3.3. What determines if a planet can have life?
A core learning question from the Astrobiology Learning Progressions
Astrobiology Learning Progressions Navigation
Grades K-2 or Adult Naive Learner
Do you know the story of Goldilocks and the Three Bears? Goldilocks thought the Papa Bear’s porridge was too hot and Mama Bear’s porridge was too cold and Baby Bear’s porridge was just right. That’s a great story but it’s also a good way to think of our home. Sometimes we can feel cold or hot but really most the time, it’s just right. When we think about our planet, the warmth that comes from the Sun keeps our world from being too cold. Earth is far enough from the Sun that it is not too hot for us to live. It’s just right.
Disciplinary Core Ideas
LS1.C: Organization for Matter and Energy Flow in Organisms: All animals need food in order to live and grow. They obtain their food from plants or from other animals. Plants need water and light to live and grow. (K-LS1-1)
LS4.D: Biodiversity and Humans: There are many different kinds of living things in any area, and they exist in different places on land and in water. (2-LS4-1)
PS3.B: Conservation of Energy and Energy Transfer: sunlight warms Earth’s surface. (K-PS3-1, K-PS3-2)
LS2.A: Interdependent Relationships in Ecosystems: Plants depend on water and light to grow. (2-LS2-1)
ESS3.A: Natural Resources: Living things need water, air, and resources from the land, and they live in places that have the things they need. Humans use natural resources for everything they do. (K-ESS3-1)
Crosscutting Concepts
Patterns: Patterns in the natural world can be observed, used to describe phenomena,and used as evidence. (K-ESS2-1)
Big Ideas: The Earth is just right for life – it is not too hot or too cold.
Boundaries: Temperature is limited to relative measures such as warmer/cooler. (K-PS3-1)
K-5 The Science of the Sun: The Source of Energy Lab. Understanding the relationship between Earth and the Sun is a fundamental concept in elementary level science. In this 30-minute lab, students focus on the Sun as the source for all energy on Earth. Students gain a perspective of how powerful the Sun is and the small fraction of its energy we receive. Students also gain an understanding of how Earth relates to the other planets in the solar system. NASA . Goddard Space Flight Center. https://sdo.gsfc.nasa.gov/assets/docs/UnitPlanElementary.pdf#page=49
K-8 Searching for the Sun. In this activity (two to four 45 minute lessons) about sunlight as an energy source, learners create a plant box and observe that a plant grows toward the Sun, its primary source of energy. This lesson also includes a hands-on activity about habitability connected to the book, The Day Joshua Jumped Too Much. NASA Goddard Space Flight Center.. https://sdo.gsfc.nasa.gov/assets/docs/Book1_resources.pdf#page=3
Grades 3-5 or Adult Emerging Learner
Do you know the story of Goldilocks and the Three Bears? Goldilocks thought the Papa Bear’s porridge was too hot and Mama Bear’s porridge was too cold and Baby Bear’s porridge was just right. That’s a great story, and it’s also a good way to think of our Earth. Even though there are really hot places on Earth, like the Sahara Desert, they still have living things there. Other places are very cold, like in Antarctica, but some living things survive there, too. Everywhere on Earth living things can survive. Our whole planet is really “just right” for life.
Do you think that there are other places beyond Earth that are too cold or too hot for anything to survive? The Earth is heated by the Sun and it happens to be not too close (too hot) and not too far away (too cold) from the Sun. Scientists sometimes call this the Goldilocks Zone. There are some planets that are too close and some that are too far away to get the right amount of heat for living things. It turns out that there are planets around other stars that are also in the Goldilocks Zone! If we want to try to find life somewhere besides Earth, then these places might be some of the best places to explore.
Disciplinary Core Ideas
PS3.D: Energy in Chemical Processes and Everyday Life: The energy released [from] food was once energy from the Sun that was captured by plants in the chemical process that forms plant matter (from air and water). (5-PS3-1)
LS1.C: Organization for Matter and Energy Flow in Organisms: Food provides animals with the materials they need for body repair and growth and the energy they need to maintain body warmth and for motion.
LS4.D: Biodiversity and Humans: Populations live in a variety of habitats, and change in those habitats affects the organisms living there. (3-LS4-4)
LS4.C: Adaptation: For any particular environment, some kinds of organisms survive well, some survive less well, and some cannot survive at all. (3-LS4-3)
ESS1.A: The Universe and its Stars: The Sun is a star that appears larger and brighter than other stars because it is closer. Stars range greatly in their distance from Earth. (5-ESS1-1)
ESS2.A: Earth Materials and Systems: Rainfall helps to shape the land and affects the types of living things found in a region. Water, ice, wind, living organisms, and gravity break rocks, soils, and sediments into smaller particles and move them around. (4-ESS2-1)
Crosscutting Concepts
Cause and Effect: Cause and effect relationships are routinely identified, tested, and used to explain change. (4-ESS2-1, 4-ESS3-2)
Big Ideas: The Goldilocks Zone is the area around a star where a planet can maintain the right temperature for life. The Earth is in the Sun’s Goldilocks Zone and is just right for life – it is not too hot or too cold. Examples of being “just right” could include moving their hands closer and further from a heat source such as a light bulb. Feeling a comfortable distance where the heat is just right. Living things can survive everywhere on Earth. Learning about life on Earth helps with the search for life beyond Earth.
Boundaries: Students in this grade band begin exploring renewable and nonrenewable energy resources. Examples of renewable energy resources could include wind energy, water behind dams, and sunlight; examples of non-renewable energy resources are fossil fuels and fissile materials. (4-ESS3-1)
K-5 The Science of the Sun: The Source of Energy Lab. Understanding the relationship between Earth and the Sun is a fundamental concept in elementary level science. In this 30-minute lab, students focus on the Sun as the source for all energy on Earth. Students gain a perspective of how powerful the Sun is and the small fraction of its energy we receive. Students also gain an understanding of how Earth relates to the other planets in the solar system. NASA . Goddard Space Flight Center. https://sdo.gsfc.nasa.gov/assets/docs/UnitPlanElementary.pdf#page=49
K-8 Searching for the Sun. In this activity (two to four 45 minute lessons) about sunlight as an energy source, learners create a plant box and observe that a plant grows toward the Sun, its primary source of energy. This lesson also includes a hands-on activity about habitability connected to the book, The Day Joshua Jumped Too Much. NASA Goddard Space Flight Center.. https://sdo.gsfc.nasa.gov/assets/docs/Book1_resources.pdf#page=3
5-9 Project Spectra: Planet Designer: Martian Makeover. This is an activity (two 50-minute lessons) about the atmospheric conditions (greenhouse strength, atmospheric thickness) Mars needs to maintain surface water. Learners use a computer interactive to learn about Mars past and present before exploring the pressure and greenhouse strength needed for Mars to have a watery surface as it had in the past. This lesson is part of Project Spectra, a science and engineering education program focusing on how light is used to explore the Solar System. University of Colorado, Boulder/NASA. http://lasp.colorado.edu/home/wp-content/uploads/2013/06/martian_makeover_teacher_20130617.pdf
Grades 6-8 or Adult Building Learner
The Sun is really important for life as we know it, since the Sun is the source of nearly all Earth’s warmth. At our distance from the Sun, it’s not so cold that the oceans freeze solid and it’s not so hot that the oceans evaporate into the atmosphere. It’s just the right temperature to have liquid water on the surface of our planet. This is such an important thing to have happen that we gave it a name. We call the area around a star where a planet can be at just the right temperature for liquid water to exist the Goldilocks Zone. This comes from the old story of Goldilocks and the Three Bears, where the main character finds that something can be too hot, too cold, or just right. Since having a planet that’s just right for liquid water is important for living things, one important place for us to look for possible alien life is on planets that are also in the Goldilocks Zone around their stars.
There are probably several hundred billion planets in our galaxy. As we keep finding more planets around other stars, a lot of astrobiologists are really interested in looking at those planets that are in the Goldilocks Zone around their stars. Also, since stars get hotter as they get older, the Goldilocks Zone around a star can actually move out over time. So, it’s also important to look at the planets that stay in the Goldilocks Zone as their stars get older. This area is called the Continuous Goldilocks Zone. Our planet Earth is in this zone around our star!
Are all stars the same as the Sun? No. Some stars are smaller, dimmer, and redder while others are larger, brighter, and white or blue. This tells us that there is a different size for the Goldilocks Zone for each type of star depending on its brightness. Larger stars have wider Goldilocks Zones, which may include more planets. However, large stars burn their fuel faster and do not exist as main sequence stars for a really long time and there aren’t a lot of them in the universe. Stars that are smaller than the Sun last a very long time and there are a lot of them, but many have smaller Goldilocks Zones with less planets or even no planets in them.
Stars that are similar to our Sun, kind of average in size, may be good planetary system candidates because their Goldilocks Zones can be big enough to have at least a few planets and they exist much longer than the really big blue and white stars. The only example of life we are aware of is around this kind of star. Categorizing stars and planets by their potential for liquid water allows researchers to more efficiently search for life. With so many planets out there to search, narrowing it down is helpful.
It also turns out that the distance from a star isn’t the only thing that matters when it comes to how hot a planet will be. The atmosphere of a planet also affects its surface temperature. On Earth, greenhouse gases like water vapor, carbon dioxide, and methane keep warmth at the surface, much like a blanket. Earth is much warmer than it would be without these greenhouse gases. But too much of an atmosphere can make a planet too hot. Venus isn’t the closest planet to the Sun (that’s Mercury), but Venus has the hottest surface because it has a really thick atmosphere.
Considering if planets have atmospheres and how close to their stars they are helps us to narrow the search for life beyond Earth. However, there are worlds in our solar system that are not in the Goldilocks Zone and yet may have had life in the past or may even have life on them right now. These are places like Mars, Titan, Europa, and Enceladus. As astrobiologists search for life out there they consider all of the possible places where life is most likely to survive and flourish.
Disciplinary Core Ideas
PS3.A: Definitions of Energy: Motion energy is properly called kinetic energy; it is proportional to the mass of the moving object and grows with the square of its speed. (MS-PS3-1) ▪A system of objects may also contain stored (potential) energy, depending on their relative positions. (MS-PS3-2) Temperature is a measure of the average kinetic energy of particles of matter. The relationship between the temperature and the total energy of a system depends on the types, states, and amounts of matter present. (MS-PS3-3, MS-PS3-4)
PS3.D: Energy in Chemical Processes and Everyday Life: The chemical reaction by which plants produce complex food molecules (sugars) requires an energy input (i.e., from sunlight) to occur. In this reaction, carbon dioxide and water combine to form carbon-based organic molecules and release oxygen. (MS-LS1-6)
LS2.C: Ecosystem Dynamics, Functioning, and Resilience: Biodiversity describes the variety of species found in Earth’s terrestrial and oceanic ecosystems. The completeness or integrity of an ecosystem’s biodiversity is often used as a measure of its health. (MS-LS2-5)
ESS1.A: The Universe and Its Stars: Patterns of the apparent motion of the Sun, the Moon, and stars in the sky can be observed, described, predicted, and explained with models. (MS-ESS1-1) ▪ Earth and its solar system are part of the Milky Way galaxy, which is one of many galaxies in the universe. (MS-ESS1-2)
ESS1.B: Earth and the Solar System: The solar system consists of the Sun and a collection of objects, including planets, their Moons, and asteroids that are held in orbit around the Sun by its gravitational pull on them. (MS-ESS1-2, MS-ESS1-3)
ESS2.A: Earth’s Materials and Systems: All Earth processes are the result of energy flowing and matter cycling within and among the planet’s systems. This energy is derived from the Sun and Earth’s hot interior. The energy that flows and matter that cycles produce chemical and physical changes in Earth’s materials and living organisms. (MS-ESS2-1)
ESS2.C: The Roles of Water in Earth’s Surface Processes: Water continually cycles among land, ocean, and atmosphere via transpiration, evaporation, condensation and crystallization, and precipitation, as well as downhill flows on land. (MS-ESS2-4) ▪ Global movements of water and its changes in form are propelled by sunlight and gravity. (MS-ESS2-4)
ESS3.A: Natural Resources: Humans depend on Earth’s land, ocean, atmosphere, and biosphere for many different resources. Minerals, fresh water, and biosphere resources are limited, and many are not renewable or replaceable over human lifetimes. These resources are distributed unevenly around the planet as a result of past geologic processes. (MS-ESS3-1)
ESS2.D: Weather and Climate: Weather and climate are influenced by interactions involving sunlight, the ocean, the atmosphere, ice, landforms, and living things. These interactions vary with latitude, altitude, and local and regional geography, all of which can affect oceanic and atmospheric flow patterns. (MS-ESS2-6) *The ocean exerts a major influence on weather and climate by absorbing energy from the Sun, releasing it over time, and globally redistributing it through ocean currents. (MS-ESS2-6)
Crosscutting Concepts
Cause and Effect: Cause and effect relationships may be used to predict phenomena in natural or designed systems. (MS-ESS2-5) Systems and System Models ▪ Models can be used to represent systems and their interactions — such as inputs, processes and outputs — and energy, matter, and information flows within systems. (MS-ESS2-6)
Big Ideas: The Goldilocks Zone is the area around a star where a planet can maintain the temperature necessary for liquid water to exist. Because Earth is in the Goldilocks Zone of the Sun, it is the right temperature to have the liquid water necessary for life. While there are billions of planets in the galaxy, planets that are in the Goldilocks Zone around their stars are of particular interest in the search for life beyond Earth. A planet’s atmosphere also helps maintain surface temperature and is critical for life.
Boundaries: Students in this grade band develop models to show gravity as the force that holds together the solar system and Milky Way galaxy and controls orbital motions within them. Examples of models can be physical (such as the analogy of distance along a football field or computer visualizations of elliptical orbits) or conceptual (such as mathematical proportions relative to the size of familiar objects such as students’ school or state). MS-ESS1-2
K-8 Searching for the Sun. In this activity (two to four 45 minute lessons) about sunlight as an energy source, learners create a plant box and observe that a plant grows toward the Sun, its primary source of energy. This lesson also includes a hands-on activity about habitability connected to the book, The Day Joshua Jumped Too Much. NASA Goddard Space Flight Center.. https://sdo.gsfc.nasa.gov/assets/docs/Book1_resources.pdf#page=3
5-9 Project Spectra: Planet Designer: Martian Makeover. This is an activity (two 50-minute lessons) about the atmospheric conditions (greenhouse strength, atmospheric thickness) Mars needs to maintain surface water. Learners use a computer interactive to learn about Mars past and present before exploring the pressure and greenhouse strength needed for Mars to have a watery surface as it had in the past. This lesson is part of Project Spectra, a science and engineering education program focusing on how light is used to explore the Solar System. University of Colorado, Boulder/NASA. http://lasp.colorado.edu/home/wp-content/uploads/2013/06/martian_makeover_teacher_20130617.pdf
6-8 SpaceMath Problem 545: Measuring Atmospheric Trace Gases Using Parts Per Million. Students convert from percentage units to parts per million and compare trace gases in the atmospheres of various planets. [Topics: percentages; unit conversions] https://spacemath.gsfc.nasa.gov/Grade67/10Page8.pdf
6-8 SpaceMath Problem 544: The Composition of Planetary Atmospheres. Students study the composition of planetary atmospheres and compare the amounts of certain compounds in them [Topics: pie graphs; percentages; scientific notation] https://spacemath.gsfc.nasa.gov/Grade67/10Page7.pdf
6-8 SpaceMath Problem 335: Methane Lakes on Titan. Students use a recent Cassini radar image of the surface of Titan to estimate how much methane is present in the lakes that fill the image, and compare the volume to that of the freshwater lake, Lake Tahoe. [Topics: estimating irregular areas; calculating volume from area x height; scaled images] https://spacemath.gsfc.nasa.gov/Grade67/6Page148.pdf
6-8 SpaceMath Problem 403: The Goldilocks Planets – Not too hot or cold. Students use a table of the planets discovered by the Kepler satellite, and estimate the number of planets in our Milky Way galaxy that are about the same size as Earth and located in their Habitable Zones. They estimate the average temperature of the planets, and study their tabulated properties using histograms. [Topics: averaging; histogramming] https://spacemath.gsfc.nasa.gov/astrob/7Page66.pdf
6-8 or 9-12 Mars Image Analysis. In this one-three hour lesson, students analyze and interpret the accompanying large-format images of Mars taken by NASA’s Mars Thermal Emission Imaging System ( THEMIS ) camera. The analysis involves identifying geologic features, calibrating the size of those features, and determining surface history. The lesson culminates in students conducting in-depth research on questions generated during their analyses. Can be used independently or part of the Mars Science Imaging Project through Arizona State University. NASA /Arizona State University. http://marsed.asu.edu/mars-image-analysis
6-9 Planet Hunters Education Guide. Lesson 3: Finding the habitable zone (page 41). This activity explores four types of stars and their characteristics, such as color, temperature, size, and lifespan. These characteristics are then used to determine the conditions for planets around each of them. Next, students compare and contrast their results to develop ideas about where it is reasonable to expect that life could be found outside our own solar system. This lesson is part of a nine lesson unit that takes learners through engaging activities that feature habitability, identifying and characterizing exoplanets, and citizen science. NASA . https://s3.amazonaws.com/zooniverse-resources/zoo-teach/production/uploads/resource/attachment/122/Planet_Hunters_Educator_Guide.pdf
6-9 Rising Stargirls Teaching and Activity Handbook: A public service announcement ( PSA ) for Life (page 57). Students work cooperatively in teams to solidify the concept of what life needs to survive. Each team pursues an in-depth study of a particular planetary environment and its prospects for life, then presents this information as a PSA to the larger class. Rising Stargirls is a 10-day workshop dedicated to encouraging girls of all backgrounds to learn, explore, and discover the universe through interactive astronomy using theater, writing, and visual art. This provides an avenue for individual self-expression and personal exploration that is interwoven with scientific engagement and discovery. Rising Stargirls. https://static1.squarespace.com/static/54d01d6be4b07f8719d7f29e/t/5748c58ec2ea517f705c7cc6/1464386959806/Rising_Stargirls_Teaching_Handbook.compressed.pdf
6-12 Astrobiology Math. This collection of math problems provides an authentic glimpse of modern astrobiology science and engineering issues, often involving actual research data. Students explore concepts in astrobiology through calculations. Relevant topics include Habitability Zones and Stellar Luminosity (page 57) and The Greenhouse Effect and Planetary Temperature (page 41). NASA . https://www.nasa.gov/pdf/637832main_Astrobiology_Math.pdf
6-12 Extreme Planet Makeover. This online interactive allows students to change the settings of a planet’s size, distance to star, age, and type of star it orbits in order to understand the habitability zone. The habitability zone is a very important concept in astrobiology and is tied to CLQ1.2 in terms of the creation of Earth as a habitable environment. NASA . https://exoplanets.nasa.gov/interactable/1/index.html
6-12 (3-5 adaptable) Project Spectra! – Goldilocks and the Three Planets. In this lesson (two class periods), students determine what some of Earth, Venus, and Mars’ atmosphere is composed of and then mathematically compare the amount of greenhouse gas and CO2 on the planets of Venus, Earth, and Mars, in order to determine which has the most. Students brainstorm to figure out what things, along with greenhouse gases, can affect a planet’s temperature which can determine its habitability. University of Colorado, Boulder/NASA. http://lasp.colorado.edu/home/wp-content/uploads/2011/08/Goldilocks.pdf
6-12 Ocean Worlds. In this web interactive, students learn about water on Earth, in the cosmos and on other planetary bodies. It tells the story of water from its creation and its delivery to the Earth, as well as up-to-date information about water on planetary bodies within the Solar System such as Mars, Europa and others and far away in a variety of places such as planet formation nebulae and exoplanets. The learner comes away with a true sense of how common water is in the universe. NASA . https://www.nasa.gov/specials/ocean-worlds/
7-8 Life Underground. This game is an interactive outreach experience for 7th and 8th grade classrooms. Life Underground is presented in a video game experience that is highly motivating for students. The goal is for students to visualize microscopic life at a range of terrestrial and extraterrestrial subsurface conditions. Students take the role of a young scientist investigating extreme subsurface environments for microbial life. They navigate through extreme conditions, including those of temperature, pressure, acidity, and energy limitations, and they begin to recognize what characterizes life in this context. NASA Astrobiology Institute. https://gameinnovationlab.itch.io/life-underground
8-10 SpaceMath Problem 124: The Moon’s Atmosphere! Students learn about the moon’s very thin atmosphere by calculating its total mass in kilograms using the volume of a spherical shell and the measured density. [Topics: volume of sphere, shell; density-mass-volume; unit conversions] https://spacemath.gsfc.nasa.gov/moon/4Page26.pdf
8-10 SpaceMath Problem 292: How Hot is That Planet? Students use a simple function to estimate the temperature of a recently discovered planet called CoRot-7b. [Topics: algebra II; evaluating power functions] https://spacemath.gsfc.nasa.gov/astrob/6Page61.pdf
8-10 SpaceMath Problem 264: Water on Planetary Surfaces. Students work with watts and Joules to study melting ice. [Topics: unit conversion, rates] https://spacemath.gsfc.nasa.gov/astrob/Astro3.pdf
8-10 SpaceMath Problem 263: Ice or Water? Whether a planetary surface contains ice or liquid water depends on how much heat is available. Students explore the concepts of specific heat and latent heat of fusion to better understand and quantify the energy required for liquid water to exist under various conditions. [Topics: unit conversion, scientific notation] https://spacemath.gsfc.nasa.gov/astrob/Astro1.pdf
Grades 9-12 or Adult Sophisticated Learner
We use the word habitable to define a planet or an environment on a planet where we think life might be able to thrive. For instance, our planet is habitable since we know that we have a biosphere of living things at the surface. But what other kinds of planets or places on planets might be habitable? An important first step in answering that question is to think about liquid water. All of life as we know it needs liquid water to survive. So, one important characteristic that might make a planet habitable is if it has liquid water at its surface like we do here on Earth. The Earth is 93 million miles from the Sun. At this distance, it’s not so cold that the oceans freeze solid and it’s not so hot that the oceans evaporate into the atmosphere. It’s just the right temperature to have liquid water on the surface of our planet. We call this region around our Sun the Goldilocks Zone, because the conditions are “just right” for liquid water at the surface of our world.
There are probably several hundred billion planets in our galaxy. As we keep finding more planets around other stars, a lot of astrobiologists are really interested in looking at those planets that are in the Goldilocks Zone around their stars. Also, since stars get hotter as they get older, the Goldilocks Zone around a star can actually move out over time. So, it’s also important to look at the planets that stay in the Goldilocks Zone as their stars get older. This area is called the Continuous Goldilocks Zone. Our planet Earth is in this zone around our star!
Are all stars the same as the Sun? No. Some stars are smaller, dimmer, and redder while others are larger, brighter, and white or blue. This tells us that there is a different size for the Goldilocks Zone for each type of star depending on its brightness. Larger stars have wider Goldilocks Zones, which may include more planets. However, large stars burn their fuel faster and do not exist as main sequence stars for a really long time and there also aren’t a lot of them in the universe. Stars that are smaller than the Sun last a very long time and there are a lot of them, but many have smaller Goldilocks Zones with fewer planets or even no planets in them. Some of our research tells us that these smaller stars may have more solar flares that could be harmful to life.
Stars that are similar to our Sun, kind of average in size, may be good planetary system candidates because their Goldilocks Zones can be big enough to have at least a few planets and they exist much longer than the really big blue and white stars. The only example of life we are aware of is around this kind of star. Categorizing stars and planets by their potential for liquid water allows researchers to more efficiently search for life. There are just so many planets out there to search that narrowing it down is helpful. It also turns out that the distance from a star isn’t the only thing that matters when it comes to how hot a planet will be. The atmosphere of a planet also effects its surface temperature. On Earth, we have greenhouse gases like water vapor, carbon dioxide, and methane. These greenhouse gases allow radiation from the Sun to enter the atmosphere and warm the surface of our planet, but then they stop the heat that is released from the surface from leaving. This keeps the surface warmer; the atmosphere works like a greenhouse or a blanket for the planet. Earth is much warmer than it would be without our greenhouse gases. But too much of an atmosphere or too much of greenhouse gases can make a planet become too hot. For instance, Venus isn’t the closest planet to the Sun (that’s Mercury), but Venus has the hottest surface because it has a really thick atmosphere with a lot of greenhouse gas. This keeps the surface of Venus around 850°F.
There are also other things to consider in the search for potentially habitable planetary systems, such as type of planetary orbit (nearly circular vs very elliptical), multiple star systems, tidal locking, and the effects of moons on a planet’s tilt and rotation. It is not thorough enough to simply say that if a planet is a certain distance from its central star (i.e., if it’s in the Goldilocks Zone), then it is habitable. What if the orbit is highly elliptical (not very circular)? It may only have an average distance that is in the Goldilocks Zone but then spends most of its time beyond the inner and/or outer range. Our solar system has planets with low eccentricity (i.e., they’re really circular) but that is not the case for all planetary systems. Our solar system also only has a single star, but it turns out that this isn’t really common. Most stars are in binary or multiple star systems. The Goldilocks Zones for liquid water for these systems are very complex. Some planets are also tidally locked to their stars. This means that the same side of the planet is always facing the star (our moon is almost tidally locked, which is why you only ever see the near side of the Moon). Could there be life on a planet that is tidally locked? We really don’t know. On Earth a lot of life uses the night and day but is this true of other planets with life? We also have to think about the importance of moons for making a planet habitable. Could having a moon make a planet more likely to have life? Computer modeling shows that having a large moon could be beneficial for a planet to have life because the planet doesn’t wobble as much. A planet whose axial tilt changes a lot likely also faces extreme climate change. The Earth goes through ice ages due to changes in its orbit eccentricity, axial tilt, and axial direction. However, life has always survived these changes. A planet without a large moon will have extreme axial tilt changes that could include a complete covering of ice or varying ice bands on the planet, both of which could be too harsh for life to survive.
Considering all of these factors helps us to narrow down possible worlds that might be habitable for life as we know it. Whether or not they’re in the Goldilocks Zone or are tidally locked, whether or not they have thin or thick atmospheres, and if they have moons and the shapes of their orbits are all important factors. However, there are also other worlds in our solar system that don’t meet some of these criteria for potentially habitable worlds and yet they may have had life a long time ago or may even have life on them right now. These are places like Mars, Titan, Europa, and Enceladus. Mars is at the outer edge of our Goldilocks Zone, it has a really thin atmosphere and is very cold, and it only has two very small moons, and yet we know that Mars once had potentially habitable environments. Likewise, Titan, a large moon of Saturn, has incredible complex organic molecules going through many processes. Could there be something alive in the organics of Titan that isn’t quite like life as we know it? Also, there are moons in our solar system like Europa and Enceladus that have oceans of liquid water below their icy crusts. Could there be living things in the oceans of Europa or Enceladus? What might be required for those environments to be habitable? As astrobiologists search for life out there they need to consider all of the possible places where life is most likely to survive and flourish.
Disciplinary Core Ideas
PS3.A: Definitions of Energy: Energy is a quantitative property of a system that depends on the motion and interactions of matter and radiation within that system. That there is a single quantity called energy is due to the fact that a system’s total energy is conserved, even as, within the system, energy is continually transferred from one object to another and between its various possible forms. (HS-PS3-1, HS-PS3-2) *At the macroscopic scale, energy manifests itself in multiple ways, such as in motion, sound, light, and thermal energy. (HS-PS3-2, HS-PS3-3)
LS1.C: Organization for Matter and Energy Flow in Organisms: The process of photosynthesis converts light energy to stored chemical energy by converting carbon dioxide plus water into sugars plus released oxygen. (HS-LS1-5) *The sugar molecules thus formed contain carbon, hydrogen, and oxygen: their hydrocarbon backbones are used to make amino acids and other carbon-based molecules that can be assembled into larger molecules (such as proteins or DNA ), used for example to form new cells. (HS-LS1-6)
PS3.D: Energy in Chemical Processes: The main way that solar energy is captured and stored on Earth is through the complex chemical process known as photosynthesis.
ESS1.A: The Universe and Its Stars: The star called the Sun is changing and will burn out over a lifespan of approximately 10 billion years. (HS-ESS1-1) *The study of stars’ light spectra and brightness is used to identify compositional elements of stars, their movements, and their distances from Earth. (HS-ESS1-2, HS-ESS1-3)
ESS1.B: Earth and the Solar System: Kepler’s laws describe common features of the motions of orbiting objects, including their elliptical paths around the Sun. Orbits may change due to the gravitational effects from, or collisions with, other objects in the solar system. (HS-ESS1-4)
ESS2.C: The Roles of Water in Earth’s Surface Processes: The abundance of liquid water on Earth’s surface and its unique combination of physical and chemical properties are central to the planet’s dynamics. These properties include water’s exceptional capacity to absorb, store, and release large amounts of energy, transmit sunlight, expand upon freezing, dissolve and transport materials, and lower the viscosities and melting points of rocks. (HS-ESS2-5)
ESS2.D: Weather and Climate: The foundation for Earth’s global climate systems is the electromagnetic radiation from the Sun, as well as its reflection, absorption, storage, and redistribution among the atmosphere, ocean, and land systems, and this energy’s re-radiation into space. (HS-ESS2-2)
Crosscutting Concepts
Scale, Proportion, and Quantity: The significance of a phenomenon is dependent on the scale, proportion, and quantity at which it occurs. (HS-ESS1-1) Stability and Change-Much of science deals with constructing explanations of how things change and how they remain stable. (HS-ESS1-6)
Big Idea: The Goldilocks Zone is the area around a star where a planet can maintain the temperature necessary for liquid water to exist. The Earth is 93 million miles from the Sun and within a habitable zone that supports life. Because Earth is in the Goldilocks Zone of the Sun, it is the right temperature to have the liquid water necessary for life. While there are billions of planets in the galaxy, planets that are in the Goldilocks Zone around their stars are of particular interest in the search for life beyond Earth because of their potential for liquid water. Planetary systems are categorized by whether or not they are in the Goldilocks Zone or are tidally locked, whether they have thin or thick atmospheres, if they have moons and the shapes of their orbits. There are other worlds in the solar system, like Mars, that do not meet some of the criteria for potentially habitable worlds and yet they show promising signs of habitability. These are places like Mars, Titan, Europa, and Enceladus.
Boundaries: Students in this grade band use basic algebraic expressions or computations to calculate the change in energy in a system. (HS-PS3-1) Students use mathematical representations for the gravitational attraction of bodies and Kepler’s Laws of orbital motions but do not involve calculus.
5-9 Project Spectra: Planet Designer: Martian Makeover. This is an activity (two 50-minute lessons) about the atmospheric conditions (greenhouse strength, atmospheric thickness) Mars needs to maintain surface water. Learners use a computer interactive to learn about Mars past and present before exploring the pressure and greenhouse strength needed for Mars to have a watery surface as it had in the past. This lesson is part of Project Spectra, a science and engineering education program focusing on how light is used to explore the Solar System. University of Colorado, Boulder/NASA. http://lasp.colorado.edu/home/wp-content/uploads/2013/06/martian_makeover_teacher_20130617.pdf
6-8 or 9-12 Mars Image Analysis. In this one-three hour lesson, students analyze and interpret the accompanying large-format images of Mars taken by NASA’s Mars Thermal Emission Imaging System ( THEMIS ) camera. The analysis involves identifying geologic features, calibrating the size of those features, and determining surface history. The lesson culminates in students conducting in-depth research on questions generated during their analyses. Can be used independently or part of the Mars Science Imaging Project through Arizona State University. NASA /Arizona State University. http://marsed.asu.edu/mars-image-analysis
6-9 Planet Hunters Education Guide. Lesson 3: Finding the habitable zone (page 41). This activity explores four types of stars and their characteristics, such as color, temperature, size, and lifespan. These characteristics are then used to determine the conditions for planets around each of them. Next, students compare and contrast their results to develop ideas about where it is reasonable to expect that life could be found outside our own solar system. This lesson is part of a nine lesson unit that takes learners through engaging activities that feature habitability, identifying and characterizing exoplanets, and citizen science. NASA . https://s3.amazonaws.com/zooniverse-resources/zoo-teach/production/uploads/resource/attachment/122/Planet_Hunters_Educator_Guide.pdf
6-9 Rising Stargirls Teaching and Activity Handbook: A public service announcement ( PSA ) for Life (page 57). Students work cooperatively in teams to solidify the concept of what life needs to survive. Each team pursues an in-depth study of a particular planetary environment and its prospects for life, then presents this information as a PSA to the larger class. Rising Stargirls is a 10-day workshop dedicated to encouraging girls of all backgrounds to learn, explore, and discover the universe through interactive astronomy using theater, writing, and visual art. This provides an avenue for individual self-expression and personal exploration that is interwoven with scientific engagement and discovery. Rising Stargirls. https://static1.squarespace.com/static/54d01d6be4b07f8719d7f29e/t/5748c58ec2ea517f705c7cc6/1464386959806/Rising_Stargirls_Teaching_Handbook.compressed.pdf
6-12 Astrobiology Math. This collection of math problems provides an authentic glimpse of modern astrobiology science and engineering issues, often involving actual research data. Students explore concepts in astrobiology through calculations. Relevant topics include Habitability Zones and Stellar Luminosity (page 57) and The Greenhouse Effect and Planetary Temperature (page 41). NASA . https://www.nasa.gov/pdf/637832main_Astrobiology_Math.pdf
6-12 Extreme Planet Makeover. This online interactive allows students to change the settings of a planet’s size, distance to star, age, and type of star it orbits in order to understand the habitability zone. The habitability zone is a very important concept in astrobiology and is tied to CLQ1.2 in terms of the creation of Earth as a habitable environment. NASA . https://exoplanets.nasa.gov/interactable/1/index.html
6-12 (3-5 adaptable) Project Spectra! – Goldilocks and the Three Planets. In this lesson (two class periods), students determine what some of Earth, Venus, and Mars’ atmosphere is composed of and then mathematically compare the amount of greenhouse gas and CO2 on the planets of Venus, Earth, and Mars, in order to determine which has the most. Students brainstorm to figure out what things, along with greenhouse gases, can affect a planet’s temperature which can determine its habitability. University of Colorado, Boulder/NASA. http://lasp.colorado.edu/home/wp-content/uploads/2011/08/Goldilocks.pdf
6-12 Ocean Worlds. In this web interactive, students learn about water on Earth, in the cosmos and on other planetary bodies. It tells the story of water from its creation and its delivery to the Earth, as well as up-to-date information about water on planetary bodies within the Solar System such as Mars, Europa and others and far away in a variety of places such as planet formation nebulae and exoplanets. The learner comes away with a true sense of how common water is in the universe. NASA . https://www.nasa.gov/specials/ocean-worlds/
8-10 SpaceMath Problem 124: The Moon’s Atmosphere! Students learn about the moon’s very thin atmosphere by calculating its total mass in kilograms using the volume of a spherical shell and the measured density. [Topics: volume of sphere, shell; density-mass-volume; unit conversions] https://spacemath.gsfc.nasa.gov/moon/4Page26.pdf
8-10 SpaceMath Problem 292: How Hot is That Planet? Students use a simple function to estimate the temperature of a recently discovered planet called CoRot-7b. [Topics: algebra II; evaluating power functions] https://spacemath.gsfc.nasa.gov/astrob/6Page61.pdf
8-10 SpaceMath Problem 264: Water on Planetary Surfaces. Students work with watts and Joules to study melting ice. [Topics: unit conversion, rates] https://spacemath.gsfc.nasa.gov/astrob/Astro3.pdf
8-10 SpaceMath Problem 263: Ice or Water? Whether a planetary surface contains ice or liquid water depends on how much heat is available. Students explore the concepts of specific heat and latent heat of fusion to better understand and quantify the energy required for liquid water to exist under various conditions. [Topics: unit conversion, scientific notation] https://spacemath.gsfc.nasa.gov/astrob/Astro1.pdf
9-11 SpaceMath Problem 181: Extracting Oxygen from Moon Rocks. Students use a chemical equation to estimate how much oxygen can be liberated from a sample of lunar soil. [Topics: ratios; scientific notation; unit conversions] https://spacemath.gsfc.nasa.gov/moon/5Page28.pdf
9-12 SpaceMath Problem 287: LCROSS Sees Water on the Moon. Students use information about the plume created by the LCROSS impactor to estimate the (lower-limit) concentration of water in the lunar regolith in a shadowed crater. [Topics: geometry; volumes; mass=density x volume] https://spacemath.gsfc.nasa.gov/moon/6Page66.pdf
9-12 SpaceMath Problem 352: Exponential Functions and Atmospheric ‘Scale heights’. A study of the way a planet’s atmosphere changes as its temperature is changed using exponential functions. [Topics: scientific notation; evaluating exponential functions; optional calculus] https://spacemath.gsfc.nasa.gov/astrob/7Page15.pdf
9-12 SpaceMath Problem 349: Exoplanet Orbits and the Properties of Ellipses. Given the formula for the orbits of newly-discovered planets, students determine the basic properties of the elliptical orbits for the planets. [Topics: properties of ellipses] https://spacemath.gsfc.nasa.gov/astrob/7Page13.pdf
9-10 Voyages through Time: Planetary Evolution. This comprehensive integrated curriculum helps students address the question, “What makes it possible for Earth to have life?” The Earth is literally filled with life and yet our neighbors; Mars, Venus and the Moon have developed so differently. Through the planetary evolution module students address that main question through a series of activities. There is a sample lesson on the website and the curriculum is available for purchase. SETI . http://voyagesthroughtime.org/planetary/index.html
Storyline Extensions
A tale of three planets:
Venus and Mars are both on the edges of the Goldilocks Zone in our solar system, but why don’t they show liquid surface waters and large apparent biospheres like we have here on Earth?
Venus is has the hottest planetary surface in our solar system. It’s about 850°F there and the pressure is about 92 times more than what we have at sea level here on Earth. That makes Venus a very different place. Venus may have once had oceans (and maybe even a biosphere!), but it appears that the entire surface of Venus heated up high enough that all of the rocks melted and turned into lava at some point long ago. On top of that, Venus has what we call a “runaway greenhouse”, where the buildup of greenhouse gases (especially CO2) in the Venusian atmosphere made it get hotter, which caused more greenhouse gases to build up, which made it warmer, and so on. Venus is a very interesting place!
We think early Mars likely had lots of water, in rivers and lakes and maybe even in an ocean. That’s because early Mars likely had a much thicker atmosphere. But, these days, the surface of Mars is very cold, very dry, and the pressure is very low. Without a thick enough atmosphere, Mars cannot sustain liquid water at its surface, even though it’s within the Goldilocks Zone.
Even though Venus and Mars are on the edges of the Goldilocks Zone, they don’t have abundant biospheres that we can see on their surfaces. This tells us that being within the Goldilocks Zone alone probably isn’t enough to guarantee that a world will have liquid water or life. However, it’s still an important place for us to look around other stars when trying to find Earth-like worlds in our galaxy.