Talk:Terrestrial planet

pluto

So what about Pluto? It's clearly not a gas giant. So is it a terrestrial planet? Or is it a third class? Or do we not know?

Well, it's not listed in the article text but it's present in the image, along with a bunch of other bodies that are mainly ice rather than silicate (and also some that have active hydrospheres, directly contradicting the article text). I don't know the answer myself but we should either find out what the official line is or we should mention that there's ambiguity. Bryan 18:21, 28 Mar 2005 (UTC)

Pluto is, at the moment, considered a planet, and would technically be designated as a terrestial planet. However, there's so much debate over Pluto's status, especially since Sedna was discovered, that I think Bryan's idea of just mentioning an ambiguity is best. Also, I was thinking of maybe adding a list of exo-terrestial-planets, too, if nobody objects? So far, I have:

55 Cancri A e 14 ME

Gliese 777 A c 18 ME

mu Arae d 14 ME

GJ 436 b 21 ME

PSR 1257+12 A .020 ME

PSR 1257+12 B 4.3 ME

PSR 1257+12 C 3.9 ME

Gliese 876 d 6-8 ME.

I'm not sure how complete this list is, as I took these from Wikipedia's "list of extrasolar planets" and then added Gliese 876 d, which despite being mentioned on most exoplanet-related pages as the lowest-mass non-pulsar exoplanet, is not listed on the "list of extrasolar planets page." Whew. Long sentence, that. ZelmersZoetrop

Pluto (and anything that far out) would probably be better classified as a 'Kuiper Object' as technically the Kuiper Belt starts from inside Neptune's orbit. ref-Kuiper_belt.

Although the label 'Minor Planet' is adequate, i would hesitate to call Pluto 'Terrestrial' in the truest sense...

Ambiguity, as you say is probably the best course for now :) Grey Area 08:44, 29 July 2005 (UTC)

Conflicting data?

I am doing a project in school. We have to design a planet and the planet that my partner and i designed is completly water except for a few scattered volcanoes. Would animals like penguins and walruses be able to survive on a planet like this? Please answer ASAP!!!!!

See planetary habitability to begin with. I think that page will have the most data for you.

If your planet is only going to have volcanic islands then its probably not tectonically active, as this is the main continent building process. This means low biodiversity and a lesser chance that animals like penguins and walruses would arise. But nothing is impossible with a hypothetical planet. Perhaps in shallow pools surrounding your islands multi-cellularity arose, and after that the sky's the limit. Perhaps you can revisit the model and add in some continents? Marskell 13:50, 4 April 2006 (UTC)

Impact Cratering

What is the role that impact cratering had in history on the formation of terrestrial planets? —The preceding unsigned comment was added by 149.169.207.32 (talk • contribs) on 00:17, 1 September 2006.

I'm not sure I understand the question. Cratering itself doesn't affect the formation much; it's usually just used as a yardstick to measure how active the geology of a planet or moon is (if it's geologically active, it recycles its surface material, reducing the number of visible craters; if it was active but then stopped being so, the crater density can give you a rough idea of how long ago this happened).

The material that's delivered in the process of bombardment is, however, important, as it's how a lot of the volatiles posessed by the inner planets got here as the solar system was forming (how much this changed the amount of volatiles is an open question, though). --Christopher Thomas 05:14, 1 September 2006 (UTC)

Intrinsic Definition ?

In the good old days, when I first learned science, all living things were either animals or plants, and there were four terrestrial planets, four gas giants, plus Pluto with a question mark. Those old dichotomies were clearly inadequate. If the definition of terrestrial planet is intrinsic, i.e., depends only on the physical characteristics of the planet itself, then the Moon must be called a terrestrial planet. The definition does not make it clear whether having a rocky surface is a requirement. What if a planet is almost all rock and iron entirely covered with a few miles of ice? What if it's rock and iron, covered with an inpenetrable atmosphere of hydrogen and helium? What about Titan, which is a planet by any intrinsic definition? And what about Io? I suspect that there will be lots of overlapping classes of planets defined in the next generation, but that consensus on this issue will not soon be achieved. Vegasprof 11:19, 30 April 2007 (UTC)

Is Ceres a terrestrial body?

Judging by its low density (2.0g/cm³, Ceres should really be called an ice dwarf and not a terrestrial (dwarf) planet.

That density is much lower than that of the Moon, or Jupiter's inner large moons Io and Europa. If Ceres is a terrestrial (dwarf) planet, what about the Moon, Io and Europa??

luokehao

Terrestrial planets

terrestrial planets include the folowing....... Mercury, Earth,Mars, and Venus so by by now and so long farewell aveters and goodbye!!!!! —Preceding unsigned comment added by Setoguchi16 (talk • contribs) 07:05, 17 December 2007 (UTC)

Most Earthlike exoplanets

What's this table doing in the article? Most of the planets in there have masses comparable to Jupiter and are probably NOT terrestrial planets! —Preceding unsigned comment added by 131.111.8.102 (talk) 11:47, 21 January 2008 (UTC)

Uncompressed Densities

I think a table of compressed and uncompressed densities of the 4 inner planets and the moon would make a nice addition to this page.

Object	mean density	uncompressed density
Mercury	5.4 g/cm³	5.3 g/cm³
Venus	5.2 g/cm³	4.4 g/cm³
Earth	5.5 g/cm³	4.4 g/cm³
Moon	3.3 g/cm³	3.3 g/cm³
Mars	3.9 g/cm³	3.8 g/cm³

I haven't found an authoritative source for these numbers or a formula to relate the density, mass and uncompressed density. So far I've found this source http://geophysics.ou.edu/solid_earth/notes/planets.html#densities but I don't believe it is original.

This is my first Wikipedia addition. Please let me know if there are things I should do to tidy up the addition. I'm still in search of a good source for the uncompressed density calculation. The uncompressed density of Ceres was an assumption based on the trend of compressed to uncompressed densities as the mass decreased.

And what is "uncompressed density"? —Tamfang (talk) 16:50, 27 October 2010 (UTC)

A planet is squeezed by its own gravity: the deeper layers are compressed by the weight of the overlying layers. This increases the density of the planet. The uncompressed density is the (lower) density that the planet would have if this gravitational squeezing did not occur. The reason one would want to estimate a planet's uncompressed density is that this gives a hint about what the planet is made of. A higher uncompressed density suggests a larger abundance of heavier elements such as iron.

Terrestial planet size

Is there a maximum size (allowing for density, composition etc) which a terrestial planet can be? If so what would happen as the limit is reached (assuming that this is smaller than that required to produce a black hole? Jackiespeel (talk) 17:30, 29 July 2008 (UTC)

Solar terrestrial planets: Planemo

I don't think the use of this term is appropriate. The planemo article says hardly anyone uses it, so I think it is not right here. Also, why apply the term to only two of the moons, all "rounded bodies" qualify for the term. HarryAlffa (talk) 18:16, 1 June 2009 (UTC)

Fusing star?

In the section on extrasolar planets, the term "fusing star" is used and linked to another article ("Solar Nucleosynthesi", I think?). The other article does not explain the term "fusing star". I recommend changing the term "fusing star" for the title of the other article. --Eddylyons (talk) 23:30, 14 August 2009 (UTC)

It refers to a star that's still undergoing fusion (as opposed to stars that have burned out). It's actually much easier to detect planets around neutron stars than around ordinary stars, so the first few extrasolar planets discovered were around "dead" stars. I've tweaked the wording to make this clear. --Christopher Thomas (talk) 23:41, 14 August 2009 (UTC)

There's another mention of a fusing star in the article. Would it be better to explain what a fusing star is?--Eddylyons (talk) 02:05, 17 November 2009 (UTC)

I don't really see how better to phrase it. A "fusing star" is a "star in which fusion is occurring". This seems pretty clear. You could call it a "main-sequence star", but a) that'd make less sense to someone who wasn't already familiar with stars, and b) that isn't strictly true (red giants aren't on the main sequence but are still undergoing fusion). The closest simplified description would be "star that's still alive". I'd expect to see that phrasing on the Simple English Wikipedia, but I don't feel it's appropriate for the main English Wikipedia.

Also, given that "star still undergoing fusion" is used in the paragraph before the one containing "fusing star", I'd think its meaning would be obvious enough from context even if one had trouble seeing what "fusing star" meant in isolation. Is the passage really that unclear as-is? --Christopher Thomas (talk) 02:40, 17 November 2009 (UTC)

I see where you're talking about now. Just my opinion, the lay reader (such as myself, admittedly) would be more familiar with the fusing star (or ordinary star as you said above) as the rule not the exception. When I think of star, my mind's eye thinks of a star, not whether it's fusing, dead, neutron or pulsar. I hesitate to use the term "regular" or "normal" star, but that's what I'm getting at. Using the term fusing star makes it sound like it's an exception to what the lay reader would come to expect. It's not tidy, but maybe "ordinary, main-sequence star"? Or "...fusing star (not a pulsar)..."? --Eddylyons (talk) 16:21, 17 November 2009 (UTC)

inner planet

Why does the first sentence of the article regard 'inner planet' as an synonym of 'terrestrial planet'? The fact that all terrestrial planets in our solar system are inner planets (that is, planets between the sun and de asteroid belt) says nothing about the situation in other planetary systems. As a result, the current article is wrong to suggest that alle terrestrial exo-planets are inner planets too. DaMatriX (talk) 22:49, 15 December 2009 (UTC)

The term "inner planet" was added to the intro in Nov 2008. It is easy enough to fix. Though as of 2009 no exo-planets have been confirmed to be terrestrial. --- Kheider (talk) 23:09, 15 December 2009 (UTC)

Is not Titan also a terrestrial world?

--MathFacts (talk) 06:30, 20 December 2009 (UTC)

Under Terrestrial_worlds, I believe any required wiggle room can be changed by changing "referred to as geophysical planets" to "referred to as geophysical worlds", if need be. Though I am going to try and think about this for a day. I also did not realize the solar terrestrial planets section contained a discussion about worlds when I made my first edit. Hmm walked into a can of worms? -- Kheider (talk) 10:01, 21 December 2009 (UTC)

"Terrestrial worlds" section

I've removed the following section from the article:

It is not uncommon for natural satellites that are in hydrostatic equilibrium to be referred to as terrestrial worlds. The seven moons that are occasionally referred to as terrestrial worlds are: Earth's Moon, Io, Europa, Ganymede, Callisto, Titan, and Triton. Planetary scientist Alan Stern has informally suggested such bodies can be referred to as geophysical planets. There are 19 known satellites that meet the geophysical requirement of a planet, though since they orbit planets they cannot be considered planets themselves. The question is should some of these objects be considered as evolved icy bodies rather than terrestrial bodies? Titan looks and behaves more like Earth than any other body in the Solar System. Titan is known to have stable pools of liquid on the surface.

• 
  Titan showing surface and atmospheric details
• 
  Volcanos on Io constantly re-surface the satellite
• 
  Europa is believed to have a subsurface ocean
• 
  False color image of Ganymede
• 
  A cloud over the limb of Triton

In addition, Earth's moon and Jupiter's satellites Io and Europa can also be regarded as terrestrial worlds. Io and Europa have mainly rocky compositions despite forming beyond the frost line. This may be because the region of the circum-Jovian disc in which they formed was kept too warm by radiation from the proto-Jupiter to contain large quantities of icy material.

As mentioned on Kheider's talk page, this appears to be putting too much weight on an unofficial, informal forum post, and needs much more in the way of verifiable sourcing. At present, it is a case of undue weight (unless, again, more sources are provided. I'd suggest that the best course is to develop the text here, rather than in the artcile, until agreement is reached as to the best approach. --Ckatz^chat_spy 10:28, 21 December 2009 (UTC)

Dictionary.com defines Terrestrial as representing the earth as distinct from other planets. So that would (IMHO) include Titan (most Earth-like body), Io (with active volcanos), and Europa (with a probable subsurface ocean). Now the question becomes what to call Ganymede and Callisto with their possible subsurface water, and Triton like bodies that have a thin atmosphere.

Dictionary.com defines World (using definition #15) as any heavenly body, of course definition #1 states, " the earth or globe, considered as a planet.". Isn't semantics fun?

We could also title the section "Worlds with Earth-like characteristics", but I do believe we are doing an injustice if we do not addreess this issue. -- Kheider (talk) 19:35, 21 December 2009 (UTC)

Others have their own definition. :) Ruslik_Zero 14:36, 22 December 2009 (UTC)

"Terrestrial" is a good word IMO, and AFAIK the one that is most frequently used for such things. However, there is no dividing line between 'rocky' and 'icy': Ceres is widely considered terrestrial, yet may be half ice. So is Callisto terrestrial? Titan? By the time you get to Pluto, 'terrestrial' would no longer seem to apply. Perhaps we could word it in such a way as to be clear that the concept is inherently ambiguous. kwami (talk) 20:18, 23 December 2009 (UTC)

Dr. James Schombert has also stated, (at bottom of page) "Large amounts of outgassing have drained the inner moons, Io and Europa of their icy materials making them rich in rocky materials." I agree that we need to make it clear that the concept is inherently ambiguous nor does it have an official definition. When does something go from rocky to icy? How do you define Earth-like? In the pre-voyager era, Earth-like pretty much meant non-gas giant since we knew almost nothing about the large moons of the solar system. -- Kheider (talk) 21:12, 23 December 2009 (UTC)

Exo-water planets could result from inward planetary migration and originate as protoplanets that formed from volatile ice-rich material beyond the snow-line but that never attained masses sufficient to accrete large amounts of H/He nebular gas. Water worlds might be thought of as a bigger and hotter version of Jupiter's Galilean moons. -- Kheider (talk) 01:56, 24 December 2009 (UTC)

"Most Earthlike exoplanets"

According to that table, Gliese 581c is smaller than 581e. But the latter is much lighter AND closer to the star (roughly implying higher density), so shouldn't 581e be the smaller one of the two? --Roentgenium111 (talk) 23:38, 3 August 2010 (UTC)