ConceptCheck 8-1: Carbon atoms can combine with other elements to form an impressively wide array of different molecules, and carbon atoms are plentiful around stars.

ConceptCheck 8-2: Carbon molecules are commonly found in meteorites and comets, which frequently crashed onto planets’ surfaces during the early formation of the solar system.

ConceptCheck 8-3: They created amino acids and other compounds that living organisms need—they did not create living entities.

ConceptCheck 8-4: The moon does not have the needed thick atmosphere to keep any liquid water needed for life from evaporating into outer space.

ConceptCheck 8-5: The scientists were looking for bubbles to be given off when water was added, hoping that the microbes would use the water for observable life processes.

ConceptCheck 8-6: On Earth, gray hematite is found in places where water exists or has existed in the past.

ConceptCheck 8-7: The SNC meteorites have trace amounts of gas trapped within them that are nearly identical to the unique Martian atmosphere.

ConceptCheck 8-8: On Earth, magnetite and pure iron sulfide crystals often occur in the presence of certain kinds of living microbes, suggesting that life once existed in this material.

ConceptCheck 8-9: Radio waves travel at more than a hundred thousand kilometers every second whereas our fastest spacecraft can only travel at tens of thousands of kilometers every hour; therefore, the vast distances between stars are simply too far to physically travel.

ConceptCheck 8-10: L is particularly difficult to estimate because at about the same time the technology to communicate beyond one’s own planet is developed, the technology of weapons of mass destruction can develop, which could end the civilization just as it starts (alternatively, the civilization can learn to live without war and exist for a very long time).

ConceptCheck 8-11: Transit telescopes are looking for an ever-so-slight dimming of a star as an orbiting planet moves between the star and the observer.

ConceptCheck 8-12: Because a transit-search telescope looks for the dimming of stars, if an orbiting planet never moves directly between the telescope and the star, as if seen from above its orbit, it will not be observed to dim the central star.

ConceptCheck 8-13: The Darwin instruments can measure a planet’s spectra, which could have a peculiar look if living organisms are influencing the planet’s atmosphere, which is what happens on Earth.

CalculationCheck 8-1: Many scientists estimate that about one Sunlike star forms in our Galaxy every year, so if this number increases three times, then the number of Sunlike stars in our Galaxy also increases to about three formed each year.