June 12, 2009 at 3:57 am #31696
note: this will be of special interest to Greatest Kan & Li adepts working with the planets. 1% chance suggests great instability in what many believe to be inviolable.
SOLAR SYSTEM’S PLANETS COULD SPIN OUT OF CONTROL
By Stephen Battersby
June 10, 2009
The solar system’s clockwork motion is by no means guaranteed: one day the
Earth could collide with Venus or tear Mars apart in a close encounter, a
new simulation has shown.
We know that the apparently reliable orbits of the planets are unstable in
the long run, because their weak gravitational effects on one another can
add up in unpredictable ways. Technically, the system is chaotic. Could this
very mild chaos lead to disaster?
Mercury is the key to catastrophe. It is especially susceptible to Jupiter’s
influence because of a small celestial coincidence: Mercury’s perihelion,
the point where it gets closest to the sun, slowly moves around at a rate of
about 1.5 degrees every 1000 years, and Jupiter’s perihelion moves around
only a little slower. One day, the two will probably fall into sync, at
which time Jupiter’s incessant gravitational tugs could accumulate and pull
Mercury off course.
A study led last year by Jaques Laskar of Paris Observatory in France found
a slim chance that Mercury’s orbit could be pulled into a highly elongated
ellipse, putting it on a potential collision course with Venus. That work
used a mathematical trick to calculate average changes over many planetary
orbits, so the method was limited. “Close to a collision, it loses its
validity,” says Laskar. He and his colleague Mickaël Gastineau have taken a
more thorough approach by directly simulating 2500 possible futures,
calculating the planets’ orbits over 5 billion years, up to when the sun
turns into a red giant.
Each of the 2500 cases has slightly different initial conditions —
Mercury’s position varies by about 1 metre between one simulation and the
next. In 20 cases, Mercury goes into a dangerous orbit and often ends up
colliding with Venus or plunging into the sun. Moving in such a warped
orbit, Mercury’s gravity is more likely to shake other planets out of their
settled paths: in one simulated case its perturbations send Mars heading
Laskar found that Mars could hit Earth directly, be thrown out of the solar
system, or come so close that Earth’s gravity would tear it into pieces
which would rain down on our heads. Alternatively, the orbits of the inner
planets could be scrambled, so Earth collides with Mercury or Venus.
“We now have the definitive answer on solar system stability,” says Gregory
Laughlin of the University of California at Santa Cruz. Fortunately, the
chance of the inner solar system one day going haywire is only 1 in 100.June 12, 2009 at 3:25 pm #31697
Doesn’t the sun’s gravity assign how the planets orbit?June 14, 2009 at 2:28 am #31699
Yes but not necessarily all the time. The current planetary orbits are “fixed” due to the Sun’s gravity. But the Sun’s gravity doesn’t negate all or any other celestial bodies gravity including the other planets in the solar system. The Moon’s gravity is still felt hear on Earth as high tides and not negated by the Sun’s gravity.
The article in one part is saying if by chance Jupiter and Mercury’s orbits fall in resonance of the same time cycle, Jupiter’s gravity will have a greater effect and be able to pull Mercury enough to change it’s orbit. Possibly sending it into Venus (Maybe somewhat like bumper cars.).
Jupiter is the biggest object only second to the Sun in the solar system (Though still not as big as the Sun). During it’s formation if it had more mass it may have ignited fusion and became a star itself.June 14, 2009 at 12:02 pm #31701
To elaborate on Derek’s post above:
The simplest explanation of the movement of
planets from grade school is that each planet circles
the sun, i.e. sun at the center, and the planets are
in fixed circles around it due to the sun pulling on the
THIS IS NOT QUITE TRUE.
To be more accurate, each planet also is pulling on the
sun with it’s own gravity. Thus each planet doesn’t
orbit the sun, but both the sun and planet together
orbit their common center-of-gravity. Since the sun
is so large compared to the planet, the center-of-gravity
is located very close to the sun’s center.
Considering the sun and planet together in this fashion,
constitutes the “two-body” problem. In this description,
the planet’s trajectory is that of an ellipse (an oval-shape),
with the sun sitting at one of the two foci of the ellipse
(two focal points of the oval that are shortly away from the
center along the elongated centerline).
This is the standard description that most people understand
about the planets motions, following Kepler’s laws.
To make things more complicated, not all planetary ellipse paths
lie in one plane (i.e. they don’t look like a pancake with the
sun in the center), each have some slight tilt above or below the
OF COURSE, this description is NOT QUITE TRUE either.
Given a planet, say Earth, we don’t just have the Sun pulling
on the Earth and the Earth pulling on the Sun (two-body description),
but ALL the planets and the sun are all pulling on each other, i.e.
they are all experiencing the gravitational potential of the other
bodies to some degree. Thus our “two-body” problem becomes an
“N-body” problem, where N is the number of different objects involved.
See the attached link for N-body problem.
In this case, each planet and the sun . . . and all the moons . . .
all have a gravitational potential they are throwing into the party.
This problem has not been mathematically solved in generality to
determine the actual “correct” motions; there have only been certain
special cases that have been determined.
Of course, our solar system is not isolated either. The sun “orbits”
the core of our Milky Way Galaxy, as well as having interactions with
other nearby stars. The galaxy itself is moving through
the Local group of galaxies and so on.
With so many different interacting variables involved, the system becomes
chaotic–and we get the same situation as what we get when try to
model global climate change. One of the ways to handle such problems
is to run millions of computer simulations with slightly different
initial conditions that are all within current parameters, and then
do an averaging over all these runs to determine different possible
outcomes, each with an assigned probability of occurrence.
This is what the people in the article did.
Apparently, from this type of analysis, we eventually get planetary
collisions about 1% of the time; and apparently it appears that
in these situations, deviations in movement which lead to
these occurrences tend to be amplified where certain
planets happen to have aligned motions–such
as the Jupiter-Mercury example.
As Derek mentioned, certain large planets can have a significant
effect. Jupiter is 1/13 as massive (resp. Saturn is 1/43 as massive)
as what we would be necessary for the planet to be a brown dwarf . . .
i.e. a gaseous substar that is not massive enough for hydrogen fusion
to have ignited it to be a star
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