Astronomy homework  :

2. Birthrate of Earth-sized planets (9 points) a. (2 pts) There are at least ~200 billion (2×10 11 ) stars in the Milky Way galaxy. Roughly 20% of them are similar to our Sun. Based on the exoplanets that have been detected so far, scientists now estimate that roughly ~20% of Sun-like stars have Earth-sized planets. Assuming there’s never more than one Earth-sized planet orbiting a given star, roughly how many Earth-sized planets are there orbiting Sun-like stars in our Milky Way galaxy? b. (2 pts) Our galaxy is ~13 billion (1.3 x 10 10 ) years old. Given your answer in (a) and assuming that planets are equally likely to be born at any time in the history of our galaxy, how often is an Earth-sized planet in our galaxy ‘born’ around a Sun-like star, on average?

c. (2 pts) There are at least 100 billion (10 11 ) galaxies in the observable universe (i.e. the Milky Way isn’t the only galaxy! Given your answer in (b), how often does an Earth-sized planet form around a Sun-like star in the observable universe?

d. (3 pts) Do you think life ever developed on a planet other than Earth, somewhere in the universe? If yes, do you think there is still life there today, and is it “intelligent”? If no, why is Earth ‘special’? There is no correct answer here – just write a few sentences summarizing your thoughts. This is one big reason we study astronomy, after all! e. (3 pts extra credit – not for the faint of heart!) Assume that the Milky Way is a flat disk with radius of 50,000 light years. Let’s also assume that one in a million Earth-sized planets around Sun-like stars in our galaxy have a civilization right now. If they were all evenly distributed, how far would it be to the nearest civilization, in light years? Can you send and receive a message from the nearest civilization in your lifetime? 

3. Planet Earth (9 points) Earth’s unique aspects have proved critical to our existence. Use the textbook and your notes from throughout the semester to answer the following questions: a. (2 pts) Plate Tectonics – Answer either (i) or (ii) but not both: i. Lithospheric plates move at a rate of ~2 cm/yr (= 2×10 -5 km/yr). At this rate how long would it take for Boulder to move around Earth once? (Earth’s radius is ~6400 km. Remember that the circumference of a sphere is 2 × ? × Radius) ii.

Why do Earth’s lithospheric plates move relative to each other? Given this, why is plate tectonics less likely to be active today on a planet like Mars or Mercury?

b. (2 pts) Liquid Water at the Surface – Give two examples of different types of water erosion in your home state / province / country. Features with names are preferred (e.g. Grand Canyon). 

c. (2 pts) Life – Earth’s early atmosphere was probably Mars- or Venus-like in composition. With this in mind, which colonized land first: animals or plants? Support your answer. 

d. (3 pts) For each of the three characteristics listed in boldface in parts (a)-(c) describe how Earth’s atmospheric composition would be different if Earth lacked only that characteristic.