The sun bounces up and down as it roams the Milky Way, and such
wavering might have hurled showers of comets Earth's way that caused mass
extinctions, including the one that killed the dinosaurs, a new study claims.
Such cosmic impacts might also have spread life to alien worlds,
scientists speculate.
However, doubts have been raised about these suggestions.
To arrive at the comet showers
idea, astronomers calculated the path of our solar system across the Milky Way
as it circles the galactic core. As we pass through the densest part of the
galactic disk, the gravitational pull of the surrounding gas and dust clouds
dislodges comets in the Oort Cloud in the outer solar system, causing these icy
goliaths to plunge toward the sun, the researchers said.
The sun passes through this galactic zone every 35 million to 40
million years, raising the chances of comets hurtling inward tenfold, according
to calculations. This cycle seems to coincide with evidence of craters and mass
extinctions on Earth, which suggest we suffer more collisions roughly every
36 million years.
"It's a beautiful match between what we see on the ground and
what is expected from the galactic record," said researcher William
Napier, an astronomer at the Cardiff Center for Astrobiology in Wales. He and
his colleague Janaki Wickramasinghe will detail their findings in a forthcoming
issue of the Monthly Notices of the Royal Astronomical Society.
Although the solar bounce cycle may have been killer news for non-avian
dinosaurs, pterosaurs, plesiosaurs, mosasaurs, and the many plant and
invertebrate species that went extinct 65 million years ago, it may also have helped
life spread to other planets. The comets may have blasted microbes off Earth
that can withstand the interstellar void.
"Microorganisms thrown into space from this barrage can pass
straight into star- and planet-forming regions within the nebula, without being
sterilized en route by cosmic rays," Napier told SPACE.com.
"This opens the door to the exciting idea that life may spread not just
within the solar system, but may be pan-galactic."
However, there is skepticism about these calculations.
"The
whole concept is wrong," said NASA Jet Propulsion Laboratory astronomer
Paul Weissman, who did not participate in this study. "As you cross the
galactic plane, it's not like you go over a speed bump. The variation in any
cratering rate would be very gentle, not in sudden pulses."
In
the past, some researchers had suggested that mass extinctions on Earth
happened roughly 30 million years apart, and that cosmic impacts also recurred at
about that frequency, and that both were linked. But Weissman noted there is
"considerable doubt" as to whether mass extinctions or cosmic impacts
do occur at such regular intervals.
Other
suggested causes for these supposed comet showers have included a red dwarf
star or a tenth planet in a distant orbit around the sun that dipped into the Oort
cloud.
"Many
researchers, including myself, responded to these ideas, pointing out faults in
the logic. The distant red dwarf star was in a highly unstable orbit that would
escape to interstellar space after about 10 orbits on average," Weissman
said. The tenth planet idea never gained any traction because it was difficult
to create an orbit with the right characteristics, he added, "and there is
no evidence for a large planet beyond Neptune, despite numerous searches."
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