Habitable Zones - National Geographic

Exercises 

Please read through the background pages entitled Life, Circumstellar Habitable Zones, and The Galactic Habitable Zone before working on the exercises using simulations below. 

Circumstellar Zones 

Open the Circumstellar Zone Simulator . There are four main panels: 

• The top panel simulation displays a visualization of a star and its planets looking down onto the plane of the solar system. The habitable zone is displayed for the particular star being simulated. One can click and drag either toward the star or away from it to change the scale being displayed. 

• The General Settings panel provides two options for creating standards of reference in the top panel. 

• The Star and Planets Setting and Properties panel allows one to display our own star system, several known star systems, or create your own star-planet combinations in the none-selected mode. 

• The Timeline and Simulation Controls allows one to demonstrate the time evolution of the star system being displayed. 

The simulation begins with our Sun being displayed as it was when it formed and a terrestrial planet at the position of Earth. One can change the planet’s distance from the Sun either by dragging it or using the planet distance slider. 

Note that the appearance of the planet changes depending upon its location. It appears quite earth-like when inside the circumstellar habitable zone (hereafter CHZ). However, when it is dragged inside of the CHZ it becomes “desert-like” while outside it appears “frozen”. 

Question 1: (1 point) Drag the planet to the inner boundary of the CHZ and note this distance from the Sun. Then drag it to the outer boundary and note this value. Lastly, take the difference of these two figures to calculate the “width” of the sun’s primordial CHZ. 

CHZ Inner Boundary	CHZ Outer Boundary	Width of CHZ
.818 AU	1.17 AU	.352 AU

Question 2: (1 point) Let’s explore the width of the CHZ for other stars. Complete the table below for stars with a variety of masses.

Star Mass (M )  	Star Luminosity (L )  	CHZ Inner Boundary (AU)	CHZ Outer Boundary (AU)	Width of CHZ (AU)
0.3	.0132	.109	.157	. 048
0.7	.134	.35	. 501	. 151
1.0	.739	.818	1.17	. 352
2.0	16.5	3.87	5.56	1.69
4.0	241	14.8	21.2	6.40
8.0	2690	49.4	70.9	21.5
15.0	19000	131	188	57

Question 3: (1 point) Using the table above, what general conclusion can be made regarding the location of the CHZ for different types of stars? 

From the table above, it can be concluded that as the mass of the star increases, the CHZ moves away further and with a reduced mass of the star, the CHZ moves closer to the sun . 

Question 4: (1 point) Using the table above, what general conclusion can be made regarding the width of the CHZ for different types of stars? 

It can be concluded that the width of the CHZ is a positive correlation as far as the mass of the star as well as the distance the CHZ is far away from the star. This means that with a higher mass of the star, the CHZ is wider and the opposite is true. 

Exploring Other Systems 

Begin by selecting the system 51 Pegasi. This was the first planet discovered around a star using the radial velocity technique. This technique detects systematic shifts in the wavelengths of absorption lines in the star’s spectra over time due to the motion of the star around the star-planet center of mass. The planet orbiting 51 Pegasi has a mass of at least half Jupiter’s mass. 

Question 5: (1 point) Zoom out so that you can compare this planet to those in our solar system (you can click-hold-drag to change the scale). Is this extrasolar planet like any in our solar system? In what ways is it similar or different? 

It is not extrasolar. It is extremely close to its star; thus, it cannot be compared to a planet in our solar system. Additionally, it is a planet similar to Jupiter that is gaseous and large; thus, it is unlike our planets. Despite that, orbits are closer compared to any other planet in our solar system. 

\ 

Question 6: (1 point) Select the system HD 93083. Note that planet b is in this star’s CHZ. Now in fact this planet has a mass of at least 0.37 Jupiter masses. Is this planet a likely candidate to have life like that on Earth? Why or why not? 

The planet does not qualify to be a candidate for Earth-like life. The size of the planet would be larger than what would be normal to have life in this habitable zone. It cannot support life, given that it is too close at its size. 

Question 7: (1 point) Note that Jupiter’s moon Europa is covered in water ice. What would Europa be like if it orbited HD 93083b? 

It could be potentially possible that Europa could have life provided that it was to orbit a planet this close to the sun. The ice water would melt without entirely drying out most probably. A water-based planet, for instance Earth would be left in an appropriate CHZ. 

Planet	Mass
e	> 1.9 M Earth
b	> 15.6 M Earth
c	> 5.4 M Earth
d	> 7.1 M Earth

Select the system Gliese 581. This system is notable for having some of the smallest and presumably earth-like planets yet discovered. Look especially at planets c and d which bracket the CHZ. In fact, there are researchers who believe that the CHZ of this star may include one or both of these planets. (Since there are several assumptions involved in the determination of the boundary of the CHZ, not all 

researchers agree where those limits should be drawn.) This system is the best candidate yet discovered for an earth-like planet near or in a CHZ. 

The Time Evolution of Circumstellar Habitable Zones 

We will now look at the evolution of star systems over time and investigate how that affects the circumstellar zone. We will focus exclusively on stellar evolution which is well understood and assume that planets remain in their orbits indefinitely. Many researchers believe that planets migrate due to gravitational interactions with each other and with smaller debris, but that is not shown in our simulator. 

We will make use of the Time and Simulation Controls panel. This panel consists of a button and slider to control the passing of time and 3 horizontal strips: 

• the first strip is a timeline encompassinging the complete lifetime of the star with time values labeled 

• the second strip represents the temperature range of the CHZ – the orange bar at the top indicates the inner boundary and the blue bar at the botom the outer boundary. A black line is shown in between for times when the planet is within the CHZ. 

• The bottom strip also shows the length of time the planet is in the CHZ in dark blue as well as labeling important events during the lifetime of a star such as when it leaves the main sequence. 

Stars gradually brighten as they get older. They are building up a core of helium ash and the fusion region becomes slightly larger over time, generating more energy. 

Question 8: (1 point) Return to the none selected mode and configure the simulator for Earth (a 1 M star at a distance of 1 AU). Note that immediately after our Sun formed Earth was in the middle of the CHZ. Drag the timeline cursor forward and note how the CHZ moves outward as the Sun gets brighter. Stop the time cursor at 4.6 billion years to represent the present age of our solar system. Based on this simulation, how much longer will Earth be in the CHZ? 

Approximately .83 Gy otherwise 8.3 hundred million years. 

Question 9: (1 point) What is the total lifetime of the Sun (up to the point when it becomes a white dwarf and no longer supports fusion)? 

At 12 Gy is when the sun becomes a white dwarf. 

Question 10: (2 point) What happens to Earth at this time in the simulator? 

At this time in the simulator, Earth has pushed away from the sun more although it has been dead for long. The CHZ is now even further even compared to the planet, Pluto. 

You may have noticed the planet moving outwards towards the end of the star’s life. This is due to the star losing mass in its final stages. 

Star  Mass  (M)	Initial  Planet  Distance  (AU )	Time in CHZ (Gy)
0.3	0.157	380
0.7	.501	29.5
1.0	1.17	8.18
2.0	5.56	1.14
4.0	21.2	174 My
8.0	70.9	32.1 My
15.0	188	11.4 My

We know that life appeared on Earth early on but complex life did not appear until several billlion years later. If life on other planets takes a similar amount of time to evolve, we would like to know how long a planet is in its CHZ to evaluate the likelihood of complex life being present. 

To make this determination, first set the timeline cursor to time zero, then drag the planet in the diagram so that it is just on the outer edge of CHZ. Then run the simulator until the planet is no longer in the CHZ. Record the time when this occurs – this is the total amount of time the planet spends in the CHZ. Complete the table for the range of stellar masses. 

Question 11: (2 point) It took approximately 4 billion years for complex life to appear on Earth. In which of the systems above would that be possible? What can you conclude about a star’s mass and the likelihood of it harboring complex life. 

Any system with a star mass of 1.0 or less could have the potential of harboring life. A star with a mass of approximately 1.3 could have a planet in a CHZ long enough for appearance of life, taking our 4 billion years that took life to appear. For anything larger, the planet does not maintain being in the CHZ long enough. 

Tidal Locking 

We have learned that large stars are not good candidates for life because they evolve so quickly. Now let’s take a look at low-mass stars. Reset the simulator and set the initial star mass to 0.3 M. Drag the planet in to the CHZ. 

Question 12: (2 point) Notice that the planet is shown with a dashed line through its middle. What has happened is that the planet is so close to its star that is has become tidally locked due to gravitational interactions. This is analogous to Earth’s moon which always presents the same side towards Earth. For a planet orbiting a star, this means one side would get very hot and the other side would get very cold. (However, a thick atmosphere could theoretically spread the heat around the planet as happens on Venus. In answering the following questions, please put aside this possibility.) 

Question 13: (2 point) What would happen to Earth’s water if it were suddenly to become tidally locked to the Sun? What would this mean for life on Earth? 

Concerning the scenario, the water on earth could undertake a few things. The heat from the side facing the sun would make the water evaporate entirely. The water would completely freeze on the side facing away from the sun. In the middle of the two halves, there is a thin strip where water could be in a liquid form. The area to support life would be minimal although it could survive in the long run. 

Mass	Tidally Locked?
0.3 M	Yes
0.5 M	Yes
0.8 M	No
1.0 M	No

Question 14: (2 point) Complete the table below by resetting the simulator, setting the initial star mass to the value in the table, and positioning the planet in the middle of the CHZ at time zero. Record whether or not the planet is tidally locked at this time. If tidal locking reduces the likelihood of life evolving on a planet, which system in the table is least conducive towards life? 

The system that has a star with a mass of 0.5 is the least as far as supporting life is concerned. It is not only tidally locked but because it has additional mass than 0.3 star, the planet having the star with a 0.5 mass will be in the CHZ for a shorter period. 

CHZ Summation 

We have seen that low-mass stars have very small CHZs very close to the star and that planets become tidally locked at these small distances. We have seen that high-mass stars have very short lives – too short for life as we know it to appear. 

The combination of these two trains of thought is often referred to as the Goldilocks hypothesis – that medium-mass stars give the optimal opportunity for complex life to appear. 

GHZ 

Now we are going to investigate habitability zones on the scale of the entire Milky Way Galaxy. The two competing factors that we will look at are 1) the likelihood of planets forming (since we assume that life needs a planet to evolve on), and 2) the likelihood of life being wiped out by a cosmic catastrophe. 

Open up the Milky Way Habitabilty Explorer. Each of the two factors described above are illustrated in a graph as a function of distance from the galactic center. 

Question 15: (2 point) What factor influences the rate of planet formation? How does this vary as a function of a star system’s distance from the center of the Milky Way? 

The rate of planet formation is influenced by the abundance of heavy elements. There are more of the heavy elements closer to the galaxy's center insinuating that the chances of planet formation are high. The heavy elements are less when it comes to regions further away from the center; thus, planet formation is less. 

Question 16: (2 point) What sort of events can wipe out life on a planet? How does the likelihood of extinction for life vary depending upon a star system’s distance from the center of the Milky Way? 

The sort of events that can wipe out life on a planet include a nearby supernova explosions and impacts emanating from larger meteors can render a planet sterile of its current life. As it gets closer to the center of the galaxy, such events as described above are more regular. Provided that you are further away from the center, the sterilization events are rare. 

Question 17: (2 point) Present a version of the Goldilock’s Hypothesis for the GHZ that is similar in character to that which we stated for the CHZ earlier. 

Given that the events that can wipe out life on a planet are a curved graph, it has a greater role in the GHZ as compared to the abundance of heavy elements which is rather graphed as a straight line. With that, it is probable that around 7-8kpc from the galaxy's center is when the events of sterilization grow very quickly. From that point, there are good amounts of heavy elements as well. This means that the distance from the center of the galaxy needs to have a higher planet count as well as more planets that can harbor life by preventing occurrences of catastrophic events.