Talk:Terrestrial planet

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Pluto[edit]

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 dwarf 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)[reply]

Geophysically, Pluto is an icy terrestrial planet, in the same class as Triton. Double sharp (talk) 08:30, 8 September 2021 (UTC)[reply]

Intrinsic Definition ?[edit]

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)[reply]

Some would say a rocky surface is required, in which case "icy planets" like Pluto (geophysically) are not terrestrial. Some would use "Terran" to include such bodies too, since the key thing is that they have not grabbed a large gaseous envelope.
"What if a planet is almost all rock and iron entirely covered with a few miles of ice?" – that's basically Europa if it had a bit less ice. That's sometimes called icy (because of the surface) and sometimes terrestrial (because of the interior).
"What if it's rock and iron, covered with an inpenetrable atmosphere of hydrogen and helium?" – not too different from the gas planets (at least we think).
Yes, by reasonable geophysical definitions Luna and Io must be terrestrial. Titan would be icy (which may or may not be a subclass of terrestrial). Double sharp (talk) 08:35, 8 September 2021 (UTC)[reply]

Is Ceres a terrestrial body?[edit]

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

This distinction is now in the article. :) Although the line between rocky and icy is not as clear as might be hoped. Eris is actually denser than Ganymede! Double sharp (talk) 16:18, 4 August 2021 (UTC)[reply]

Terrestial planet size[edit]

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)[reply]

A terrestrial planet cannot be larger than about 10 Earth masses (or ≈2.2 Earth radii) according to current thinking. Above this limit (maybe earlier) it will attract enough hydrogen to turn into a gas giant planet. But that's still far below the size needed for a black hole, it's not even enough to form a star. --Roentgenium111 (talk) 17:42, 23 June 2011 (UTC)[reply]
And according to newer research, likely the limit is about 2 Earth masses only in general. And then indeed a hydrogen envelope should form, although the result would be more ice giant (like Uranus and Neptune) than gas giant. Exception would be those planets so close to their stars that they cannot form this envelope. But Roentgenium111's point stands, of course. Double sharp (talk) 09:32, 4 August 2021 (UTC)[reply]

Why have the Moon and Ceres in the lead picture?[edit]

What exactly is the purpose of that? I replaced it with the exact same picture, minus the moon and ceres, with a much shorter caption where we dont have to explain that the moon and ceres are in fact not planets. The one minus those two is superior and less confusing. Cadiomals (talk) 00:05, 24 September 2012 (UTC)[reply]

Others may agree with you, but I happen to think it's more interesting to have a couple of smaller bodies for contrast, and the caption is perfectly clear. Rothorpe (talk) 00:10, 24 September 2012 (UTC)[reply]
I don't think it's about what certain people deem to be interesting. One man's trash is another man's treasure. Images in Wikipedia are meant not just to be interesting but to educate and convey facts. Eliminating these two non-planetary bodies would eliminate the need for a long caption explaining that these bodies are in fact not considered planets. If you don't mind I would like to replace that picture with an alternate minus those two, as it is more crystal clear than any caption could be. Cadiomals (talk) 01:00, 24 September 2012 (UTC)[reply]
Since the moon and Ceres are excellent examples of what small "terrestrial" planet-like-bodies look like, I think it is better with them included. -- Kheider (talk) 11:28, 24 September 2012 (UTC)[reply]
I tend to agree with Cadiomals; the lede image of the article should stick closely to the subject, and Moon and Ceres are no terrestrial planets, not being planets. I think I wouldn't even call Ceres a "terrestrial" object, since it has such a large proportion (~25%) of water ice. (Vesta is presumably more "terrestrial" than Ceres.) --Roentgenium111 (talk) 19:07, 24 September 2012 (UTC)[reply]
Yes, it's likely that Ceres will turn out to be not very terrestrial, but it's a commonly given example. Both it and Luna are "planets" according to Stern, though that's a small-minority position. — kwami (talk) 09:01, 31 December 2013 (UTC)[reply]

I would actually have liked this picture if it had Vesta instead of Ceres. It is often called the smallest terrestrial planet, after all! Double sharp (talk) 11:10, 14 August 2021 (UTC)[reply]

Rocky planets may orbit double stars[edit]

Recently, a few known gas-giant uninhabitable planets orbit double stars. Scientists believe that rocky planets do not do so. Now, scientists believe that, it is possible for rocky planets too. Read more on this website:

http://phys.org/news/2015-03-rocky-planets-orbit-stars.html

MansourJE (talk) 09:20 31 March 2015 (UTC)

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