Astronomers have discovered what may be five planets orbiting Tau Ceti, the closest singl

Astronomers have discovered what may be five planets orbiting Tau Ceti, the closest single star beyond our solar system whose temperature and luminosity nearly matches the suns. If the 【M1】______ planets are there, one of them is about the right distance from the star to support mild temperatures, oceans of liquid water, and even life. Tau Ceti is only 12 light-years from Earth, just three times as far as our suns nearest stellar neighbor, Alpha Centauri. Tau Ceti assembles the sun so much that astronomer Frank Drake, who has 【M2】______ long sought radio signals from possible extraterrestrial civilizations, made it his first target back in 1960. Unlike most stars, that are 【M3】______ faint, cool, and small, Tau Ceti is a bright G-type yellow main-sequence star like the sun, a trait that only one in 25 stars boasts of. Moreover, unlike Alpha Centauri, which also harbors a 【M4】______ G-type star and even a planet, Tau Ceti is single, therefore theres 【M5】______ no second star in the system whose gravity could yank planets away. Astronomer Mikko Tuomi of the University of Hertfordshire in the United Kingdom and his colleagues analyzed more than 6,000 observations of Tau Ceti from telescopes in Chile, Australia, and Hawaii. As the researchers will report in Astronomy & Astrophysics, slight changes in Tau Cetis motion through space suggest that the star may be responding to gravitational tugs from five planets that are only about two to seven times as massive as Earth. If thats right, all five planets lie close to their star than Mars 【M6】______ does ours; however, Tau Ceti emits only 45% as much light as the 【M7】______ sun, so each planet receives less warmth than a planet would at the same distance from our sun. Tau Cetis three innermost planets — designated b, c, and d — are probably too hot to support life, being such close to the star that they require only 14, 35, and 【M8】______ 94 days to complete an orbit. The farthest of the three, d, is about as close to Tau Ceti as Mercury is to the sun. Its fourth 【M9】______ planet — planet e — that the scientists suggest might be another life-bearing world, even though its about four times as massive as Earth. If you live there, youd see a yellow sun in the sky, but your 【M10】______ year would last just 168 days.

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