Purveyors of doom often look to the heavens for their protagonists. During the 1980s, we were briefly captivated by Nemesis, a supposed companion of the Sun that triggered a death-dealing rain of comets every 26 million years. During the 1990s we endured wild speculations about Nibiru, which managed somehow not to destroy Earth in 2003.
Now there's a new threat — but unlike Nemesis and Nibiru, this one's real. It's called Gliese 710 (pronounced GLEE-zuh), an obscure, 10th-magnitude orange dwarf star situated about 63 light-years away in the constellation Serpens. Astronomers first took note of this modest star about a decade ago, when Joan García-Sánchez (Jet Propulsion Laboratory) and others found, based on positional observations from the Hipparcos satellite, that in roughly 1½ million years Gliese 710 should pass about 1.3 light-years from the Sun.
That's not close enough for the gravitational pull of a 0.6-solar-mass star to unhinge the planets from their current orbits. But it would stir up some trouble in the Oort Cloud, the storehouse of perhaps a trillion comets that theoretically extends to the limit of the Sun's gravitational grip. García-Sánchez and her team estimated that 2.4 million Oort Cloud denizens might be perturbed into Earth-crossing orbits over a couple million years, increasing the risk of our planet being hit by a long-period comet by only 10%. No big whoop.
But a revised Hipparcos catalog of stars' distances and motions came out in 2007, and a fresh analysis of that and other datasets by Vadim V. Bobylev (Pulkovo Astronomical Observatory) shows that Gliese 710 is a little more problematic than first thought.
After running a million computer simulations that factored in various observational errors, Bobylev confirms that the star has an 86% chance of passing through the Oort Cloud's outer limit, assumed to be 1.6 light-years from the Sun. Moreover, the simulations yield a 1-in-10,000 chance of skirting within 0.02 light-year, or about 1,000 astronomical units. A star passing by only 50 times farther out than Pluto would wreak havoc within the Oort Cloud, sending a long-lasting pulse of comets throughout the planetary system.
Yet all the comets stashed in the Kuiper Belt, and the planets themselves, would not likely be affected, since the Sun's gravitational force on a typical comet in the belt would still be thousands of times stronger than the tug from the passing star.
"The way to think about what is happening," notes dynamicists Hal Levison (Southwest Research Institute), "is to realize that the star gets closer to the sun than to the vast majority of comets. So the Sun's velocity is changed while the Oort cloud in not. The Sun heads off in a new direction while to Oort cloud keeps going in the original one."
Could such a close encounter really happen? It probably already has. Dynamicists point to the fact that the Kuiper Belt appears to end abruptly at a distance of about 60 a.u. Then there's the strange 12,000-year orbit of 90377 Sedna, whose solar distance ranges between 76 and 976 a.u. No mere planetary encounter could have flung Sedna out that far; the only plausible explanation is the gravitational yank from a close-passing star long ago.
On the plus side, Bobylev's analysis of 35,000 stars within 100 light-years of the Sun didn't turn up any new interlopers of consequence within the past or forthcoming 2 million years. He's added nine new stars to the list of past or future encounters of 6½ light-years or less, none of which come nearly as close as Gliese 710. Some 27 and 28 million years from now, respectively, the stars Proxima and Alpha Centauri sail past our solar system but come no closer than 3 light-years.