Introduction and Background

Earth is the only planet known to harbor life of any kind, past or present. As part of the search for evidence of life on other planets, both in our solar systems and in other planetary systems, we are looking not just for evidence of the living organisms itself, but for evidence of the conditions that might even be hospitable to life as we know it. One feature that astronomers consider to determine whether a planet MIGHT have the conditions necessary for life as we know it is whether the planet falls within a 'habitable zone' of its host star. (Note: This lab will explore circumstellar habitable zones only, not galactic habitable zones.) If you have already learned about habitable zones in another astronomy class or from your own general knowledge, great. If not, or if you'd like to get a stronger background before proceeding, please read more about habitable zones at astro.unl.edu, universetoday.com, astronomynotes.com, or space.com.

1.

What is a "habitable zone"? Give a good, complete definition, in your own words. If you're not sure and need to look anything up, use a reliable resource (such as one of the four linked above), and be sure to reference the resource you use. Be sure to include in your definition:

a) what is meant by "habitable",

b) why it is a "zone" and not one specific location,

c) what object it is surrounding, and

d) what object(s) may be located within it.

e) Use complete sentences.

Part 1 of 5: The Habitability of the Earth

To begin, load up the Habitable Zone simulator written by the University of Nebraska at the following URL in a new window:

http://astro.unl.edu/naap/habitablezones/animations/stellarHabitableZone.swf

You will need this Angel question window and the unl.edu habitable zone simulator window open simultaneously, as you will need to switch back and forth frequently.

The flash simulator will show you a visual diagram of the solar system in the top panel, a set of simulation settings in the middle panel, and a timeline of the habitability of the Earth in the bottom panel. The timeline units will either be Megayears (Myr) which means millions of years, or Gigayears (Gyr) which means billions of years. To run the simulation, click run in the bottom panel. This button immediately becomes a pause button which will allow you to pause the simulation at any time.

The simulation runs pretty quickly by default. To adjust the speed use the rate slider bar to the right of the run button. You can also manually advance the simulation forward or backward by clicking and dragging the upside-down dark grey triangle above the timeline. To restore the simulation to the original default settings, press the reset button at the very top of the simulation.

2.

The simulation is currently set to zero-age - this is the Solar System as it was when it first formed, about 4.5 billion years ago. Which planet(s) were in the Habitable Zone at this time, if any?

3.

The blue region marked on the diagram is the Habitable Zone around our Sun. Notice how there is both an inner edge and an outer edge - the planets interior to the habitable zone are too hot to support liquid water, while the planets exterior to it are too cold. Why?

4.

Press the start button and watch the Habitable Zone change with time. Pause the simulation when it reaches an age of 4.5 billion years (you can keep track of the time by looking at the timeline marker in the bottom panel). This is the Solar System as it is today - which planet(s) are in the Habitable Zone now, if any?

5.

Allow the simulation to run until the Earth is no longer in the Habitable Zone. At what age does this happen? AND How long from now until this happens? You can use the timeline bar in the bottom panel to determine your answers. Type both answers in the box below, being sure to include units with your numbers.

6.

After the Earth is no longer within the Habitable Zone, what do you think the conditions on Earth will be like, and why?

7.

Resume the simulation and let it run until the end. Which planets other than the Earth will fall within the Habitable Zone at any point during the Sun's life, if any?

8.

Why does the habitable zone change during the Sun's lifetime? Pay attention to how the properties of the Sun change, and explain how this can affect the habitability of planets. One or two full sentences please.

9.

Optional Question: Around 12 billion years, the Earth's distance from the Sun suddenly changes. Why? (Draw from your knowledge of what you learned in your previous astronomy class that was a prerequisite or corequisite for taking this lab.) Extra credit is possible for very good answers.

Part 2 of 5: The History of Life on Earth

As you saw in the simulations above, the Earth has been within the Habitable Zone of our Sun nearly since its formation. Complex life, however, did not develop immediately. And humans did not appear until later still. The timeline shown below delineates several milestones in the history of life on Earth.

10.

For each of the events on the timeline, determine how long after the formation of the Earth this event occurred (in Gigayears -- "Giga" means billion) AND THEN calculate what fraction of its current age (4.5 billion years) the Earth was at that time. Looking at the second row for example, the first (primitive) life arose 3.8 billion years ago, which was 0.7 billion years after the Earth formed (4.5 billion years ago - 3.8 billion years ago = 0.7 billion years), so fill in ``0.7'' in the first column next to this event. And at this time, when the Earth was 0.7 billion years old, that was 0.7/4.5 = .155 = 15.5% of Earth's now current age, so fill in ``15.5'' in the second column. Do not include units or the % sign with your numbers; the units are already included.

Milestones in the Emergence of Life on Earth

11.

Think about your answers to the previous timeline question. What do you think was the purpose of that exercise? What is the take-home message? (Think about whether primitive life arose early or late. What about humans?) One or two full sentences please.

Part 3 of 5: The Habitability Different Kinds of Stars

Now that you've simulated the Habitable Zone around our Sun, we'll run the same simulation for other stars. Astronomers classify stars with letters: O, B, A, F, G, K, and M. The O stars are the hottest and most luminous, while the M stars are the coolest and dimmest. Every types of star has its own Habitable Zone, but the brighter the star the farther out the Habitable Zone. Imagine putting an extra log on a campfire - the campers all have to back off a few feet to maintain the same comfortable temperature.

Below is a table of the different types of stars, using this common single letter classification scheme. Notice how they each have a different mass - in fact, the mass of a star is the underlying determining factor for all other stellar properties (luminosity, temperature, etc.), and therefore dictates what type it will be classified as.

Reset the Habitable Zone simulator with the reset button at top, and then adjust the star mass with the initial star mass slider bar in the middle panel. The units of star mass are Solar Masses; our Sun's mass is exactly one Solar Mass by definition. Notice how the Habitable Zone immediately changes in size. Notice also that you can adjust the orbit of ``Earth'' (i.e. the planet under consideration) by adjusting the initial planet distance slider bar in the middle panel. You can also adjust it by clicking on the planet itself and dragging it closer or farther from the star. The units of distance from the star are AU - astronomical units, which is defined as the distance of the Earth from the Sun. The Earth is one AU from the Sun by definition.

For each of the star types in the table below, your job is to find the planet orbit that remains in the habitable zone the longest. This will take some time! This is the main part of this lab.

Note: The first two questions in this section may appear out of order. Please fill in the table of star types first, and then answer the question about the best place to look for planets harboring life.

12.

Given your answers in the table above, and keeping in mind that the Universe is only 13.7 billion years old, what type of star do you think would be the best place to look for planets harboring life, and why? One or two full sentences, please.

13.

For each of the types of stars, run the habitable zone simulator with the closest mass you can find to that listed as "typical". Indicate what mass you chose in the first column, even if it was identical to the typical mass listed. Adjust the initial planet distance (I suggest dragging the planet back and forth slowly through the HZ while keeping an eye on the total length of the blue bar, indicating time of habitability, on the bottom) until you find the one that gives the longest amount of time CONTINUOUSLY in the habitable zone; record both the initial planet distance used and the corresponding TOTAL time in the habitable zone. Note, you are recording the TOTAL time continuously in the habitable zone for the longest stretch, NOT necessarily just the time when the planet leaves the habitable zone, as these may be different. For some of the lower mass stars, you should find that the planet becomes tidally locked even while it is still in the habitable zone. Ignore tidal locking, and just pay attention to when the planet is in the HZ.* WARNING: Sometimes the numbers on the timeline are shown in Myr (Megayears, where "Mega" = million) instead of Gyr (Gigayears, where "Giga" = billion) in cases where the star lives are short enough to warrant these units. Be sure to convert these times to Gyr as necessary before you enter your answer! If you need help with this conversion, ask other students or else the professor!

Finally, in the last column, record the most advanced life that could develop in this amount of time, if any, using your answers from the table in the previous section.

For the Star Mass, Orbit Size, and Habitable Lifetime columns, enter a number only -- units are already provided in the column header. If you enter anything but numerical digit(s), Angel will mark it wrong because it is automatically graded and is expecting only a number. For the last column, type out the word or words corresponding to the most advanced life that could develop. Pay attention to spelling, because again, it will be automatically graded.

Different Types of Stars

14.

What do you notice about the TOTAL lifetimes of the different types of stars? (That is, the lifetimes of the stars themselves, ignoring any planets and the habitable zone.) Which live the longest, and which the shortest?

15.

Which type of star is most luminous? Which is least? So which is easiest to detect and monitor?

16.

What type of star is our Sun?

OBAFGKM

17.

Compared to our Sun's type (see above)... what do you think the development of life on planets orbiting hotter types of stars would be like? What about cooler types of stars? Do you think that life in such conditions is even possible? Justify your answers either way. Several full sentences please.

18.

If you were the director of a NASA program to search for life beyond Earth, toward which type of star would you direct your attention, and why? Consider your responses to ALL the previous questions, and justify your answer. You may use any additional lines of reasoning you like. Several full sentences please.

Part 4 of 5: Practical Considerations

19.

Jupiter's moon Europa is currently covered with water ice (H2O), and possibly liquid water beneath. How is this possible, given that Jupiter is well outside our Sun's current habitable zone? Be sure to consider on which side of our Sun's habitable zone Jupiter and Europa are located, and include in your answer what assumptions go into the standard definition of "habitable zone" as used by this simulator. Two or three full sentences would be appropriate.

20.

In the last two decades, astronomers have developed the ability to detect planets around other stars. Do you think MOONS will be easier or harder to detect than planets, and why?

21.

If a planet or moon IS inside the habitable zone, does that necessarily mean it is habitable? Why or why not? (Hint: Earth's Moon is inside our Sun's habitable zone. Is it habitable?)

22.

Most of the stars we can see with the unaided eye in our night sky are hundreds or even thousands of lightyears away from Earth. (The very closest ones are only a few dozen lightyears away, but most are much further.) The vast majority of stars in our galaxy are many tens of thousands of lightyears away. IF intelligent life existed on planets orbiting some of these stars -- and that's a huge IF! -- comment on the likelihood and practicality of (a) visiting, (b) communicating with, or (c) verifying the existence of those life forms. Describe how would you might go about approaching EACH of these three tasks, or if you think they are even possible. (Two or three sentences for EACH part would be appropriate.)

Part 5 of 5: Reflection and Summary

Now I would like you to reflect on what you have learned about habitable zones around stars and their implication for the possibility of life in the universe.

23.

In about 50-100 words, summarize what you have learned (or what the main take-home points were) about habitable zones and the possibility of life on other planets around other stars. For full credit, mention main points from each part of this lab, and do so with full sentences. These were the parts of the lab:

Introduction and Background

Part 1 of 5: The Habitability of the Earth

Part 2 of 5: The History of Life on Earth

Part 3 of 5: The Habitability Different Kinds of Stars

Part 4 of 5: Practical Considerations