30 (3) Flashcards
Since the discovery of the first exoplanet in ………….., there have been thousands of confirmed detections, and many more candidates that are not yet confirmed.
1995
In evaluating the prospect for life in distant planetary systems, astrobiologists have developed the idea of a ………… …………..—a region around a star where suitable conditions might exist for life.
habitable zone
the habitable zone is generally thought of as the range of distances from the central star in which water could be present in ………. form at a planet’s surface. In
liquid
Mars has a very thin atmosphere and thus very little greenhouse warming (about …….. °C worth), while Venus has a massive carbon dioxide atmosphere that creates very strong greenhouse warming (about …….. °C worth).
2 / 510
planet. Earth’s natural greenhouse effect, which comes mostly from water vapor and carbon dioxide in the atmosphere, raises our average surface temperature by about ……… °C over the value it would have if there were no greenhouse gases in the atmosphere.
33
The ……….. of stars like the Sun also increases over their main-sequence lifetime, and this means that the habitable zone migrates ………….. as a star system ages.
luminosity / outward
Calculations indicate that the power output of the Sun, for example, has increased by at least 30% over the past 4 billion years. Thus, Venus was once within the habitable zone, while Earth received a level of solar energy insufficient to keep the modern Earth (with its present atmosphere) from freezing over.
Of the confirmed or candidate exoplanets known at the time of writing, nearly ……….. are considered to be orbiting within the habitable zone and more than ……… of those are roughly Earth-size.
300
10%
What types of observations might constitute good evidence for life?
To be sure, we need to look for robust ………….. (3) capable of creating planet-scale change.
biospheres
atmospheres, surfaces, and/or oceans
Indeed, ………….. and ………….. microorganisms are so abundant at Earth’s surface that they affect the color of the light that our planet reflects out into space—we appear greener in visible wavelengths and reflect more near-infrared light than we otherwise would.
plants and photosynthetic
Moreover, photosynthesis has changed Earth’s atmosphere at a large scale—more than ……….. of our atmosphere comes from the photosynthetic waste product, oxygen.
20%
If we manage to separate out a clean signal from the planet and find some features in the light spectrum that might be indicative of life, we will need to work hard to think of any nonbiological process that might account for them. “Life is the hypothesis of ………. …………,” noted astronomer ……….. ………..—meaning that we must exhaust all other explanations for what we see before claiming to have found evidence of extraterrestrial biology.
last resort / Carl Sagan
While space travel by living creatures seems very difficult, robot probes can travel over long distances and over long periods of time. Five spacecraft—two …………, two …………, and ……….. ………..—are now leaving the solar system.
Pioneers / Voyagers / New Horizons
We have access to a wide spectrum of electromagnetic radiation, ranging from the longest-wavelength radio waves to the shortest-wavelength gamma rays. Which would be the best for interstellar communication?
……….. ……….. turn out to be the best answer.
radio waves
Why are we using Radio Waves for interstellar communication?
- Being the lowest-frequency (and lowest-energy) band of the spectrum, they are not very ………… to produce, and
- we already use them extensively for communications on Earth.
- They are not significantly ……….. by interstellar dust and gas.
- With some exceptions, they easily pass through Earth’s ………… and through the ………….. of the other planets we are acquainted with.
- expensive
- FYI
- absorbed
- atmosphere / atmospheres
The very first radio search was conducted by astronomer ………. ……… in 19……., using the ……-foot antenna at the National Radio Astronomy Observatory
Frank Drake / 60 / 85
