Is Mercury a Former Gas Giant?
windbag
close an orbit as Mercury when stellar radiation reaches different
levels?
Could the shockwave of radiation actually remove a gas giant's
atmosphere -- if it were close enough to the emerging sun -- leaving
behind the core?
Jacob Krolo
"windbag" <jaunty.a...@gmail.com> wrote in message
news:38f0e5bb-8f2e-494d...@k1g2000prb.googlegroups.com...
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Obviously, Mercury is tidally locked to the Sun, but, if we would like to
speculate about its previous life we
should re-examined our views about, so-called "established theories of
planetary evolution".
Before the discovery of the 51 Pegasi b, a Jupiter-class planet (0.468 Mass
of Jupiter) which
was confirmed in 1995., it was entrenched theory (created on one example) of
planetary evolution allowing to
giant planets to form at distances greater than several AU from the parent
star. That's meaning, narrow-minded
perceptions presumed, at about the distance of Jupiter from the sun.
51 Pegasi b orbits its host star in just 4.23 days (which is only 4.8
percent of the period of 88 days, what Mercury
need to complete a trip around the Sun) at distance of 0.05 AU, (7.5
million kilometres, 4.687 million miles), which
is much closer than Mercury orbits our Sun (12.8 percent Mercury's distance
in his elliptical orbit of 47 million km to
70 million km from the Sun).
That discovery was the proof that astronomical community at the time, didn't
have a slightest idea (regarding
planetary evolution) what's really going on over there.
At least that forced them to re-examine their theories J
How is possible to planet of this size could finished at this proximity to
the parent star?
Is it formed at its present position or in the outer regions of the system
and then interacted with a second giant planet
and migrated to its current position. Who knows? Your guess is good as
anybody else's and you are entitled to
come-up with your own "plausible" theory like some posters did in previous
treads.
However, in comparison to planet Mercury, 51 Pegasi b is much closer to its
parent star and
its thick atmosphere is not blown away by the solar wind, only because of
the gravitational force of the gas.
If Mercury ever had a "sufficiently massive atmosphere" it would be much
easier to retain it.
Obviously, that would be valid if Sun is constant as it is at it's present
stage. In later evolutionary stages, in next few
billion years, when Sun runs out of hydrogen fuel, will swell into its 'red
giant' stage. It's easy to imagine that in the
process of expansion would incinerate, not only Mercury and its presumed
"sufficiently massive atmosphere", but
the rest of the inner rocky planets.
Jacob
windbag
Jacob Krolo
> The age of 51 pegasi could be an issue.
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Applying the "Theory of stellar structure and evolution", scientists
(astronomers) have been
able to calculate that the Sun is 4.5 billion years old, which is close to
the figure of 4.6 billion
years, for the rest of the solar system.
Using the same "Theory of stellar structure and evolution", astronomers
calculated that the
51 Pegasi (yellow dwarf star) is older (7.5 billion years) than the Sun.
What is most important for us in order to try to find the answer on
question, how is possible
for gas giant 51 Pegasi b, like Mercury tidally locked and so close to its
parent star, to retain
the atmosphere without having been stripped away.
I mentioned already that gravitational forces of the gas, actually prevented
atmosphere not to
be blown away.
Surprisingly, recent calculations after planetary atmospheres temperature
measurements, indi-
cated that planet under such a conditions, could hold to its Hydrogen/Helium
atmosphere
over the period of the lifetime of its parent star. In the same period,
loosing only about 5%
of its atmosphere.
Once again, if Mercury ever had a "sufficiently massive atmosphere", I'm of
opinion that it
would be possible holding it to the present time.
Jacob
windbag
away in a solar system, regardless of proximity, star age or type?
David Williams
-> away in a solar system, regardless of proximity, star age or type?
Yes. If a significant fraction of the molecules of any gas in the
atmosphere are moving with speeds approaching the gravitational escape
velocity of the planet, then some of them will escape into space, and
that gas will be lost from the atmosphere. So the higher the
temperature, the lighter the molecules, and the weaker the planet's
gravity, the faster the gas will escape.
dow
windbag
that its current mass should now be considerably less than it had been
when this system was in its infancy.
Jacob Krolo
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Yes, some of the gas will escape into space, but obviously there is
another aspect which should be considered!
Not only the planet's gravity, but in addition a gravitational forces
of the
gas, preventing atmosphere not to be significantly depleted during the
time.
Jacob
David Williams
-> Yes, some of the gas will escape into space, but obviously there is
->
-> another aspect which should be considered!
->
-> Not only the planet's gravity, but in addition a gravitational forces
-> of the
->
-> gas, preventing atmosphere not to be significantly depleted during the
-> time.
->
-> Jacob
->
The gaseous atmosphere is part of the planet! Yes. Molecules near the "top"
of the atmosphere would feel gravity from the rest of the atmosphere as
well as whatever solid or liquid material may be below it, and this would
increase the local escape velocity, making it less likely that the
molecules will escape.
The rate of escape is never zero. There are always a few molecules that, by
chance, are moving at more than the escape velocity. But the rate of escape
is very small if the escape velocity is much greater than the average speed
of the molecules. I think I remember seeing that, if the escape velocity is
more than twice the average velocity, then the rate of escape is so low
that the atmosphere will last for billions of years.
Let's see if I can figure something out in my head. The escape velocity
from a giant planet such as Jupiter is something like 50 km/sec. At "room
temperature", molecules of air in the Earth's atmosphere move at something
like 0.3 km/s (the speed of sound). Hydrogen molecules weigh about 1/16 as
much as air molecules, so they will move 4 times faster, i.e. at 1.2 km/s,
at 300 K. If the temperature is such that hydrogen molecules move at 25
km/s or slower, a hydrogen atmosphere will be retained for billions of
years by a Jupiter-like planet.
So the velocity can be 20 times higher than it is at 300 K, so the
temperature an be as high as 300 x 20^2 kelvins, which is more than 100,000
K. Even if the atmosphere of the planet is heated by a "solar wind", it's
not likely to get that hot, even very close to a star. So, yes, the
atmospheres of giant planets very close to stars can last for long periods
of time.
This is all very approximate, of course, but I think it's in the right
ball-park.
dow