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 system's 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.

That's probably because no one was looking， de Wit said. Most exoplanet searches， like those involving NASA's 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. Its size is as big as Jupiter in our solar system

B. It is known as an ultracool dwarf

C. It is the star that the three newly discovered planets are orbiting around

D. It is not bright enough to warm the three Earth-sized planets

3. It seems that the most essential element for life is ____.

A. warm air B. liquid water

C. enough radiation D. the size of a planet

4. According to de Wit， what is the possible reason for astronomers failure to detect a planet orbiting an ultracool dwarf？ ____.

A. Astronomers didnt take notice of such cool small sized stars

B. Such dwarfs are invisible to most detecting devices

C. Such dwarfs are shrouded by special mist

D. Such small sized stars are not valuable enough

Fake Sugar can Lead to Overeating

People trying to lose （A） w___________may turn to fake sugars—or foods that contain them. But these sugar substitutes have provoked controversy in the past few years. One reason： studies have shown that they can actually boost someone's appetite. A new （1） s___________now confirms that.

In the new study， scientists showed that eating a popular （2） a___________sweetener made fruit flies and mice hungrier， says Herbert Herzog. He works at the Garvan Institute of Medical Research in Sydney， Australia. If people respond the same way—and Herzog's team thinks they will — long term consumption of fake sugar could encourage people to overeat.

The researchers fed a diet of （B） n___________sugar and yeast to two groups of fruit flies. One group ate nothing else. The other group also got a bonus. For five days， its diet was sweetened even more with sucralose. This is the fake sugar sold under the name Splenda.

On day 6， the second group went back to eating meals of only sugar and yeast. Throughout the day， the scientists carefully measured how much each group of flies ate. Fruit flies who had recently been eating the artificial sweetener now ate 30 percent more than the flies that had eaten the （3）n___________diet all along. However， a few days after sucralose was removed from the diet of those flies， they returned to eating normal （C） a___________of food.

The scientists were also curious to see if the same thing would happen in other （D） a___________. So they repeated the （4） e___________in mice. They again fed some a diet with sucralose for a week. And compared to those not getting the fake sugar， these mice consumed more food after that artificial sugar was taken out of their food.

This new study shows artificial sweetener can lead to eating more， so scientists suggest that most people should avoid starting the use of artificial sweeteners.

（A， B， C， D FOR CROSS， 1， 2， 3， 4 FOR DOWN. The first letters of the absents were given.）