Tiny Star Warms Earth― like Planets etc..docVIP

Tiny Star Warms Earth― like Planets etc. 　　Astronomers have discovered three Earth-sized planets orbiting a small red sun， just 40 light-years away， according to research published in the journal Nature. 　　Researchers say all three worlds are potentially habitable and could represent the best targets yet in the search for life beyond the solar system. 　　These are not massive， hot， super-exotic worlds， said Julien de Wit， a postdoctoral associate in planetary science at the Massachusetts Institute of Technology （MIT） who worked on the study. Each of them could be suitable for life. 　　The systems host star， called TRAPPIST-1， is what is known as an ultracool dwarf. It is about one-tenth the size of our sun and just a bit bigger than Jupiter. 　　The star is just 0.05 percent as bright as our sun， but scientists say it could still give off enough radiation to warm all three planets so liquid water could exist on their surfaces. 　　A planet that is neither too hot nor too cold to sustain liquid water on its surface is generally considered to have the potential for life. 　　Ultracool dwarfs make up about 15 percent of the astronomical objects in our immediate neighborhood， but until now， no one had ever discovered a planet orbiting one. 　　Thats probably because no one was looking， de Wit said. Most exoplanet searches， like those involving NASAs Kepler telescope， target hotter， bigger stars. 　　For a while， there was some debate in the astronomy community about whether it was possible for planets to form around such tiny and dim stars. 　　The team took a big risk even looking for planets around these stars， de Wit said. But it has really paid off. 　　1. The three Earth-sized planets discovered by astronomers ____. 　　A. can serve as possible habitats for life 　　B. are orbiting the sun in the solar system 　　C. are potential threats to human beings 　　D. are massive， hot， super-exotic worlds 　　2. Which of the following is NOT true about TRAPPIST-1？ ____. 　　A. It