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 to 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.
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 post-supernova 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.
In 2006, however, NASA’s Spitzer Space Telescope discovered unexpected infrared emission from the neutron star 4U 0142+61. The infrared light might arise from the star’s magnetosphere or from a circumstellar disk. This star formed in a supernova explosion about 100,000 years ago, and it typically takes about a million years or so for planets to agglomerate, so if the radiation does signal the presence of a disk, this system may one day resemble that revolving around PSR 1257+12.
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发表于 2024-10-9 09:36:08
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