Saturday, March 21, 2009

A thirsty southern star

When our solar system formed, the terrestrial planets when allow the practice of geology were assembled from solid material which condensed from the solar nebula. The composition of this nebula is hypothesized to have been the same as the composition of the sun, as spectroscopic measurements of the solar photosphere yield similar elemental ratios as analysis of meteorites for condensable elements.

The most common elements in the solar nebula were H, He, O, C, and N. At high temperature, they form the molecules H2, CO, H2O, and N (or NH3), which remain gaseous until very low temperatures. The combination of CO + H2O is possible because the molar C/O ratio is less than one- it is about 0.5 in the sun. Stars where C/O is greater than one will condense carbon and carbide to form carbon planets, as explained by Kuchner and Seager 2005. No such stellar systems have been identified to date. In our solar system, the abundance of all other elements is sufficiently low that all the metals and rock forming elements condensed from the solar nebula in the presence of H2O and CO gas. A simple stoichiometry exercise will show that condensing all the metal oxides with the reaction M+H2O -> MOx + xH2 will not deplete the solar nebula’s supply of H2O, so the CO + H2O buffer will remain in place until the H2O condenses, the CO reacts with H2 to form H2O + CH4, or the solar nebula dissipates.

The first 2 conditions require fairly low temperature, and are generally confined to the outer solar system. So condensation in the presence of H2O and CO is a good first order assumption for our solar system. Not surprisingly, H2O is pretty common in the solar system, and is found everywhere from the mantle of Earth and the acids of Venus all the way out to the Kupier belt.

The bulk solar composition is not too different to that of other nearby stars. So one might be tempted to assume that all planets condense under similar conditions. Is this valid? Let’s have a look at an interesting nearby system for comparison.

HD28185 is a sun-like star 138 light years away, in the constellation Eridanus. It is orbited by a 5 jupiter mass planet in a low eccentricity orbit with a radius similar to our own. This has led some folks to believe that any moon orbiting this planet may have liquid water present, and may be suitable for life as we know it. Does the star’s bulk composition have any bearing on this hypothesis?

HD28185 has higher concentration of most metals and carbon than the sun. But the oxygen, as reported by Gonzalez and Laws (2008) is only slightly elevated. Does this matter? A simple calculation below shows something interesting.

Unlike the solar nebula, the HD 28185 nebula does not contain enough water to buffer the condensation of the refactory major elements. Somewhere around 1400K 900K, the condensation of silicate minerals will suck all of the H2O out of the nebula. What will that do? I don’t know. Figuring it out requires a complex thermodynamic model, a plasma condensation apparatus, or an interstellar spaceship to find extrasolar asteroids. But one thing is certain. It will be different than what happened here. And taking the presence of water for granted may not be a safe assumption.


Chris said...

It is also unsafe to assume that a terrestrial body in the goldilocks-zone would harbour liquid. Look at Venus - a little too close to the sun, but about the right size and built from a nebula with exactly the right combination of ingredients to produce a planet with liquid water and life.

Perhaps if the giant impact that resulted in the Moon had been a little more giant (blasting away any atmosphere and pulverising both bodies), we wouldn't have anywhere in the solar system with liquid water.

There's more variables than is comfortable to consider when assessing a solar system's ability to harbour liquid water and life.

qraal said...

Oh you guys are such optimists! Why'd you have to bring us back down to reality? And I was looking forward to those Earth-like moons around HD28185

Anonymous said...

This would presumably raise issues for forming the giant planet. Standard view says planets form beyond the ice line from the water-rich material found there, but if there's no water, the ice line becomes irrelevant. Maybe the "tar line" is a more relevant concept for this system?

C W Magee said...

Actually, the lowest H2O partial pressure happens around 900K, depending on the total gas pressure, of course...