1 . Shining just 12 light-years from Earth, the star Tau Ceti so resembles the sun that it has appeared in numerous science- fiction stories and was the first star astronomers ever searched for signs of intelligent life, half a century ago. In 2012 Tau Ceti grew still more interesting when astronomers reported five possible planets somewhat larger than Earth circling closer to the star than Mars orbits (围绕……运动) the sun—one of which is in the star’s habitable zone. Newly released images taken by the Herschel Space Observatory provide even more insight about Tau Ceti’s solar system: greater detail about its dust belt.
Dust arises when asteroids and comets (小行星和彗星) crash into one another, so its location reveals where these dust- creating objects—which are too small to be seen directly—orbit a star. In Tau Ceti’s case, “it’s quite a wide dust belt,” says Samantha Lawler of the University of Victoria in British Columbia. As her team reported in November, the belt’s inner edge is roughly two to three astronomical units (AUs) from the star, which is the position of our own sun’s asteroid belt. (An AU is the distance from Earth to the sun.) Tau Ceti’s dust belt extends out to 55 AU, which would be just beyond our system’s main Edgeworth-Kuiper belt, the zone of small bodies whose largest member is probably Pluto. Presumably full of asteroids and comets, Tau Ceti’s dust belt most likely lacks a planet as large as Jupiter, Lawler says. The gravity of such a massive planet would have driven away most small space rocks.
Within a year a new series of radio telescopes in Chile called ALMA should provide a sharper view of the disk, especially of its inner edge. The ALMA images will help astronomers confirm whether the star’s five proposed planets are indeed real. If the disk overlaps the planets’ hypothesized (假设的) orbits, then they probably do not exist; they would have kept away most asteroids near the star, removing the source of dust.
If those planets do exist, however, Lawler’s team suggests that Tau Ceti’s planetary system may resemble what our solar system would have looked like had the four giant planets—Jupiter, Saturn, Uranus and Neptune- never formed: small planets orbiting close to the star, and nothing but asteroids, comets and dust beyond.
1. According to astronomers, the five possible planets of Tau Ceti________.| A．resembled Earth in size |
| B．functioned like a solar system |
| C．were located in Tau Ceti’s habitable zone |
| D．were nearer to Tau Ceti than Mars to the sun |
| A．It is narrower than the asteroid belt in our system. |
| B．The bodies inside it are all smaller than Jupiter. |
| C．The gravity of Tau Ceti makes it get denser. |
| D．It is over 55 astronomical units in width. |
| A．they don’t move into the dust belt while orbiting Tau Ceti |
| B．they have kept away most asteroids and comets |
| C．they don’t crash into any asteroid or comet |
| D．they can be seen clearly by ALMA |
| A．is useful because it stops asteroids or comets crashing into the star |
| B．makes Tau Ceti different from the sun because it extends farther |
| C．is interesting because it keeps other planets away from Tau Ceti |
| D．plays a role in helping decide whether the five planets are real |
