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  Night Sky
 

Enjoy the starry night skies
Be a considerate neighbor. Reduce nighttime glare.
Shield your outside lights downward.
Let the stars light up the night.

March 2015 night sky

—Charlie Christmann

Nemesis

The Oort cloud, named for Dutch astronomer Jan Oort, who postulated the existence of an icy cloud containing a trillion comets circling the sun well beyond the orbit of Pluto. Today, astronomers are finding large objects well beyond Pluto and are convinced the Oort cloud exists. With orbits around the sun measured in hundreds of thousands to millions of years, the farthest Oort objects may be as far away as two light-years from the sun, halfway to the nearest star. Out there, the sun’s gravity has only a feeble hold on any object in a solar orbit; it does not take much of an external gravitational push or pull to change an orbit, either nudging the object toward the sun or throwing it out into interstellar space.

In 1984, paleontologists David Raup and Jack Sepkoski theorized that massive extinction events occur on Earth about every 27 million years. To explain these periodic events, several groups of astronomers, almost simultaneously, suggested that perhaps large meteors from the Oort cloud smashing into Earth were to blame. Science is fairly sure the dinosaurs met their demise this way.

The astronomers needed to find a massive object orbiting far from the sun to periodically kick Oort cloud comets onto an inward path toward the sun. So, they conceived of a theoretical dark star, too dim to be seen easily with telescopes, and named it Nemesis, after the Greek goddess of divine retribution. This dark star would need to have an elliptical orbit circling the sun every 27 million years. Nemesis would swing as far out as 1.5 light-years from the sun. To be effective, but yet undetected, the object must be a brown dwarf, an almost star larger than Jupiter, or a red dwarf, a small cool star. Science decided to see if it could find Nemesis.

The Two Micron All Sky Survey (2MASS) telescope found 173 brown dwarfs close to our solar system, but nothing close to home. NASA’s Wide-field Infrared Survey Explorer also took a look. That survey found brown dwarfs within twenty light-years, but again, none near the solar system. No Nemesis.

Even if we have no death star orbiting the sun, we did have a visit from a red dwarf star about seventy thousand years ago, just about the time early humans arose. Scholz’s star was recently discovered speeding almost directly away from our solar system. After seventy thousand years, the star is now about twenty light-years away. At its closest, it was about 0.8 light-years away, well within the Oort cloud. But impact records on Earth and other planets do not show a large change in the number of meteor impacts. Scholz’s star had little impact on the Oort cloud and does not seem to have rained any large numbers of meteors upon the inner solar system. No increase in the rate of impact on Mars, the moon, or Earth has been found correlating with the passage of Scholz’s Star.

Unfortunately, this close shave experiment will repeat, like it or not. Coryn Bailer-Jones, an astrophysicist at the Max Planck Institute for Astronomy in Heidelberg, Germany, has found 14 stars heading in this direction. One, named Hip 85605, will pass through the Oort cloud in about 250 thousand years. That event may have another opportunity to change the orbit of some of the Oort cloud comets, sending some toward the sun and the inner planets. Hopefully by then, we will have a defense thought out to deflect any comets on a collision course with Earth.

 
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