The Search for Life: Are We Alone? || Teacher's Guide

The investigation of life on other worlds embraces concepts drawn from the Physical, Earth and Planetary, and Life sciences. What is life? What are the requirements for life? How do living things survive? What kinds of environments—on Earth or elsewhere—are suitable for life? Where in the universe should we look for life? The Search for Life: Are We Alone? invites viewers to ask these questions and begin to formulate answers. Presented below is background on some significant topics from the Space Show.

Physical Science
Detection of Exoplanets

Until 1995, scientists had speculated about the existence of exoplanets, but had not discovered any. Now astronomers are finding them at the rate of one per month, which suggests that exoplanets are quite common.

Exoplanets are too small and faint to be observed directly, so astronomers look instead for their effect on the motion of the stars they orbit. All objects with mass exert a gravitational force on each other, so not only does a star keep a planet in orbit, but a planet also pulls on its parent star. When we detect a planet orbiting a star, the exoplanet sometimes pulls the star slightly toward us, sometimes slightly away from us. Astronomers can detect this "wobble" by studying the light from the star. Thus far, the exoplanets discovered are giants (at least the size of Jupiter). Better technology may enable us to detect planets and moons of sizes approximating Earth or Europa.

Earth's Radio Emissions

The expanding sphere of our civilization's radio waves has already reached many of the exoplanets we have discovered. If intelligent life exists on these worlds, our signals could be detected there.

All electromagnetic radiation, which includes light, radio, television, and radar waves, possesses certain characteristics. It travels at the speed of light, 300,000 kilometers or 186,000 miles per second, along a linear path. A radiation source emitting in all directions creates a "bubble" of radiation spreading outward at the speed of light. Several decades of radio emissions from Earth have created just such a bubble—expanding into space and washing over many exoplanets.

Distances to other stars, and their planets, are tremendous. To make the numbers easier to manage, astronomers use light years. A light year is the distance light travels in one year, about 9.5 trillion kilometers (5.9 trillion miles). The distance to the brightest star, Sirius, is nine light years. In other words, the light from Sirius reaching us today left that star nine years ago.

Earth and Planetary Science
Solar System and Earth Formation

Dense regions of interstellar clouds can collapse, becoming denser and hotter and possibly forming stars and planetary systems.

Earth and the rest of the solar system began as a rotating cloud of hydrogen and helium, with trace amounts of heavier elements, dust, and ice grains. Gravity collapsed the cloud inward. As the cloud shrank, it began to spin faster and flattened into a disk with a central bulge. Planets would eventually form in the disk. Collisions in the bulge raised its temperature. When the central temperature became high enough to fuse the hydrogen in the bulge, the Sun ignited. Meanwhile, temperatures in the disk away from the Sun were cool enough to allow heavier elements and compounds such as iron and silicates to condense into solids. These particles in the disk began to accrete, forming small planet-like bodies called planetesimals, which then merged together to form larger bodies, culminating in Earth and the other bodies of the solar system. Our current Earth is the result of 4.6 billion years of planetary evolution from the time of its formation.

Comparative Planetology

Mars was probably more like our own watery blue planet in the past, but now its surface is a barren desert.

Both Earth and Mars orbit at a distance from the Sun at which water can exist as a liquid. However, the more massive Earth is better able to retain its atmosphere than the less massive Mars. Earth's greater gravitational pull prevents heavier molecules such as nitrogen and oxygen from escaping its atmosphere. Thus, Earth's atmosphere is relatively dense, making its temperature and pressure high enough to support liquid water. The present Martian atmosphere is less dense, and thus its atmospheric pressure and atmosphere are too low for liquid water to exist stably on the surface. Data returned from the Mars Odyssey spacecraft indicates that huge amounts of water ice lie within the upper regions of soil around the Martian South Pole. This finding supports the possibility of life existing on Mars—not on its surface, but beneath it.

Life Science
Life Fills Every Niche

Earth's ocean floor seems to be as alien as another planet, yet life exists there.

Basically, something is alive if it carries on life processes with the aim of sustaining itself and its species: A living thing obtains energy, responds to its environment, grows, and reproduces. The information on how to carry on life processes is encoded in DNA (deoxyribonucleic acid) found in fundamental units called cells. Scientists know that life can thrive even in the most inhospitable environments, such as around "black smokers"—sulfide chimneys formed near hydrothermal vents on the sea floor. (Hydrothermal vents were discovered in 1977. In 1998, the Museum mounted an expedition to collect sulfide chimneys—now on display in the Hall of Planet Earth.) Certain types of microbes, which form the food-chain base, convert energy from the hot, mineral-rich hydrothermal fluids to food as it mixes with seawater. Microbes that live below Earth's surface obtain their energy from Earth's interior, rather than from the Sun. These microbes may be the most common life form on Earth. Microscopic organisms arose on Earth much earlier than plants and animals and thrive in places where plants and animals cannot survive. Hence, it is reasonable to hypothesize that microbial life could exist in similar, seemingly hostile environments beyond Earth.

Does All Life Look Alike?

Could there be living forms on other worlds that don't need or can't stand water, but instead need something else?

We have seen that life is resilient. But what forms might life on other worlds take? Although extraterrestrial life in movies and on television often resembles humans, this depiction is limiting. Life may not need to be based on DNA, for example. Or perhaps not even carbonbased, but silicon-based. Some scientists have theorized that liquids other than water, such as ammonia, methane, and ethane, could possibly sustain life on other planets. These life forms, however, would be different chemically from those on Earth. Thus, the criteria used to search for life on other worlds should reflect the understanding that our conception of life is based on a single example—the life that arose here on Earth.

	SPACE SHOW the search for life: are we alone?
background synopsis come prepared! key concepts and orientation background before your visit while you're at the museum back in the classroom references credits	The investigation of life on other worlds embraces concepts drawn from the Physical, Earth and Planetary, and Life sciences. What is life? What are the requirements for life? How do living things survive? What kinds of environments—on Earth or elsewhere—are suitable for life? Where in the universe should we look for life? The Search for Life: Are We Alone? invites viewers to ask these questions and begin to formulate answers. Presented below is background on some significant topics from the Space Show. Physical Science Detection of Exoplanets Until 1995, scientists had speculated about the existence of exoplanets, but had not discovered any. Now astronomers are finding them at the rate of one per month, which suggests that exoplanets are quite common. Exoplanets are too small and faint to be observed directly, so astronomers look instead for their effect on the motion of the stars they orbit. All objects with mass exert a gravitational force on each other, so not only does a star keep a planet in orbit, but a planet also pulls on its parent star. When we detect a planet orbiting a star, the exoplanet sometimes pulls the star slightly toward us, sometimes slightly away from us. Astronomers can detect this "wobble" by studying the light from the star. Thus far, the exoplanets discovered are giants (at least the size of Jupiter). Better technology may enable us to detect planets and moons of sizes approximating Earth or Europa. Earth's Radio Emissions The expanding sphere of our civilization's radio waves has already reached many of the exoplanets we have discovered. If intelligent life exists on these worlds, our signals could be detected there. All electromagnetic radiation, which includes light, radio, television, and radar waves, possesses certain characteristics. It travels at the speed of light, 300,000 kilometers or 186,000 miles per second, along a linear path. A radiation source emitting in all directions creates a "bubble" of radiation spreading outward at the speed of light. Several decades of radio emissions from Earth have created just such a bubble—expanding into space and washing over many exoplanets. Distances to other stars, and their planets, are tremendous. To make the numbers easier to manage, astronomers use light years. A light year is the distance light travels in one year, about 9.5 trillion kilometers (5.9 trillion miles). The distance to the brightest star, Sirius, is nine light years. In other words, the light from Sirius reaching us today left that star nine years ago. Earth and Planetary Science Solar System and Earth Formation Dense regions of interstellar clouds can collapse, becoming denser and hotter and possibly forming stars and planetary systems. Earth and the rest of the solar system began as a rotating cloud of hydrogen and helium, with trace amounts of heavier elements, dust, and ice grains. Gravity collapsed the cloud inward. As the cloud shrank, it began to spin faster and flattened into a disk with a central bulge. Planets would eventually form in the disk. Collisions in the bulge raised its temperature. When the central temperature became high enough to fuse the hydrogen in the bulge, the Sun ignited. Meanwhile, temperatures in the disk away from the Sun were cool enough to allow heavier elements and compounds such as iron and silicates to condense into solids. These particles in the disk began to accrete, forming small planet-like bodies called planetesimals, which then merged together to form larger bodies, culminating in Earth and the other bodies of the solar system. Our current Earth is the result of 4.6 billion years of planetary evolution from the time of its formation. Comparative Planetology Mars was probably more like our own watery blue planet in the past, but now its surface is a barren desert. Both Earth and Mars orbit at a distance from the Sun at which water can exist as a liquid. However, the more massive Earth is better able to retain its atmosphere than the less massive Mars. Earth's greater gravitational pull prevents heavier molecules such as nitrogen and oxygen from escaping its atmosphere. Thus, Earth's atmosphere is relatively dense, making its temperature and pressure high enough to support liquid water. The present Martian atmosphere is less dense, and thus its atmospheric pressure and atmosphere are too low for liquid water to exist stably on the surface. Data returned from the Mars Odyssey spacecraft indicates that huge amounts of water ice lie within the upper regions of soil around the Martian South Pole. This finding supports the possibility of life existing on Mars—not on its surface, but beneath it. Life Science Life Fills Every Niche Earth's ocean floor seems to be as alien as another planet, yet life exists there. Basically, something is alive if it carries on life processes with the aim of sustaining itself and its species: A living thing obtains energy, responds to its environment, grows, and reproduces. The information on how to carry on life processes is encoded in DNA (deoxyribonucleic acid) found in fundamental units called cells. Scientists know that life can thrive even in the most inhospitable environments, such as around "black smokers"—sulfide chimneys formed near hydrothermal vents on the sea floor. (Hydrothermal vents were discovered in 1977. In 1998, the Museum mounted an expedition to collect sulfide chimneys—now on display in the Hall of Planet Earth.) Certain types of microbes, which form the food-chain base, convert energy from the hot, mineral-rich hydrothermal fluids to food as it mixes with seawater. Microbes that live below Earth's surface obtain their energy from Earth's interior, rather than from the Sun. These microbes may be the most common life form on Earth. Microscopic organisms arose on Earth much earlier than plants and animals and thrive in places where plants and animals cannot survive. Hence, it is reasonable to hypothesize that microbial life could exist in similar, seemingly hostile environments beyond Earth. Does All Life Look Alike? Could there be living forms on other worlds that don't need or can't stand water, but instead need something else? We have seen that life is resilient. But what forms might life on other worlds take? Although extraterrestrial life in movies and on television often resembles humans, this depiction is limiting. Life may not need to be based on DNA, for example. Or perhaps not even carbonbased, but silicon-based. Some scientists have theorized that liquids other than water, such as ammonia, methane, and ethane, could possibly sustain life on other planets. These life forms, however, would be different chemically from those on Earth. Thus, the criteria used to search for life on other worlds should reflect the understanding that our conception of life is based on a single example—the life that arose here on Earth.
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