Monday, December 6, 2010

Planets: Water, Water, Everywhere

           Nothing makes the popular science press giddier than the mention of the word “water”.  Whether it is on the Moon, Mars, Mercury, or distant planets of other stars, the mere mention of the word leads to mock-eloquent flights of fancy, in which the words “Earth-like” and “life” figure prominently  But perspective is lacking.  Here’s the real situation.

The Universe is overwhelmingly composed of hydrogen and helium.  For every atom of silicon (think rocks) there 28000 atoms of hydrogen and 2700 atoms of helium.  What about the rest of the Universe?  Add 24 atoms of oxygen, 10 atoms of carbon, and 3 atoms each of neon and nitrogen. 

Our fixation with Earth-like worlds leads us to wonder about the rest: add 1 atom each of silicon, magnesium, and iron to the mix.  All the other elements combined would add up to a single atom.  Terrestrial (rocky) planets are mostly made of these relatively rare elements in the form of oxides of these three elements plus metallic iron.  The most abundant element in Earth (and Mercury, Venus, Mars, the Moon and most asteroids) is actually oxygen.  After accounting for elemental hydrogen and helium, oxygen is next in order of abundance.  Its presence makes chemistry (the formation of chemical compounds) possible, including the formation of water (2 atoms of hydrogen combined with one of oxygen).  It also means that water is the most abundant chemical compound in the Universe.  We therefore should expect to find it everywhere.  If water is absent from a planet, we need to explain why.

Consider this week’s feeding frenzy about water in the atmosphere of an extrasolar plant (GJ 1214b) with about 6.5 times the mass of Earth.  This planet is in a very close orbit (about 2 million km, or 1.2 million miles) around a faint red M4.5-class star with a luminosity of only 0.003 times that of the Sun.  The planet is so close to its star that its orbital period (year) is only 38 hours long. 

Such a faint star imposes certain limitations on its solar system: Any planet of such a faint star must be very close in order for it to have “Earthlike” temperatures (in the liquid water range; not boiled, not frozen).  Any such planet must be despun by tidal interactions with its parent star.  Even worse, the planet and sun must be so close that they are on the verge of being torn apart by those tidal forces.   The best data show no evidence of a massive hydrogen (and helium) atmosphere, which has almost certainly been stripped away by the tidal influence.  So what should be left behind?  Throughout the Universe, candidate #1 is water.  What we see is probably water clouds.  The interior is likely much hotter and may or may not have a global ocean, the reality depending on factors not yet known from observation. 

This planet is not the first, and certainly not the last, to be implicated as water-rich.  The first such “super-Earth” was discovered several years ago.  I also wrote about them in my 1998 book, Worlds without End.  But “water” is not a synonym for “ocean” or “inhabited” or “life-supporting”.

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