[meteorite-list] Worlds in Collision
From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Thu, 25 Sep 2008 18:01:11 -0500
The press release says:
> collisions of Mercury with Earth or Venus sometime
> in the next billion years or more. Of course by then the
> sun will have expanded and we might be toast anyway.
Not quite. When the Sun formed 4.5 billion years ago,
it was only 70% as bright as now, 90% of its present
diameter, and about 250 K. cooler. The Sun has been
getting bigger and brighter ever since, and will continue
to do so for a long time.
About 1.1 billion years from now, it will be 10% brighter,
and that would be enough to start a runaway Greenhouse
that would eventually cause the loss of all the Earth's water
out the top of the atmosphere. Of course, that far in the
future, any intelligent life will easily be able to orbit a sun
screen and prevent the developing catastrophe...
By 3.5 billion years from now, when the Sun gets to 40%
brighter than present, all the Earth's water would be gone in
runaway Greenhouse if there's no intelligent life to prevent
it. (And if there's no intelligent life left, does it matter?) The
Earth would now resemble present-day Venus very closely.
6.35 billion years from now, the Sun's core runs out of
hydrogen to burn, and the Sun starts to expand, burning
the final outer layer of hydrogen. At the point the Sun
starts expanding, it's 2.2 times brighter than today and
58% bigger. It turns into a Red Sub-Giant. Over the
next 0.7 billion years, it gets 130% bigger but no brighter
and actually cools off. The solar wind intensifies.
Over the next 0.6 billion years, it will lose 28% of its mass,
become 2350 times brighter and 166 times bigger than it is
today, and cool down to 3100 K., 7.65 billion years from now.
The sun will swallow Mercury, and fry the other inner planets,
even though all that solar wind will have pushed them much
further out than they are now.
The solar system does get fried, but not for quite a while;
it takes longer than they say.
After this, the Sun's life gets really complicated. There's a
"Helium Flash" when it gets hot enough to burn helium, it
shrinks and heats up, then expands and cools, uses up all
the helium, starts losing mass faster, then blows off much of
its mass in a series of short fast pulses, and collapses into
a hot little White Dwarf star with about 54% of its present
mass and a diameter about the size of the Earth! There might
be a short-lived nebula to delight the amateur astronomers
of other species in this neck of the galaxy. I hope it's pretty.
The White Dwarf Sun will cool and shine for the next 100 billion
years and the next 100 billion after that and so forth, ever fainter,
for a trillion years or so... But, for the next 5 or 6 billion years,
we should be OK, assuming good Planetary System Management
and Maintenance. (You gotta do better than we are now, though...)
Therefore, avoiding collisions of the existing planets over the
next billion years is important, yes, but it's not good enough.
I have no idea (well, some ideas) of how you would fiddle
with the dynamics of a solar system to avert collisions but
I bet, a hundred million years from now, Earth Life (if there
is any) will be able to manage it, just as we hope that in a
century or two, we will be able to "manage" to avoid those
pesky big asteroid collisions.
Then, the authors are assuming that the collision of two
independent Earth-sized planets is the only way to produce
this much dust and debris. There is another way. Probably
rare, but far from impossible, are double planets. The Earth
and Moon have been called that, but the Moon only masses
1.25% of the Earth's mass. Pluto and Charon (11% of Pluto's
mass) are a much better example (and would have been so
defined... well, almost).
Imagine a 1.35 Earth mass planet with a 0.65 Earth mass
moon, in a temperate place in a solar system, with oceans
of volatiles and all the rest of it. The tidal forces generated
on such a locked satellite would be enormous. The smaller
body would first become tidally locked to the larger, then
the tidal drag would slow the rotation of the larger planet,
transferring momentum to the orbit of the smaller planet and
"raising" its orbit.
Even the piddling force of the lunar tides will drive our
Moon away eventually, but a high-mass satellite might well
have its orbit "pumped up" to the point where it would
escape or be perturbed free, in only a handful of billions
of years. Then, there would be TWO planets sharing nearly
identical orbits. This never turns out well.
The short-term probability of collision is close to 100%.
There are some escape scenarios: the smaller planet is ejected
into an eccentric orbit but such orbits have a high probability
of future collision, or possibly into a Trojan orbit, also not
stable over the long term. Once any collision occurs, total
rubblization of the planets involved (and any planets that
stood too close) is certain.
Another large-moon disaster is a large retrograde double
planet. The tidal interaction takes momentum from the smaller
planet's orbit and the orbit shrinks until it "crashes." Yes, the
Moon falls from the sky! Aren't you glad we don't have a
retrograde satellite? Perhaps all warm terrestrial double planets
everywhere are a disaster waiting to happen.
And, lastly, all simulations of solar system formation from the
earliest days to the latest most sophisticated supercomputer
models produce a certain percentage of solar systems with a
large number of small terrestrial planets (and no big ones), like
a gigantic zone of very large asteroids. A solar system with
10-20 planets 3000 to 8000 kilometers in diameter crammed
into its inner system doesn't last very long, a few billion years,
and once collisions start, the large pieces precipitate new
collisions, and so on... until there's nothing left but a zone of
dust and debris.
There is more than one way for a solar system to go wrong.
To paraphrase what Tolstoy said about families: "All happy
solar systems are happy in the same way, but all unhappy
solar systems are unhappy in their own way."
Sterling K. Webb
----- Original Message -----
From: "Darren Garrison" <cynapse at charter.net>
To: "Meteorite Mailing List" <meteorite-list at meteoritecentral.com>
Sent: Thursday, September 25, 2008 12:04 AM
Subject: [meteorite-list] Worlds in Collision
Huge Planetary Collision Left Tons of Space Debris
Around a distant star, two planets similar to Earth collided and were
astronomers said today.
The somewhat speculative scenario is based on the leftovers: a ring of
around the star that includes a million times more dust than now circles our
"It's as if Earth and Venus collided," said researcher Benjamin Zuckerman,
professor of physics and astronomy. "Astronomers have never seen anything
this before. Apparently, major catastrophic collisions can take place in a
mature planetary system."
The researchers used X-ray data and other observations of a star called
307. They had assumed it was a young star, just a few hundred million years
and the debris was leftovers from planet formation.
But earlier this year, another study showed the star was actually a binary
and that the stars were billions of years old.
So why all the debris? The dust is about the same distance from the stellar
as Earth is from the sun, and given current theories of planet formation,
debris should have been swept up into planets by now or pushed away by
radiation. It simply shouldn't be there.
A colossal collision must have created all that dust sometime in the past
hundred thousand years and perhaps much more recently, the astronomers
It would have been a whopper.
"If any life was present on either planet, the massive collision would have
wiped out everything in a matter of minutes - the ultimate extinction
said Gregory Henry, an astronomer at Tennessee State University (TSU) who
with Zuckerman on the research. "A massive disk of infrared-emitting dust
circling the star provides silent testimony to this sad fate."
To put the collision into context, Zuckerman said: "By contrast with the
crash in the BD+20 307 system, the collision of an asteroid with Earth 65
million years ago, the most favored explanation for the final demise of the
dinosaurs, was a mere pipsqueak."
The work was funded by the National Science Foundation and NASA, and also by
and the State of Tennessee. It will be detailed in the December issue of the
The conclusion has the astronomers thinking about home.
"This poses two very interesting questions," said TSU astronomer Francis
"How do planetary orbits become destabilized in such an old, mature system,
could such a collision happen in our own solar system?"
It has already happened here, in fact. Our moon is thought to have been
when a Mars-sized object slammed into Earth.
Henry points out that computer models done by other researchers suggest that
planets in our solar system migrate over time, there is a "small probability
collisions of Mercury with Earth or Venus sometime in the next billion years
Of course by then the sun will have expanded and we might be toast anyway.
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Received on Thu 25 Sep 2008 07:01:11 PM PDT