A．exerting effects on their birthplaceB．gravitational p-找考题网

A．exerting effects on their birthplaceB．gravitational p……

When the end of the world comes, we’ll know what to blame. Scientists have found compelling evidence that the Sun has a baby brother, a dark star whose eccentric orbit is responsible for periodically showering the Earth with comets and meteorites.
The dark star -- named Nemesis by astronomers -- is thought to be a brown dwarf that spins round the Sun in an orbit so large it is measured in light years, the distance light travels in a year, equivalent to about 6,000 billion miles.
The research suggests that, every 26m years, the star’s eccentric orbit brings it within one light year of the solar system. There it causes havoc in the Oort Cloud, a huge region surrounding the solar system that contains billions of bits of cosmic rabble left over from the formation of planets.
Of the millions of rocks it throws out of orbit at each visit, some hurtle Earthwards, and have several times nearly wiped out life on Earth.
Astronomers have long wondered if the Sun has a smaller partner. Recently, two independent groups of researchers have found evidence of one.
One group, led by John Matese, professor of physics at the University of Louisiana at Lafayette, confirms the notion that it is most likely to be a brown dwarf, a star that never accumulated enough mass to ignite and which has simply sat in space smoldering for billions of years.
Matese studied 82 comets from the Oort Cloud and found common elements in the shape of their orbits that could only be explained if they had been influenced by the gravitational pull of an object several times the size of Jupiter and existing about 25,000 times farther from the Sun than the Earth.
Matese said: "A companion to the Sun orbiting at these distances would have little effect on the planets. But it would play a big role in the way comets ’made their way’ from their birth places in the planetary disc out to the Oort Cloud and in how they can return to the inner solar system."
Further research was published by Richard Muller, professor of physics at the University of California, Berkeley, following analyses of moon rock samples brought back to Earth by Apollo 14.
The absence of a protective atmosphere means the moon has been subjected to intense bombardment in its 4.5 billion-year existence.
Muller’s breakthrough was to find a way to date how long ago any particle was melted -- meaning he could build up a picture of whether the moon gets a constant barrage or suffers spells of intense bombardment.
He said, "The evidence clearly shows that the moon has gone through spells of relatively frequent impacts and others of reduced intensity. I believe it is likely that this is because the Oort Cloud is being disturbed by a massive body that is throwing comets out of stable orbits, a small fraction of which could reach the Earth."
Muller and others believe that the dark star probably takes about 26m years to complete an orbit around the sun.
Other scientists have already noted that mass extinctions of life on Earth seem to occur in a pattern with gaps equivalent to multiples of 26m, suggesting some regular event is causing the comets to come Earth’s way.
The best-known such event was the one that wiped out the dinosaurs 65m years ago, but that was not the worst; the planet has suffered several such large mass extinctions.
Astronomers have found the first Earth-sized planet orbiting another star. The discovery raises the chances of finding planets that could support life as we know it.
The Nemesis influences the comets by ______.

A．exerting effects on their birthplace
B．gravitational pull of planets
C．affecting the Oort Cloud
D．influencing their way to the Oort Cloud and the inner solar system

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