Tuesday, May 29, 2012

Like a Diamond in the Sky?

Spitzer measurements suggest the atmosphere of planet WASP 12-b has carbon monoxide, excess methane, and not much water vapor. Image credit: NASA/JPL-Caltech/CFHT/MIT/Princeton/UCF

I have learned that there could be some astronomical truth in the picture conjured when singing the refrain of "Twinkle, Twinkle Little Star". David and Edward love this song and so do many other generations of children.  I love it, too and I am glad to have an excuse to include it in a post in some form.

"Twinkle, twinkle, little star,
     How I wonder what you are.
     Up above the world so high,
     Like a diamond in the sky."
              (Ann and Jane Taylor, 1806)

  Do stars have a diamond-like composition?... well,  not as far as I know, but some planets do. At the end of April I went to a seminar on the chemical characterization of Extraplanetary Atmospheres by Nikku Madhusudhan, a prize postdoctoral fellow at Yale University. I found out that the atmosphere and the core of some planets have more Carbon than Oxygen. In fact, this is not so new anymore, but it is the first time I have heard of it.

It is important to remember that planets do not have any light of their own, but they do masquerade as stars because they reflect the light of the sun. However, in general, they do not appear to twinkle in the sky.  The stars twinkle because we see them through many layers of thick, turbulent air known as the Earth's atmosphere. As the light from the star travels through the atmosphere the light is bent or refracted. This bending of light many times and in random directions causes the twinkling effect. Planets do not appear to twinkle unless the air is extremely turbulent because they are much closer to us. Stars do not twinkle when viewed from the Moon, for example, because the Moon does not have an atmosphere or from the International Space Station.
Hubble Image of a young sun-like star surrounded by a disk
  Let's get back to what Nikku said. He pointed out that models for planet formation and planetary atmospheres assume solar abundance ratio. Our star, the sun, has Carbon to Oxygen ratio of 0.5, i.e., twice as much oxygen as Carbon.  If we assume this ratio, water should be the most abundant element in planetary atmospheres. Nikku and his collaborators found several planets with atmospheres that are Carbon rich vs. Oxygen rich. So, this assumption needs to be relaxed.

How do scientists measure the composition of atmospheres of extrasolar planets? Telescopes use spectroscopic measurements of light reflected by the planetary atmosphere, i.e., they measure the intensity of the light as a function of wavelength, to find the chemical composition. One opportunity to make this type of measurements is when we observe the planet transiting across its parent star. The planet blocks some of the star's light and produces a noticeable dip in the measured light output. The advantage of this method is that scientists can probe the atmospheres of planets discovered in this way, and obtain information on their chemical composition, in addition to that of the star. They can also look at the depth of the transit and estimate the ratio of the radius of the planet with respect to that of the star. The disadvantage of this method is that there are relatively few sources because we can only detect planetary systems that are close to edge-on with respect to the observer.

 Another way to study the atmospheres of planets is to directly image them. However, planets are extremely faint sources relative to their parent stars. So, telescopes need to block the light from the star while leaving the planet detectable in order to detect the planets this way.  So, while the number of planets detected this way is growing, this method remains very challenging. 

What about planets in our solar system?
The Galileo satellite was destroyed during a controlled impact with Jupiter in 2003. Its findings are consistent with a Carbon to Oxygen ratio greater than or equal to 1. So, we could have diamond planets/moons in our own solar system. However, it is believed that the satellite landed in a particular dry spot on Jupiter.  There are several missions that will tell us more about Jupiter and its moons in the near future. The Juno mission, which was launched last year, will test this hypothesis in a few years. It takes 5 years to get to Jupiter in its current configuration. There is another mission that will be launched to study Jupiter's moons by the Europeans by 2022.  It's called Jupiter Icy Moon Explorer (JUICE).  However, it is important to remember that Jupiter is very different from extrasolar planets because it is cold. Jupiter is cooler than the water condensation curve, which means water would be condensed, i.e., buried deep in the atmosphere. Most of the discovered extrasolar planets are Hot Jupiters, Hot Neptunes and SuperEarths.

The first diamond planet and possible consequences
The first giant planet believed to have its atmosphere and core dominated by Carbon, WASP-12b, was discovered by astronomers in 2008. Nikku and his collaborators showed that this planet has low levels of water and high levels of methane. The carbon to oxygen ratio (C/O) was bigger than or equal to one by three standard deviations from the mean. If a planet with this composition is found orbiting a sun-like star with a C/O = 0.5, then this means that there are planetesimals with both this composition and a C/O grater than or equal to 1 composition.  In this case, the protoplanetary can not have the same composition everywhere - it must fluctuate as a function of distance. An alternative model proposes that the water condensed early on in the protoplanetary disk, while the C/O in the gas is higher (Oberg et al. 2011). Giant planets form by accreting this gas and hence would have a composition that is Carbon rich.

Can rocky planets have diamond cores?
Some authors (Rogers and Seager 2010, Fressin et al. 2012) show that rocky planets like our Earth could have Carboids instead of Si at their center. However,  SuperEarths are smaller and their atmospheres are harder to observe. Their chemical composition needs to be studied in order to understand which planets can support life and which can not. So, it's a question that hopefully will be answered sometime in the future.

Overall, the main message of the talk was that it is not OK to assume that all planets and stars have the same composition as our sun. This ratio varies and the variation can have impact on our predictions for habitability and on our understanding of planet formation.

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