A、The interference of their stars. B、The extremely long distance. C、The insufficient lightness. D、The huge amount of calculation

Moderator:
    Hello, ladies and gentlemen. It gives me great pleasure to introduce our keynote speaker for today’s session, Dr. Anomawa Shields. Dr. Aomawa Shields received her PhD in Astronomy and Astrobiology from the University of Washington in 2014.(19)Dr. Shields is the founder of Rising Stargirls, an organization dedicated to encouraging girls of all colors and backgrounds to explore and discover the universe. She uses her writing background to communicate science to the public in engaging, innovative ways.
Dr. Shields:
    Thank you for that instruction. Today, I’m going to talk about how we’ll find life on other planets.
     (20)On our planet, where there’s water, there’s life. So we look for planets that orbit at just the right distance from their stars. At this distance, shown in blue on this diagram for stars of different temperatures, planets could be warm enough for water to flow on their surfaces as lakes and oceans where life might reside. Some astronomers focus their time and energy on finding planets at these distances from their stars. What I do picks up where their job ends. I model the possible climates of exoplanets. And here’s why that’s important: there are many factors besides distance from its star that control whether a planet can support life.
     (21)We don’t know what the atmospheres of these planets are like because the planets are so small and dim compared to their stars and so far away from us. For example, one of the closest planets that could support surface water is 23 light years away. So that’s more than 100 trillion miles. Trying to measure the atmospheric composition of an exoplanet passing in front of its host star is hard. So I use computer models to calculate the kind of atmosphere a planet would need to have a suitable climate for water and life.
    Ice on a planet’s surface is also important for climate.(22)Ice absorbs longer, redder wavelengths of light, and reflects shorter, bluer light. That’s why the iceberg in this photo looks so blue. The redder light from the sun is absorbed on its way through the ice. Only the blue light makes it all the way to the bottom. Then it gets reflected back to up to our eyes and we see blue ice. My models show that planets orbiting cooler stars could actually be warmer than planets orbiting hotter stars.
    My organization, Rising Stargirls, teaches astronomy to middle-school girls of color, using theater, writing and visual art. Science and art don’t often go together, but interweaving them can help these girls bring their whole selves to what they learn.
    I hope more girls can join the Rising Stargirls. Thank you.
19. What is the aim of Dr. Shields Rising Stargirls?
20. What kind of planets do astronomers focus on?
21. What makes it difficult to measure the atmospheric composition of planets with surface water?
22. Why do icebergs look blue?