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

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

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

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

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? 

Question 6:Select the system HD 93083. Note that planet b is in this star’s CHZ.  This planet has a mass of at least 0.37 Jupiter masses (which is greater than the mass of Saturn, Uranus, and Neptune, making it a gas giant). Is this planet a likely candidate to have life like that on Earth? Why or why not? 

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

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 MEarth 

b

> 15.6 MEarth 

c

> 5.4 MEarth 

d

> 7.1 MEarth 

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 selectedmode 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? 

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)?

Question 10:What happens to Earth at this time in the simulator? 

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. 

Star Mass (M) Sun

Initial Planet Distance (AU)

Time in CHZ (Gy) 

0.3

0.157 

380 

0.7

1.0Open 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

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

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

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

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? 

Question 6:Select the system HD 93083. Note that planet b is in this star’s CHZ.  This planet has a mass of at least 0.37 Jupiter masses (which is greater than the mass of Saturn, Uranus, and Neptune, making it a gas giant). Is this planet a likely candidate to have life like that on Earth? Why or why not? 

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

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 MEarth 

b

> 15.6 MEarth 

c

> 5.4 MEarth 

d

> 7.1 MEarth 

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 selectedmode 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? 

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)?

Question 10:What happens to Earth at this time in the simulator? 

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. 

Star Mass (M) Sun

Initial Planet Distance (AU)

Time in CHZ (Gy) 

0.3

0.157 

380 

0.7

1.0

2.0

4.0

8.0

15.0

2.0

4.0

8.0

15.0