Astronomer David Nesvorny from the Southwest Research Institute
in Texas believes
that the solar system might have once contained a fifth gigantic
planet, which was ejected deep into the galaxy in a moment of
By looking at the population of the Kuiper belt -- the icy-cold ring of asteroids beyond Neptune --
and by studying the historical fingerprints left on the craters of
the Moon, Nesvorny was able to piece together clues about our solar
He found that a dynamic instability, which occurred about 600
million years into the solar system's life, greatly affected the
orbit of our giant planets and scattered smaller bodies. Some moved
into the Kuiper belt and others traveled inwards, marking their
course as impacts on the Moon and planets.
But that scenario has a flaw. Slow changes in Jupiter's orbit
would have had a large effect on the orbits of the terrestrial
planets. All hell would have broken lose, and the Earth could have
collided with Mars or Venus. Something had to change.
"Colleagues suggested a clever way around this problem," says
Nesvorny. Instead of a slow movement, Jupiter's orbit could have
quickly changed, which would have altered the outer solar system
but been less harmful to the inner planets.
Unfortunately, this too caused problems. Computer simulations,
run thousands of times, showed that Jupiter's quick jump had the
intended effect, but Uranus or Neptune was always knocked out of the solar system.
"Something was clearly wrong," Nesvorny explains.
So perhaps, instead, the early solar system could have had five
giant planets instead of four. By plopping an additional giant
planet with a mass similar to that of Uranus or Neptune the simulation
worked as planned. Jupiter jumped into place, the inner planets
remained unharmed and the outer planets stayed behind.
"The possibility that the solar system had more than four giant
planets initially, and ejected some, appears to be conceivable in
view of the recent discovery of a large number of free-floating planets in
interstellar space, indicating the planet ejection process could be
a common occurrence," says Nesvorny.