Lesson 7 Lab - Habitable Zone Worksheet WORD

Name: Thomas Sanche Lesson 7 Lab - Habitable Zones 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 Dinner Douter Douter-Dinner 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) NAAP – Habitable Zones 1/8

0.3 0.00006 0.07 0.7 0.63 0.7 0.0003 0.14 1.4 1.26 1.0 0.001 0.21 2.1 1.89 2.0 0.004 0.35 3.5 3.15 4.0 0.016 0.56 5.6 5.04 8.0 0.064 0.9 9.0 8.1 15.0 0.25 1.4 14.0 12.6 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? As the mass of the stars grows, the extent of the circumstellar habitable zone (CHZ) similarly expands. This occurs due to the fact that stars with more mass possess a more powerful radiation pressure, which in turn causes the outer limit of the Circumstellar Habitable Zone (CHZ) to be pushed to a greater distance. 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? The table above shows that star mass and luminosity increase circumstellar habitable zone (CHZ) width. The CHZs of stars with higher masses and luminosities are wider, allowing planets to support liquid water and life at farther distances. Planetary systems orbiting huge, brilliant stars may host livable homes at specific orbital distances. We need to know the CHZ for different star types to determine if exoplanets in other solar systems are habitable. 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? NAAP – Habitable Zones 2/8

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? Earth evolved in the circumstellar habitable zone. Moving the timeline cursor forward represents the Sun's ageing, which increases the star's brightness, expanding the CHZ. Stopping the simulation at 4.6 billion years shows Earth's current position relative to the CHZ limits. This lets you estimate how long Earth will be habitable before star luminosity changes affect it. The simulation shows how habitable zones change over a star's lifetime. 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)? From formation to becoming a white dwarf and no longer supporting fusion, the Sun probably lived 10 billion years. The Sun's main-sequence phase involves hydrogen- helium fusion in its core. This era has lasted 4.6 billion years. It's projected that the Sun NAAP – Habitable Zones 4/8

will stay in its main sequence for 5 billion years. After exhausting its hydrogen fuel, it will develop into a red giant and subsequently become a white dwarf. Based on stellar evolution models, these stages and timeframe are detailed. Question 10: (2 point) What happens to Earth at this time in the simulator? The Sun will become a red giant after exhausting its hydrogen fuel. In this phase, the Sun's outer layers expand, making it larger. The Sun's rising brilliance and size will affect Earth and the solar system. Earth will likely shift dramatically in the simulator during the red giant period. Earth's atmosphere may heat and lose heat when the Sun expands. Planetary surface temperature will rise, and oceans may evaporate. The Sun's outer layers will eventually swallow Earth. Earth's existence as we know it ends when the red giant Sun's expanding outer layers destroy it. White dwarfs, dense, cooling stellar remnants, will form from Sun leftovers. The simulator parameters determine the details of these occurrences, but this basic scenario mirrors Earth's fate in the distant future as the Sun evolves. 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 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. The circumstellar habitable zone (CHZ) period of planets in star systems with different masses was simulated to estimate the potential for sophisticated life. The NAAP – Habitable Zones 5/8 Star Mass (M  ) 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

could freeze on the dark side due to falling temperatures. The extreme circumstances on both sides might destroy ecosystems and make life difficult. Tidally locked Earth would significantly limit heat distribution and Earth's atmosphere, which maintain habitability. 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? Each star mass is established in the simulator with the initial star mass from the table, and the planet is placed in the circumstellar habitable zone (CHZ) at time zero to complete the table. At this first time, observations show if the planet is tidally locked. A planet gravitationally locks onto its star, displaying the same side. In a tidally locked planet, large temperature changes between the lighted and dark sides may impede life evolution. According to the table, the earth around the 1.0 M star is not tidally locked at the start. If tidal locking reduces life's potential, this system with a 1.0 M star may be better for life than systems with a tidally locked planet. 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. NAAP – Habitable Zones 7/8 Mass Tidally Locked? 0.3 M  0.5 M  0.8 M  1.0 M  No

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? Star system metallicity affects planet formation. A star's metallicity is its abundance of heavier components like carbon, oxygen, and metals. Heavy elements are needed to generate planets. Thus, stars with higher metallicity are more likely to have them. As you move away from the Milky Way's centre, star systems become less metallic. Because of their increased metallicity, star systems closer to the galactic centre are more likely to create planets. 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? A planet's distance from the Milky Way centre determines its extinction risk. Star density increases towards the galactic centre, increasing supernova explosions and other catastrophic events. As you move further from the galactic centre, these events decrease. Therefore, planets farther from the Milky Way's centre may have a lesser risk of catastrophic catastrophes that may wipe out life. This regional variance in cosmic catastrophe probabilities complicates galactic habitability calculations. 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. The Galactic Habitable Zone (GHZ) Goldilocks Hypothesis states that sophisticated life thrives in the Milky Way Galaxy within a medium distance from the galactic centre. The metallicity of star systems affects the possibility of planets forming. In contrast, the density of stars and accompanying events like supernovae and gamma-ray bursts affect the likelihood of cosmic catastrophes wiping out life. Closer to the galactic centre, systems with higher metallicity have more planet-forming potential but are more susceptible to cosmic disasters. Farther from the galactic centre, systems may have lower metallicity and fewer catastrophic events, which may affect planet formation. The Circumstellar Habitable Zone (CHZ) around individual stars is similar to the Goldilocks Hypothesis, which proposes that star systems at intermediate distances provide perfect conditions for sophisticated life to form. NAAP – Habitable Zones 8/8