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第二篇
P1 发现一个星球KL-3547（？）什么的 这个星球有三个卫星绕转
P2 忘了 有一句是三个卫星的距离比月球到地球的距离近 考题了
P3 这种星球通常会有一个disk，这个disk是前一个恒星/dust/星云/之类的爆炸或毁灭之后的遗留物 太阳系里就有这种例子（细说） 那么，类似的这颗星球是否有disk是值得期待的（不确定有没有）
是原文吗？
Astronomers have always suspected that planets might orbit stars other than our sun. We imagined, though, that we would find systems much like our own solar system, centered on a star much like the sun. Yet when a flood of discoveries began 15 years ago, it was apparent right away that extrasolar planetary systems can differ dramatically from our solar system. The first example was the sunlike star 51 Pegasi, found to have a planet more massive than Jupiter on an orbit smaller than that of Mercury. As instruments became more sensitive, they found ever stranger instances. The sunlike star HD 40307 hosts three planets with masses between four and 10 Earth masses, all on orbits less than half the size of Mercury’s. The sunlike star 55 Cancri A has no fewer than five planets, with masses ranging from 10 and 1,000 Earth masses and orbital radii ranging from one tenth that of Mercury to about that of Jupiter. Planetary systems imagined in science fiction scarcely compare.
The white dwarf systems demonstrate that the stars do not even need to be sunlike. Planets and planetary building blocks can orbit bodies that are themselves no larger than planets. The variety of these systems equals that of systems around ordinary stars. Astronomers hardly expected the ubiquity of planetary systems, their hardiness and the apparent universality of the processes by which they form. Solar systems like our own might not be the most common sites for planets, or even life, in the universe.
Phoenix from the Ashes
It is sometimes forgotten today, but the first confirmed discovery of any extrasolar planets was around a very unsunlike star: the neutron star PSR 1257+12, an even more extreme type of stellar corpse than a white dwarf. It packs a mass greater than the sun’s into the size of a small asteroid, some 20 kilometers across. The event that created this beast, the supernova explosion of a star 20 times the mass of the sun, was more violent than the demise of a sunlike star, and it is hard to imagine planets surviving it. Moreover, the star that exploded probably had a radius larger than 1 AU (astronomical unit, the Earth-sun distance), which is larger than the orbits of the planets we see today. For both reasons, those planets must have risen up out of the ashes of the explosion.
Although supernovae typically eject most of their debris into interstellar space, a small amount remains gravitationally bound and falls back to form a swirling disk around the stellar remnant. Disks are the birthing grounds of planets. Astronomers think our solar system took shape when an amorphous interstellar cloud of dust and gas collapsed under its own weight. The conservation of angular momentum, or spin, kept some of the material from simply falling all the way to the newborn sun; instead it settled into a pancake shape. Within this disk, dust and gas coagulated into planets [see “The Genesis of Planets,” by Douglas N. C. Lin; Scientific American, May 2008]. Much the same process could have occurred in the postsupernova fallback disk.
Astronomers discovered the system around PSR 1257+12 by detecting periodic deviations in the timing of the radio pulses it gives off; such deviations arise because the orbiting planets pull slightly on the star, periodically shifting its position and thus altering the distance the pulses must travel. Despite intensive searches of other stars’ pulses, observers know of no other comparable system. Another pulsar, PSR B1620–26, has at least one planet, but it orbits so far from the star that astronomers think it did not form in a fallback disk but rather was captured gravitationally from another star.
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发表于 2018-7-30 19:21:40
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