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Location: UFOUpDatesList.Com > 2011 > Aug > Aug 10

Moons Not Needed For Habitable Planets

From: Ray Dickenson <r.dickenson.nul>
Date: Tue, 9 Aug 2011 19:19:49 +0100
Archived: Wed, 10 Aug 2011 06:04:44 -0400
Subject: Moons Not Needed For Habitable Planets

Source: Space,Com


09 August 2011

Moonless Earth Could Potentially Still Support Life, Study Finds

Scientists have long believed that, without our moon, the tilt
of the Earth would shift greatly over time, from zero degrees,
where the Sun remains over the equator, to 85 degrees, where the
Sun shines almost directly above one of the poles.

A planet's stability has an effect on the development of life. A
planet see-sawing back and forth on its axis as it orbits the
sun would experience wide fluctuations in climate, which then
could potentially affect the evolution of complex life.

However, new simulations show that, even without a moon, the
tilt of Earth's axis - known as its obliquity - would vary only
about 10 degrees. The influence of other planets in the solar
system could have kept a moonless Earth stable

The stabilizing effect that our large moon has on Earth's
rotation therefore may not be as crucial for life as previously
believed, according to a paper by Jason Barnes of the University
of Idaho and colleagues which was presented at a recent meeting
of the American Astronomical Society.

The new research also suggests that moons are not needed for
other planets in the universe to be potentially habitable.

As the world turns

Due to the gravitational pull of its star, the axis of a planet
rotates like a child's top over tens of thousands of years.
Although the center of gravity remains constant, the direction
of the tilt moves over time, or precesses (as astronomers call

Similarly, a planet's orbital plane also precesses. When the two
are in synch, the combination can cause the total obliquity of
the planet to swing chaotically. But the gravity of Earth's moon
has been shown to provide a stabilizing effect. By speeding up
Earth's rotational precession and keeping it out of synch with
the precession of Earth's orbit, it minimizes fluctuations,
creating a more stable system.

As terrestrial moons go, Earth's moon is on the large size -
only about a hundred times smaller than its parent planet. In
comparison, Mars is over 60 million times more massive than its
largest moon, Phobos.

The difference is substantial, and with good cause - while the
Martian moons appear to be captured asteroids, scientists think
that Earth's moon formed when a Mars-sized body crashed into the
young planet, blowing out pieces that later consolidated as the
lunar satellite - a satellite which affects the planet's tilt.

Scientists estimate that only one percent of any terrestrial
planets will have a substantial moon.  This means that most such
planets are expected to experience massive changes in their

The pull of the planets

While Earth's moon does provide some stability, the new data
reveals that the pull of other planets orbiting the sun -
especially Jupiter - would keep Earth from swinging too wildly,
despite its chaotic evolution. [10 Extreme Planet Facts]

"Because Jupiter is the most massive, it really defines the
average plane of the solar system," said Barnes.

Without a moon, Barnes and his collaborators have determined
that Earth's obliquity would only vary 10 to 20 degrees over a
half a billion years.

That doesn't sound like much, but the changes of 1 to 2 degrees
the planet presently exhibits are thought to be partly
responsible for the Ice Ages.

According to Barnes, the present shift is "a small effect, but
in combination with Earth's present climate, it causes big

Still, a 10-degree change is not a huge problem when it comes to
life. "(It) would have effects, but not preclude the development
of large scale, intelligent life."

Furthermore, if Jupiter were closer, Barnes explains, the
Earth's orbit would precess faster, and the moon would actually
make the planet fluctuate more wildly, rather than less.

"A moon can be stabilizing or destabilizing, depending on what's
going on in the rest of the system," he said.

The benefit of a backspin

The team also determined that planets with a retrograde, or
backward, motion should have smaller variations than those that
spin in the same direction as their parent star, a large moon

"We think the initial rotation direction should be random,"
Barnes said. "If it is, half the planets out there would not
have problems with obliquity variations."

What determines which way a planet spins? He suspects that
"whatever smacks the planet last establishes its rotation rate."

A 50/50 shot at retrograde precession, combined with the
likelihood of other planets in the system keeping the planet
from tipping on its side, means more terrestrial planets could
be potentially habitable. Barnes ventured an estimate that at
least 75 percent of the rocky planets in the habitable zone may
be stable enough for life to evolve, though he notes that
additional studies are needed to confirm or disprove that.

In comparison, the previous idea that a large moon was necessary
for a constant tilt meant that only about 1 percent of
terrestrial planets would have a steady climate.

"A large moon can stabilize (a planet)," Barnes said, "but in
most cases, it's not needed."

This story was provided by Astrobiology Magazine, a web-based
publication sponsored by the NASA astrobiology program.

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