It certainly captures the imagination: a star passing
silently by our solar system knocks a deadly barrage of comets towards Earth.
However, recent simulations by one group of researchers has shown that these
star-induced comet showers may not be as dramatic as once thought.
The idea of nearby stars influencing comets goes back to
1950, when the astronomer Jan Hendrik Oort hypothesized an invisible repository
of comets — the so-called Oort cloud — swarming around the solar system out to
a distance of 100,000 AU (one AU is the distance between the sun and the
Earth).
Oort assumed that stars passing through the cloud would
cause a fresh batch of comets to fall in towards the sun, where they become
visible to astronomers. Such a disturbance could have long-term effects.
"The comets we see now could be from a stellar passage
hundreds of millions of years ago," said Hans Rickman of the Uppsala
Astronomical Observatory in Sweden.
However, Rickman and his colleagues have confirmed that star
encounters alone cannot explain comet behavior. Using a computer model of the
Oort cloud, they show that gravity effects from the galaxy are equally
important. The results are reported in a recent article in the journal Celestial
Mechanics and Dynamical Astronomy.
Two stars passing in the night
Although Earth has almost certainly been hit by comets
throughout its history, it is not all that clear how often that has happened.
Much of the crater
history on Earth has been erased because of erosion or tectonic activity.
The remaining craters could have come from asteroids instead of comets.
"It's quite difficult to tell a comet-induced crater
from an asteroid one, since the impactor gets essentially vaporized,"
Rickman said.
Comet
impacts are, however, likely to be more energetic (and therefore more
damaging), since comets are moving much faster than asteroids when they pass by
Earth.
Comet orbits can be altered whenever another star comes
within 10,000 AU of our sun. Such a close encounter — occurring every 100
million years or so — will not typically disturb asteroids or planets, but it
definitely "shakes up the whole Oort cloud," Rickman said.
Most scientists have presumed that these star crossings will
lead to a shower of comets raining down on the Earth and the rest of the inner
solar system. Some have even claimed to find evidence of periodic mass
extinctions that might be explained by a single (as-yet-unidentified) star in
an elliptical orbit around the sun.
To study the effect of stellar perturbations, Rickman and
his colleagues model the Oort cloud with a sample of one million comets (the
true number of cloud comets is unknown, but certainly much higher). The
simulations are allowed to run for a time period corresponding to the
5-billion-year age of the solar system.
The results show that stars can induce comet showers, but
the contrast with non-shower periods is less than what people have thought
before, Rickman said. This leveling out in comet activity is due to the
influence of the gravitational field of the Milky Way.
Galactic tide
Astronomers have known for some time that our galaxy's
gravity has an influence on the Oort cloud. Specifically, the cloud experiences
a tidal effect due to the fact that the gravitational field is stronger the
closer one is to the plane of the galaxy.
The simulations by Rickman and colleagues show how the
galactic tide constantly gives a small nudge to the cloud's comets. Some of
these comets are in rather unstable orbits to begin with, so the slight push
can send them on a sun-bound trajectory. Eventually, however, all these
unstable comets are ejected from the solar system.
And this is where stellar encounters become important. They
scramble the Oort cloud, so that the galactic tide has a new crop of unstable
comets to funnel into the inner solar system.
"The general picture spawned by our results is that
injection of comets from the Oort Cloud is essentially to be seen as a teamwork
involving both tides and stars," the scientists write in their paper.
This star-tide collaboration keeps a relatively steady
supply of comets zooming nearby, so the threat from comet impacts probably does
not change much over time.
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