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雅思听力材料：新行星的发现

2018-04-09 15:32:12来源：网络 柯林斯词典

　　Images from Keck II show that the newfound planet is also a gas giant, about seven times Jupiter's mass. But it orbits closer to the star, at a mere 1.4 billion miles (2.2 billion kilometers), equivalent to between the orbits of Saturn and Uranus.

　　The new Keck data also indicate there may be an asteroid belt just inside the newfound planet's orbit.

　　Widespread "Jupiters" Creating Dilemma

　　Overall, the HR 8799 system is starting to look like a scaled-up version of our solar system, the astronomers note. But the setup doesn't fit either of the two currently accepted models for how gas giant planets form, Marois said.

　　It's thought planets in general form out of the disks of gas and debris that surround many young stars. One model for how gas giants form is called gravitational instability, in which a perturbation in the disk causes a clump of material to suddenly coalesce into a gassy planet.

　　The other model is core accretion, in which material first clumps into a rocky core, and the core then gravitationally gathers nearby gases. (See "New Model of Jupiter's Core Ignites Planet Birth Debate.")

　　According to Marois's team, the problem with such widespread gas giants is that they all couldn't have formed the same way. Too far from the star and there's not enough gas for core accretion to work, but too close and it's too hot for debris to become gravitationally unstable.

　　"The system is either just too young and the planets are just too far away and don't have time to form before the gas in the disk is depleted, or they are too close and the disk is too warm to form planets," Marois said.

　　Gas giant planets have been found in orbits close to their parent stars, sometimes even closer than Mercury is to the sun. In these cases, many experts believe the so-called hot Jupiters may have formed farther away from their stars and then migrated closer over time.

　　"That is one possible way out in explaining the problem. There is probably more to it than that—but this will require more work," Marois said.

　　"But the fact that we are seeing this planetary system at this early stage of evolution is definitely giving us insight on what happened to arrive at its current configuration."

　　The HR 8799 planetary system is described in a paper published online this week by the journal Nature.



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