astronomy exam 5
How much solar energy reaches earth?
As energy moves away from the sun, it is spread over a greater and and greater area. Intensity decreases as Inverse Square: 1/d^2
There are many things that could happen to Earth sometime during the next 5 billion years that could cause the extinction of the human species. For example: (i) impacts by a large asteroid; (ii) some incurable viral pandemic; (iii) nuclear way... are some of the possibilities that come to mind. However, there are certain things that will happen as the Sun ends it life. Which one below is inevitable?
As the Sun slowly heats up, theEarth will lose its water as the greenhouse warming increases. Eventually, the Sun will turn into a Red Giant about 5 billion years from now. Its expanding outer layers will envelope the Earth and fry its surface. Finally eh Sun will end its life as a White award and its habitable zone will shrink to nothing. The Earth will freeze over, becoming a cold, and blackened, dead sphere.
How do you measure mass of star?
Measuring the mass of stars in binary systems in easy. Binary systems are sets or two or more stars in orbit about each other. By measuring the size of the orbit, the stars' orbital speeds, and their orbital periods, we can determine exactly what the masses of the stars are.
How many ETs?
N = 10^-3 L (only in Milky Way)
What are the prospects of finding habitable, terrestrial planets in extrasolar systems that contain a "hot" Jupiter?
Not good- the "hot" Jupiter likely formed far enough away from its parent star that it formed as a "cold" Jupiter. It then migrated closer inward towards its parent star, becoming the "hot" Jupiter of the kind we have detected. This inward migration would either lead to the terrestrial planet being "eaten" or ejected from the extrasolar system... at least into the outer regions of its Oort Cloud.... no terrestrial planet would likely survive the inward migration of a Jovian planet in the process of being a "hot Jupiter". Furthermore, even if we assume that the "hot Jupiter." Furthermore, even if we assume that the "hot Jupiter" formed close in to its parent Sun via some process we don't understand at the moment, no terrestrial planet would then form or exist within that Sun's habitable zone.
What is nE in the Drake equation?
Number of those planets that fall within the CHZ
The stellar classification scheme, _____, is a temperature, luminosity-ordered sequence, with both temperature and luminosity decreasing. In this scheme, our sun is a class _____ star. Stars that might have habitable planets probably fall within the classification range ______.
OBAFGKM G2III A through F
Even though we have now detected hundreds of extrasolar planets, (i) almost all of them are Jovian-sized planets, (ii) most orbit very close to their parent star or if further away -- are in highly elliptical orbits and (iii) those that might be terrestrials are still much larger than Earth and have been found in very close orbit (less than 1 AU) around very small M-type stars. Why haven't we detected any extrasolar terrestrial planets in circular orbits at about one AU from sun-like stars?
Our technology is not good enough! A terrestrial planet (i) doesn't cause a detectable Doppler shift in a sun-like star (ii) doesn't produce a measurable wobble in a sun-like star's position and (iii) doesn't Blok enough of a sun-like star's light during a transit. We need larger aperture telescopes with better resolving power to detect such planets... for example, by putting telescopes in orbit about the Earth and using wide baseline interferometry.
Of the techniques described below, which was the first one to result in the detection of exoplanets (extrasolar planets)?
Periodic Luminosity - Doppler Shift Technique... Astronomers detected the first exoplanets by making accurate Doppler shift measurements of a parent star's spectrum to see if it indicates that the star "wobbles" about its axis - caused by the gravitational pull of the unseen planet. Also, they observed a few cases in which "wobbling star" dimmed periodically indicating that a planet passed in front of the star (transits) and blocked some of its light.
Why is it better to search for extrasolar planets directly using infrared (IR) rather than visible radiation?
Planets emit more IR radiation than visible radiation relative to their parent star
What if the Earth had no moon?
Planets without large moon may have the direction of their spin axis shift over time, this may produce long term climatic shifts. If you would take away the Moon suddenly, it would change the global altitude of the ocean! We might not of had convection on Earth that led to multi-plate tectonics! The level of adaptation of night vision would be very different without the moon. The Earth would be rotating more rapidly!
70% (...not 50% like your text says) of the stars in our stellar neighborhood are members of multiple star systems. Most are members of binary systems, but some are members of triple, quadruple, and even quintuple systems. What are the prospects of finding habitable planets in such systems?
Possible in some of them - In Binary star systems, where the two stars orbit each other at least about 5 times the distance of a planetary orbit about one of the stars, a planet might lie within a stable habitable zone.
Perhaps the most critical factor that affects the temperature of any planet is ....
how far the planet is from its parents Sun
The habitable zone is the area where
terrestrial planets could have liquid water on their surfaces
What is N in the Drake equation?
the number of intelligent civilizations in the milky way
Life on Earth appears to have arisen quite easily and quite rapidly after the Earth became habitable. This suggests that the factor fL (f life) in the Drake equation - the probability that life arises on a habitable planet - could be close to
1.0
How long will the sun generate energy by fusing hydrogen into helium?
10 billion years
51 Pegasi ... Using the Doppler technique, astronomers found an extrasolar planet orbiting that star, 51 Pegasi, with a period of p = 4.23 days. 51 Pegasi has about the same mass as our Sun, which means that we can determine orbital properties of its planets by applying the reduced version of Kepler's 3rd law: p^2 = a^3 where p is the orbital period in years and a is the mean radius of the orbit AU (1 AU = 150 million kilometers). Use Kepler's 3rd law to calculate the planet's mean orbital radius. Compare this to Mercury's distance from our Sun, which is 0.39 AU (hint: don't forget to use years for the time unit for orbital period and AU for the distance unit for orbital radius in Kepler's 3rd law).
4.23 days x 1year/365.25 days = .0116 year (0.0116)^2 = a^3 a = (0.0116)^2/3 AU = 0.051 AU a = 0.051 AU a = 0.051 / 0.39 a Mercury = 0.13 a Mercury
Habitable planets in the multiple star systems?
50% F and G (estimated fractions stars in multiple star systems)
How Far Away is ET? Assume that the galaxy is a large cube, made up of 200 billion stars, each star positioned in the center of a small cue arranged together in a lattice. The entire lattice of small cubes makes up a larger cube representing the volume of the entire galaxy. Let the distance between each neighboring star be D = F LY. Therefore, D is also the length of the side of one of the small cubes. If 100 currently signaling civilizations are randomly distributed 'little cubes' somewhere in this much larger 'cubic' galaxy, on average how far apart in LY are these civilizations?
A little abstract thought processing here - something members of every intelligent civilization can do, ummmm? First, let N = 200 x 10^ 9, the number of stars in our "cubic" galaxy. If each star is located at the corner of a cube in this lattice, separated by a distance D = 5 LY, then the total volume of the cubic galaxy is V gal - ND^3. The number of civilizations in the galaxy is n civ = 100. Let each one of them be surrounded by an "empty" cube of volume V civ = x ^3, in which there are no other civilizations -- on the average. If they are evenly spread throughout the galaxy, then the volume of the galaxy is V gal = n civ. V civ = n civ x^3 Equation these two numbers gives: n civ x ^3 = ND^3... Solving for x... x = (N/nciv)^1/3 D = (2 x 10^9) ^1/3 = 6,30000 LY Thus, each civilization is most likely separated from its nearest neighbor by a rather sizable distance.
What exactly do astronomers look for when using the radial velocity method to detect extrasolar planets?
A periodic shift in the absorption spectrum of a star as it moves toward then away from an observer while 'wobbling' about a center of mass displaced from its own center
What is a Habitable Planet?
A planet that is not big, not too small, not too hot or too cold
What if the Earth had no Jupiter @ 5 AU?
Devastating impacts would be more frequent!
A _____ is a structure that completely surrounds the home star of a Type II civilization designed to capture the entire energy output of its star. Of course, the sphere would radiate infrared hat as waste into the space surrounding it. The enormity of the undertaking required to build such a sphere in our ow civilization can be ascertained by a quick calculation of the material needed simply to build one, 1 meter thick, at 1 AU around our Sun. The material could only be obtained by completely dismantling Jupiter and somehow changing the hydrogen and helium that you would mostly get, into the structural material needed, such as titanium.
Dyson sphere
What is fp in the Drake equation?
Faction of those stars that have planets in stable orbits
What is fi in the Drake equation?
Fraction of those planets where intelligence evolves
What is fc in the Drake equation?
Fraction of those planets where intelligent life develops a civilization capable of communication
What is fL in the Drake equation?
Fraction of those planets where life actually evolves
What is f. in the Drake equation?
Fraction of those stars that are sunlike
What is the spectral type of the sun?
G2V
What is L in the Drake equation?
How long the civilization exists
Guiseppe Cocconi and Philip Morrison noted that hydrogen atoms (H) and hydroxyl radicals (OH) in space naturally emit radio waves at the frequencies of 1420 and 1720 Mhz. They called the region of the radio spectrum between these two frequencies The Water Hole (since H-OH = H2O). Fortuitously, it is also a region where galactic radio noise and atmospheric attenuation of radio signals is minimal. Cocconi and Morrison argued that intelligent aliens would most likely transmit radio signals to other likely habitable places at some frequency that lies within this range. They made the case that we should search for signs of intelligent civilizations by targeting likely habitable stars and scanning them for strong radio signals in this frequency range. Does this make sense or would you argue that intelligent aliens would most likely try to establish contact in some other way?
It makes good sense.... Radio waves travel at the speed of light. It makes good sense.. Radio waves, unlike visible light, penetrate galactic gas and dust, and therefore travel much further with little attenuation. It makes good sense... It's "cheap" to make radio waves... the energy cost of making one optical photon is equivalent to that required to make 10 million radio photons! Thus, the complexity achievable in a radio message compared to an optical message is huge for a given cost outlay. It makes good sense ... any civilization that understand physics should realize the importance of water for life and the corresponding significant of the Water Hole. It ought to anticipate that another intelligent alien civilization would come to the same conclusion and would likely try to signal others with beamed radio waves somewhere in that region of the spectrum. ALL OF THE ABOVE MAKE GOOD SENSE.
Assuming primitive life emerges on a planet, why is the emergence of intelligence not given, i.e., fi < 1 (we picked 0.1)
It took ~4 billion years for intelligent life to emerge on Earth under favorable circumstances, which might be extremely rare. The Earth acquired a sizable Moon .. by accident .. that stabilized its axis of rotation, which established a stable environment for life for billions of years. The Earth was close ... but not too close .. to a giant planet, Jupiter, that protected it from frequent devastating impacts. If an impact hadn't triggered the extinction of dinosaurs 65 million years ago, intelligent humans would never have evolved. ALL OF THE ABOVE WERE CONTRIBUTING FACTORS.
What will happen to the Sun's habitable zone over time?
It will slowly move further away as the Sun slowly heats up (due to the buildup of its helium core and the outward migration of its hydrogen-burning zone). .... the Sun slowly heats up as its helium core expands and its hydrogen burning shell moves outward around the core. Thus, the habitable zone slowly migrates outward away from the Sun. When the Sun enters its helium burning phase, the habitable zone will behave erratically, but the Sun will not "burn increasingly heavier elements." The heaviest element it generates via nuclear fusion is Carbon. The Sun is not large enough to generate elements heavier than this.
The least accurately known term in the Drake equation is probably the factor _____ because who knows how long an intelligent civilization, capable of beaming radio messages to the stars, survives -- 100 years -- 1 billion years? we can't even use ourselves as a very good example. After all, who knows how long humans will survive... yet?
L When Frank Drake wrote down his equation, the terms proceeding from left to right went from most to least accurately known. Now, knowing the orbital radius from (a), and given the orbital period, p, calculate the planet's average speed, v, around 51 Pegasi. (Hint: The planet's average speed is just the distance it travels around a circle, C = 2πa divided by its orbital period, p.) Why this number is quite a bit larger than the measured Doppler shift of the star 51 Pegasi itself (53 m/sec)?
What is the Drake equation?
N = R* x f. x fp x nE x fL x fi x fc x L
Do you think that an intelligent civilization that is 10,000 LY away.... for example, one that has achieved a technological capability similar to our own.... could have direct knowledge of our existence here on Earth right now at this time? Why or why not?
No ... Our civilization achieved operational intelligence within the past 100 years. Prior to that time, we were 'radio and communication' quiet. Information indicating that an intelligent species inhabited our planet could have travelled at the speed of light no more than 100 LY. Therefore this alien civilization is too far away to be aware of our existence... yet. Furthermore, even if its technology was advanced enough to detect us in some way other than by intercepting communications, any information it obtained would have come from a human population that existed 10000 years ago! Brightly lit cities did not exist...nor did much of anything else exist that would have given us away.
If you're looking for an E.T., should you look around a white dwarf or giant?
No.
Does It Make Sense? If the Sun were three times brighter than it actually is, the habitable zone would include Jupiter's orbit.
No.... Jupiter is 5 times further way from the Sun than is the Earth, so it receives about 1/25 of the light intensity that Earth receives. The sun would have to be about 25 times brighter for Jupiter to lie within the habitable zone. Even if we want Jupiter to lie just within the outer boundary of the habitable zone, the Sun would ave to be about 10 times brighter. Both of these factors are a lot more than "three". You might argue that a "Sun-like" star will eventually become 10 times brighter than it is as it moves into the red giant stage, but it doesn't stay in that stage very long ... a few million years or so ... not long enough for life so the existence of a habitable zone this far out would be fleeting.
The first extrasolar planets detected used the radial velocity method and most are 'Hot Jupiters'. Does this mean that our solar system is 'an anomaly'?
No...the radial velocity method is 'biased' since large planets very close to a parent star cause the largest spectrum shift and therefore are most likely to be detected.
Why are radio waves the optimum way to transmit information through space?
Radio photos are CHEAP, you can make 10 million of them for the same cost as 1 visible photo. Fast - they travel the speed of light. They are penetrating, i.e., they easily Tavel through intergalactic dust and gas.. and planetary atmospheres.
How much solar energy reaches Earth?
Solar radiation would pass through the area of a circle defined by the radius of Earth (r e)... A = pie r e ^2 ... if the Earth weren't there! ... but the Earth is there.. and blocks its passage E in = (L/4 pie d^2) x (pie r e ^2) BUT THIS IS NOT QUITE CORRECT! ** some energy is reflected away **
Why is the Earth Habitable?
Solid surface (useful for concentrating chemicals & reactions), not located in a "bad" neighborhood (low supernova rate, no nearby gamma ray bursters or "death rays"), SUN had plenty of HEAVY ELEMENTS to make terrestrial planets from (not Pop || star), relatively low major impact rate (major "killers" only every 100Myr, planet-sterilizer s less frequent), Large Moon (stabilizes rotation axis to prevent some huge changes in climate), LOCATION x 3
The region in which a planet could remain habitable for some specified period of time is called
The Continuously Habitable Zone
How can we explain the presence of extrasolar planetary systems with Jovian-sized planets at distances where we normally find terrestrial?
The Jovian planets formed farther out and then migrated inward.
An advanced alien civilization residing on a planet orbits Alpha Centauri A, a class G2V star only 4 LY away, detects radio emanations from the star Sol (our Sun) and concludes that it is of intelligent origin. The aliens decide to try and establish contact with the inhabitants of the star system and the beam a repeating sequence of high intensity, sharp, red laser light pulses of wavelength 656 nm toward the star. Digital 1's are represented by a pulse of Burt duration 1 ns. Digital 0's are represented by the absence of a pulse duration 1 ns. The pulse sequence (which is continually repeated) is as follows: ... 0011 0001 0100 0001 0101 1001 0010 0110 0101 0100 ... Four years later, optical SETI workers on Earth detect these optical pulses from Alpha Centauri at the Lick Observatory, in the mountains just outside Santa Cruz, California. The observers are fairly certain that the signal is coming from an alien civilization. The observers discover that there is a "message" in the signal which convinces them that it is of intelligent origin. What is the "message?"
The base ten system is the number system we humans currently use. Is it because the system is so "nice" in terms of multiplication and division, etc. ... or is it because humans have 10 fingers??? (By the way, the ancient Babylonians used the base 60 number system!) Anyway, there are 10 digits in the base 10 system. Each digit can be encoded using 4 binary bits in the base 2 system, obviously the simplest system that can be used for communication. 10 x 4 = 40, the number of bits in the message. Divide the message into 10 groups of 4, decode using binary ...for example, the first 4 bits are 0011 = 3; the second 4 bits are 0001 = 1, ...continue and you come up with the message ... π --- 3141592654 Presumably, the aliens figure we know where to put the decimal point!
Even though K- and M-type stars have very long lifetimes, why might they not have many habitable planets around them?
Their habitable zones are very narrow
The Copernican Principle and Rare Earth. The Copernican revolution taught us that our planet was not "special," i.e., it wasn't the center of the solar system ... let alone the "universe" ... a premise that had been dogmatically accepted for centuries. Taking this lesson to heart, we now assume that our planet is not special in any way. Instead, we assume that we live on a not very special planet in a not very special place in the universe. This principle is commonly called The Copernican Principle (or The Principle of Mediocrity). Cemeteries are littered with the graves of those who have argued against it. For example, at the beginning of the 20th century, many astronomers supported the hypothesis that our galaxy constituted the entire known universe and that our Sun was located at its center. Observations with increasingly powerful telescopes have disabused us of that notion. For a while after that, not having learned the lesson of The Copernican Principle, we believed that our galaxy was at the center of the universe. Like many beliefs that have no tangible supporting evidence, this one also fell by the wayside as we examined the universe with ever more powerful telescopes --- indeed we now know that the visible universe has no center at all! So --- what do you think about the Rare Earth hypothesis? Could it possibly be right?
Yes, it could! It all boils down to probabilities. There are about 100 billion stars in our galaxy and about 100 billion galaxies in the visible universe. That adds up to about 1022 stars! Suppose that the rate at which intelligent life emerges in the universe is as low 10 -32 per year ...a small but finite probability! See if you can figure out how many stars in the universe would now serve as the home base for an intelligent life form? Would this life consider itself special and the "Earth" on which it lived --- rare. · Let's talk about intelligent life here, that is, life that became complex enough that it developed the ability to reason and ultimately to do things like build radio telescopes. Such a life form might come to the conclusion that its emergence was very special. It certainly took time for intelligent life to emerge in the Universe. First, there was the Big Bang. Then, the formation of first generation stars and galaxies. Life could not yet begin --- the building blocks were not yet there. Eventually, some of those first generation stars "cooked up" the heavy elements necessary for second generation stars with habitable terrestrial planets and life on such planets. Intelligent life on one of those planets --- somewhere --- had to be the first to emerge, even if the emergence of intelligent life proves to be a very common event. If such life develops the capability of looking around for signs of other life with sophisticated instruments and finds none, wouldn't it conclude that its own place of habitat was indeed a "rare Earth?" It just might be that we live at a special time in the universe ... when conditions for the emergence of life finally are "just right." Indeed, the fact that we're here (and see no evidence of other similar beings; see the Fermi Paradox coming up in Chapter 13) suggests that the "Rare Earth Hypothesis" just might be true ... at least at the present time. · Ultimately, observational evidence is the final arbiter!
Why do scientists not take the "UFO hypothesis" seriously? (The UFO hypothesis claims that those UFO's that have not been explained by conventional means are spaceships piloted by aliens, a hypothesis that many people believe to be true)
Your text makes an interesting observation... cops don't solve all murders - ergo, the unsolved ones that must have been committed by aliens! Likewise, for "unsolved" UFO's. The long and the short of it - THERE IS NO UNEQUIVOCAL PROOF OF THE UFO HYPOTHESIS... and the burden of proof lies upon the shoulders of its supporters.
Which type of planetary orbit might possible be stable within a habitable one in a wide binary system (a system in which the two component stars are separated by a great distance)?
a circular orbit within the habitable zone of one of the stars
Assume that all the multiplicative factors in the Drake equation lead to the conclusion that N=L/10^3 (L in years). If the lifetime of humans is typical of the lifetime of other species - we estimate L = 10^6 years and thus N = 1000. Given this, roughly how long would it take to exchange a single message with one of those civilizations?
about 1-10 thousand years
The "water hole" radio frequency region between 1420 MHz generated by neutral hydrogen atoms and 1720 MHz generated by hydroxyl (OH) radicals is a good choice for interstellar communications because...
all operationally intelligent civilizations would be aware of its significance and would think of sending and receiving signals within this spectral region
The _____ is a spherical region of space that surrounds a star, within which an orbiting planet could maintain water in the liquid state on its surface
habitable zone
A star more massive than our Sun will
have a shorter lifetime
The Drake equation allows us to estimate the number of
intelligent civilizations capable of interstellar communication that currently exist in our galaxy
If a terrestrial planet is too far away from the Sun, it freezes over - like Mars. Current estimates for our Sun place the _____ at 1.4 - 1.7 AU. Your test argues that this limit is based on the planet having a thick atmosphere. However, the actual limit is probably less than 1.4-1.7 AU for the following reason... large, terrestrial planets capable of holding a thick atmosphere are not likely to form in this region because of the disruption of the formation process by the gravity exerted by an innermost large Jovian planet, which models suggest, will form quite rapidly. Even if a large terrestrial planet did form there the dominant gasses in its atmosphere would be CO2 and H2) vapor, since these gasses are emitted from volcanoes. Early on, the CO2 would stay in the atmosphere as a gas. The liquid water would absorb it! As the gas is removed, the greenhouse effect that a planet needs to keep the water liquid, would lessen.. unless it was continually replenished by frequent volcanic activity associated with plate tectonics. But a planet has to be close to Earth-sized to do that. An Earth-sized planet is large enough to hold a thick enough CO2 dominated atmosphere together with liquid oceans but not a significantly smaller planet likes Mars! So continual volcanic activity would eventually cease and no more CO2 or H2O vapor would be injected into the atmosphere and the remaining CO2 and H2O vapor would freeze and fall to the surface. In other words, the presence of surface water during the early history the planet would create the seeds of its own destruction!
outer limit of its habitable zone
What is R* in the Drake equation?
rate at which stars form in the milky way
Venus formed closer to the Sun than did Earth and since it formed more or less in the same way as Earth, it once had water that it lost (this hypothesis is supported by the observation of the deuterium-hydrogen ratio in the little water vapor that remains today). Its disappearance happened something like this: Venus' surface water evaporated... rose up high in its atmosphere where H2O molecules were broken apart by ultraviolet (uv) light in their constituent atoms. This break up process is called photo-dissociation. The light weight hydrogen atoms (Atomic weight 1) that are freed up from the water molecules in the high temperature of Venus... have enough speed to escape into space. For every hydrogen atom that was lost, so was the water molecule to which the hydrogen atom had been bound. As Venus continued to lose water this way, Venus was less able to absorb atmospheric CO2. As CO2 was released into its atmosphere by its volcanoes, the greenhouse effect increased... which raised surface temperatures... which increased the rate of water evaporation, etc, etc. The process of heat buildup, water loss and CO2 buildup accelerated via this positive feedback effect. This scenario is an example of what happens to a planet subjected to the ____ effect
runaway greenhouse ..... the Earth exhibits a greenhouse effect that raises the EArth's surface temperature enough to keep its surface water liquid. Venus lost all of its water because of a RUNAWAY greenhouse effect.
If a planet is too close to the Sun, the same thing will happen to it that happened to Venus (see the question above). In the case of our own solar system, if the Earth were perhaps only 5% closer to the Sun, it might avoid the Venus scenario described above, yet nonetheless lose its water slowly by uv photo-dissociation and thus ultimately be rendered uninhabitable. This would happen because more water would evaporate from its oceans, which would warm the surface even more than one would estimate based solely upon its closer position to the Sun because of an enhanced greenhouse effect. Thus, the water molecules would rise higher in the atmosphere than they do now, where they would be attacked more strongly by ultraviolet light from the Sun. Succumbing to Ascenario like this is probably the essential factor that determines ....
the location of the inner boundary of the habitable zone.... if a planet lies within the inner boundary of the habitable zone that surrounds its parent Sun, it will loses its surface liquid water
If the Earth were moved to where Venus is today...
the oceans would evaporate and CO2 would build up in the atmosphere triggering a runaway (or moist) greenhouse causing the temperature to rise as high as the one that exists on Venus today
A planet's temperature reaches equilibrium when ....
the solar energy absorbed by the planet is balanced by the total energy that it emits back into space
If you know the distance and apparent brightness....
then you can calculate "absolute brightness"
51 Pegasi ... Using the Doppler technique, astronomers found an extrasolar planet orbiting the star, 51 Pegasi, with a period of p = 4.23 days. 51 Pegasi has about the same mass as our Sun, which means that we can determine orbital properties of its planets by applying the reduced version of Kepler's 3rd law: p2 = a3 where p is the orbital period of the planet in years and a is the mean radius of the orbit in AU (1 AU = 150 million kilometers). Now, knowing the orbital radius from (a), and given the orbital period, p, calculate the planet's average speed, v, around 51 Pegasi. (Hint: The planet's average speed is just the distance it travels around a circle, C = 2πa divided by its orbital period, p.) Why this number is quite a bit larger than the measured Doppler shift of the star 51 Pegasi itself (53 m/sec)?
v = 2 pie a / p = 6.28 x 0.051 AU x 1.5 x 10^8 m / AU _________________________________________________ 4.23 day x 24hr / day x 3600 s / hr = 132 km/s The measured Doppler shift is due to the motion of the star. Since the planet is much smaller than the star, it moves faster about their common center of mass.
Compared to today, a billon years in the future, the Sun's habitable zone will be ...
wider and farther from the Sun
An advanced alien civilization residing on a planet orbits Alpha Centauri A, a class G2V star only 4 LY away, detects radio emanations from the star Sol (our Sun) and concludes that it is of intelligent origin. The aliens decide to try and establish contact with the inhabitants of the star system and the beam a repeating sequence of high intensity, sharp, red laser light pulses of wavelength 656 nm toward the star. Digital 1's are represented by a pulse of Burt duration 1 ns. Digital 0's are represented by the absence of a pulse duration 1 ns. The pulse sequence (which is continually repeated) is as follows: ... 0011 0001 0100 0001 0101 1001 0010 0110 0101 0100 ... Four years later, optical SETI workers on Earth detect these optical pulses from Alpha Centauri at the Lick Observatory, in the mountains just outside Santa Cruz, California. The observers are fairly certain that the signal is coming from an alien civilization. Why do they come to this conclusion?
· The signal comes from a point source in the sky · The signal is repetitive · The bursts are of such short duration that they are not likely to have been produced by a natural phenomenon (at least any that we know of) · The bursts have a wavelength of 656 nm, the hydrogen Balmer 3à 2 atomic transition, a well-known wavelength if you know some physics and possibly a likely wavelength to use for a signal. The observers discover that there is a "message" in the signal which convinces them that it is of intelligent origin.
