About 10% of planets with small orbits have such alignment, and the fraction decreases for planets with larger orbits.

The transit method also makes it possible to study the atmosphere of the transiting planet.

When the planet transits the star, light from the star passes through the upper atmosphere of the planet.

By studying the stellar spectrum carefully, as it filters through that planet’s atmosphere, one can detect elements present in that atmosphere. Missing frequencies through the spectroscope are clues, indicating elements

or compounds that absorb light at those frequencies are present in the atmosphere. For example, if the light frequencies corresponding to methane and carbon monoxide are missing from an analysis of the starlight, the atmosphere contains methane and carbon monoxide, which absorbed the missing light.We use similar methods to determine atmospheric components around planets in our own solar system.

Being able to identify the gases in an orbiting planet’s atmosphere is most important to a branch of science known as astrobiology.

Carl Sagan

Neil deGrasse Tyson

Michio Kaku

These scientists are interested in finding Earth-like planets, and in doing so, finding the signatures of life. What types of elements and molecules would you expect to find in the atmospheres of these planets?

Oxygen?Water Vapor?Ozone?Carbon Dioxide?Nitrogen?

The extrasolar planets that have been easiest to detect are large, and in small, close orbits. 

Many of these are gas giants like the planet Jupiter but orbiting as close to their sun as planet Mercury. 

This type of planet has been called a “hot Jupiter”. 

Why couldn’t planets like hot Jupiters have formed close to the sun in our solar system?At this stage, it appears to be fairly common for Sun-like stars to have planets.  Our current estimates are that it is possible as many as 40% of Sun-like stars have some type of orbiting planet.  This would mean there is an enormous number of planets in our galaxy, given there are at least 200 billion stars.

7. Why must scientists resourt to “indirect methods” to detect extrasolar planets?

8. What indirect methods do we use to detect extrasolar planets?

9. Why is it more difficult to detect planets farther away from the star, than close-up?

10. What is the main problem with the Pulsar Timing method of extrasolar planet detection?

11. What is the transit/photometric method of extrasolar planet detection?

12. What characteristics about a planet can you determine using the transit method?

13. What limitation does the transit method pose? 14. What is a Hot Jupiter?