Planetary Physics Question

Hi,

What determines the upper limit to the size of a terrestrial Earthlike
planet
(i.e., one with a solid surface and thin atmosphere)?

There was a recent announcement as to the possible discovery of a
"Super-Earth" planet with a mass of about 14 Earth masses. Is this
near the upper mass limit for an Earthlike planet?

Thanks

[[Mod. note -- You might also try posting over in sci.space.science,
as a number of planetary-science experts seem to regularly read/post
there. -- jt]]

In article , "R. Mark Elowitz"
wrote:


"R. Mark Elowitz" :

What determines the upper limit to the size of a terrestrial Earthlike
planet
(i.e., one with a solid surface and thin atmosphere)?

There was a recent announcement as to the possible discovery of a
"Super-Earth" planet with a mass of about 14 Earth masses. Is this
near the upper mass limit for an Earthlike planet?


I'm not a terrestrial planet modeler, but I read the literature,
and the article that I've seen that best describes the process of
terrestrial planet formation was published last April in Physics
Today:

[1] "Origin of Terrestrial Planets and the Earth-Moon System"
by Robin Canup, pgs. 56-62.

[[Mod. note -- Some of the planetary-science articles from that issue
seem to be available free from the Physics Today web pages, but others
(including Canup's) seem to require a subscription):
http://www.physicstoday.org/pt/vol-5.../contents.html
-- jt]]

More details on terrestrial planet formation can be found online
in the KITP seminars. These are particularly relevant:

[2] Terrestrial Planet Planet Formation in BInary Star Systems
by Jack Lissauer
http://online.itp.ucsb.edu/online/pl...uer/oh/01.html

[3] Implications of Planet Formation Models for the Initial State
of the Earth by David Stevenson
http://online.itp.ucsb.edu/online/pl...son/oh/01.html

[4] Two Fluid Flights of Fancy: From Dust to Planetesimals
by Andrew Youdin
http://online.itp.ucsb.edu/online/pl...din/oh/01.html

see also:
J.E. Chambers (2001), "Accretion in the Solar System" Icarus 152 205.


Terrestrial planet accretion in our solar system is typically
described in three stages [1, 2]:


I. Early Stage:

Growth of dust grains (about micron) -- planetesimals (1-10 km)
Timescale: Fast [3]

This early stage is the least well-understood process [1],
coagulation (sticking) is difficult, gravitational instability is
hard [4].

II. Middle Stage:

Growth by accretion of planetesimals -- planetary embryos (1000s km)
Timescale: ~ 10^7 years [1, 3]

Canup's paper describes this part well. She says:

This next stage is much better understood due to extensive modeling
work by the theoreticians. The rate of accretions (and hence the
growth of the planetesimals) is controlled by the rate of collisions
among the orbiting planetesimals in the solar nebula.

The rate of collisions depends on the local orbital velocity (which
increases with decreasing distance from the Sun, so that regions
closer to the Sun generally accrete more rapidly), the number
density of the planetesimals and their sizes and relative
velocities. A source of velocity damping for the smaller objects is
the gaseous nebula.

There is a possible 'runaway growth' that can happen, due to the
largest objects growing the fastest, with a single object running
away with most of the available mass in its annular region in the
solar nebula disk. In this case, then an object of roughly 1% of
Earth mass can grow in as little as 10^5 years.

Eventually we run out of solar nebula material. so that becomes is
our final limiting factor for growth at this stage.


III. Late Stage:

Collision of tens to hundreds of embryos to yield -- planets

Occurs in the absence of the solar nebula, but not always.

Current modeling work suggests that solid planets are 'sculpted' by
a violent, stochastic final phase of giant impacts. She says in [1]
that a seemingly inherent feature of the late stge is giant impacts,
in which lunar-to-Mars-sized objects mutually collide to yield the
final few terrestrial planets. (Canup has worked extensively on the
Earth-Moon formation process, that occured at this stage.)

The bodies come from outside of the original accretion zone. Note
that in our own solar system, these events were strongly influenced
by Jupiter's gravity. The implication of this last stage is that our
terrestrial planets (and Moon) may only represent one possible
outcome in a wide array of potential solar-system architectures.[1]



In summary, I don't know if your question can be answered clearly,
there are many many variables. I hope this helps.


Amara

--

************************************************** *****************
Amara Graps, PhD |Istituto di Fisica dello Spazio Interplanetario
| INAF, Rome, Italy
www.amara.com | http://www.mpi-hd.mpg.de/dustgroup/~graps
************************************************** ****************
"We came whirling out of Nothingness scattering stars like dust."
- Rumi