I'm trying to avoid an opinion-based question, so before I outline the comparison I'm proposing, I will qualify the specific facts that yield this comparison. By focusing answers on the relative accuracy of those facts, hopefully we can avoid primarily opinion-based answers.


  1. Over the course of millions of years, planetary bodies, especially those closer to the sun, tend to lose their water and atmosphere as it is blown away by solar wind, and radiated or boiled off by other forces, unless they have a strong magnetosphere to repel the bulk of that solar wind, or enough gravity to hang on to their matter. On a much faster time scale, this also happens to comets as they approach the sun; thus, the tail.
  2. Over the course of millions of years, bombardment of planets by comets can increase the water and gas on a planet, or at least counter-balance the gradual loss of the same (depending on other factors like proximity to the sun, size, and magnetic protection).
  3. The Oort cloud is composed mostly of icy bodies (including solids that would be a gas at Earth temperatures). And it is from this region that comets "fall" toward the inner solar system.

Also Fact?
So I started wondering what ultimately happens to the water and gas that gets blown off of planetary bodies. I would guess that given the outward pressure of solar wind and solar radiation, the direction of such material is generally outward, away from the sun. Well, where to? Media coverage of the departure of Voyager 1 from our solar system into the realm of interstellar space alerted me to the idea that there is an outer edge to solar influence, and I would guess that this is where the lighter molecules, like water and gases would tend to land. And indeed there is a collection of similar material out there at that solar radius.

Given these facts, is it accurate to view the Oort cloud as behaving similarly to a terrestrial cloudscape? Our clouds form at the altitude where water tends to deposit after it evaporates from the heated surface of the Earth. It gathers there and coalesces to a form which is then overcome by the opposing force of gravity. Then it returns from whence it came. Similarly, I imagine water and gases ultimately being driven from the heat of the sun to an “altitude above its surface”, where ultimately, over a much longer time scale, that material coalesces until other forces eventually drive them back toward the hot sun. Is this an accurate understanding? Is this comparison justified?

  • 1
    $\begingroup$ This is a fairly interesting notion I haven't seen before. Are you thinking that water and other volatiles get blown off the planets, can't coalesce near the Sun because of the warm solar wind, but then coalesce far away, like into the icy asteroids in the Kuiper Belt or Oort Cloud? Thus explaining how those regions formed? $\endgroup$
    – DrZ214
    Commented Jun 13, 2015 at 3:37
  • $\begingroup$ The key is in the age of the comets. And your idea is consistent with the age of comet Wild 2, if you move the events back to the first few million years of the solar system. First measurement of the age of cometary material $\endgroup$
    – eshaya
    Commented Jun 13, 2015 at 22:33
  • $\begingroup$ @DrZ214 That's the idea, basically. Although I'm sure there is no one all-encompassing explanation, I'm wondering if this could be a significant part of the picture. $\endgroup$ Commented Jun 15, 2015 at 16:18
  • $\begingroup$ All this gaz leaving atmosphere still have a velocity close to Earth velocity, so I would say that at first order it blurs around the Earth orbit (and that we get some back the next year :-) ). But ok, solar wind must probably spiral it outward. (+ all the complicated dynamics that surely occurs on the way). Now, forces effect is incremental, and at distance where no extra solar force acts, you still have the acquired velocity (excepted if the gaz has impacted or be satellited, if these things happen). $\endgroup$ Commented Oct 29, 2015 at 15:10

1 Answer 1


Dating cometary material is tricky, just because you have to go to space to get it. But the dates from the "stardust" mission show that comets are very very old. As old as the oldest asteroids, and date back to the origin of the solar system.

Now while there is evidence that material that formed in the inner solar system when the sun was young, and the planets still forming, was carried to the outer solar system. There is no suggestion that this process is ongoing. There is no astrononomical water cycle.

Although the planets might lose a little water to solar wind. It is very very little. Earth is protected by a magnetic field. Mars has lost all its water already, as has Venus, and Mercury probably never had much.


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