_Habitable zone
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Name: Habitable Zones - Student Guide 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: 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 .820 AU
1.17 AU
.35 AU
NAAP - Habitable Zones 1/7
Question 2: 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
0.0132
0.109
0.157
0.048
0.7
0.134
0.349
0.500
0.151
1.0
0.739
0.817
1.17
0.353
2.0
16.5
3.87
5.56
1.69
4.0
241
14.8
21.2
6.4
8.0
2690
49.3
70.9
21.6
15.0
19000
131
188
57
Question 3: Using the table above, what general conclusion can be made regarding the location of the CHZ for different types of stars? The table above shows that the bigger and hotter the star is, the further away the habitable
zone is going to be from the star.
Question 4: Using the table above, what general conclusion can be made regarding the width of the CHZ for different types of stars? One general conclusion would be that the bigger the star is, then the wider the habitable zone is. This would be because the massive temperature of the star would be able to heat planets that are further away.
NAAP - Habitable Zones 2/7
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: 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? This planet is not very similar to any other planets in our solar system except for Mercury, but only because it is very close to its star and also most likely very hot and dry. This planet sits at a distance of only .0520 AU away from the star.
Question 6: 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? This planet sits comfortably within the boundaries of this system’s CHZ. It would make sense that this planet would be able to sustain life due to its orbit being within the CHZ.
Question 7: Note that Jupiter's moon Europa is covered in water ice. What would Europa be like if it orbited HD 93083b? If Europa was orbiting this planet, you would probably see that Europa would have polar ice caps similar to earth while having oceans around the equator. This would mostly rely on if Europa was able to generate a sustainable atmosphere.
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.
Planet Mass e
> 1.9 M
Earth b
> 15.6 M
Earth c
> 5.4 M
Earth d
> 7.1 M
Earth
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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 encompassing 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 bottom 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: Return to the none selected mode and configure the simulator for Earth (a 1M
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? Earth will remain in the CHZ for approximately 820 million more years.
Question 9: What is the total lifetime of the Sun (up to the point when it becomes a
white dwarf and no longer supports fusion)? 12 Billion years
Question 10: What happens to Earth at this time in the simulator
? The Earth is destroyed at this point
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. We know that life appeared on Earth early on but complex life did not appear until several billion 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: 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. The first three simulations in the table above would be within the CHZ for more than 4 billion years and theoretically have enough time to develop complex life. The conclusion can be made that the smaller the star’s mass is the longer the planet will remain in the CHZ given that it is in the CHZ to begin with.
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. Star
Mass
(M
)
Initial
Planet
Distance
(AU
)
Time in CHZ
(Gy)
0.3
0.157
380
0.7
.502
29.4
1.0
1.17
8.21
2.0
5.56
1.16
4.0
21.2
.174
8.0
71
.0322
15.0
188
.0113
Question 12: 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
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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:
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? Earth’s water would begin to constantly evaporate on the sunny side of Earth while
the other side would have frozen oceans. Life could still be possible along the border between these two regions where temperatures might be warm enough to not have all the water frozen.
Question 14: 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?
A planet orbiting a star with a mass on 0.3 M
would be
the least conducive toward life from this table.
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. Mass
Tidally
Locked?
0.3 M
YES
0.5 M
YES
0.8 M
NO
1.0 M
No
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. NAAP - Habitable Zones 6/7
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 Habitability Explorer. Each of the two factors described above are illustrated in a graph as a function of distance from the galactic center. Question 15: 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? Catastrophic events influence the rate at which planets form. This means that the further from the center you look the less planets you will find.
Question 16: 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? Extinction events could be supernovae, solar weather, orbital drift towards host star, or even impact from another body. The likelihood of extinction decreases as you get further away from the center of the Milky Way.
Question 17: 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. A possible Goldilocks’ zone for the GHZ could range from a distance of 5kpc to 15 kpc. Any distance smaller than 5kpc would begin to exponentially increase in extinction risk while an distance further than 15kpc would be less likely to have planets form in order for
life to develop.
NAAP - Habitable Zones 7/7
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