Carbon planets and diamond clouds

Could diamond planets exist? Probably not, but what about carbon-planets in more general terms? So far, almost all discovered planets were oxygen-rich. The quest about carbon-rich planets touches fundamental question of astrophysics, including how stars formed in the early universe and how planets form in disks around young stars. Ultimately and somewhat surprisingly perhaps, we are required to understand what kind of clouds form in atmospheres of carbon-rich planets in order to disentangle existing observations or to predict possible observations, because clouds do act like an optical and a thermal blanket to the planet below it. Therefore, Helling, Tootill, Woitke & Lee (2016) set out to model cloud formation in carbon-rich atmospheres. They find that the material composition and, hence, the appearance of carbon-rich planets (possibly WASP-12b or HR 8799 b, c, d, e) drastically differs from the well-studied oxygen-rich giant gas planets (like HD 189733b or HD 209458b). They show, that only the very inner parts of a carbon-cloud could possibly be made of diamond crystals, making it a rather darkish type of planet.

Stars come with all sorts of chemical ‘tastes’: Those with more oxygen than carbon (called ‘oxygen-rich’), and those with more carbon than oxygen (called ‘carbon-rich’). Stars in the present universe are born as oxygen-rich and only change to carbon-rich at later stages in their life-time when they become giant stars with diameters of hundreds of stellar radii. Therefore, extrasolar planets are expected to be oxygen-rich.

Observations start to indicate that some extrasolar planets maybe carbon-rich, or at least have enough carbon-molecules in their atmospheres so that our telescopes can see them. For example, WASP-12b was suggested to be carbon- rich (Madhusudhan et al. 2011) but a second observation could not confirm that finding (Kreidberg et al. 2015). Four planets of the planetary system HR 8799 have tentative detections of C2H2, CH4, and CO2, all rather large carbon-binding molecules (Opennheimer et al. 2013).

Planets only form in disks around young stars and young stars are oxygen-rich. Stars only turn carbon-rich at later ages. So, why should carbon-rich planets exist at all?

Figure 1: The top panels show the material composition that we predict from our cloud models in per cent: 1.0 means that the cloud particles are made to 100% of this material (e.g. Al2O3 – light blue, carbon – grey) and 0.2 means that the cloud particles contain 20% of that material (e.g. Mg2SiO4 or MgSiO3 – yellow/orange). Different lines represent different materials. The lower pictures visualize how the cloud would look like if we were able to observe it more closely: An oxygen-rich giant planet (left) would sparkle in different colours while a carbon-rich planet would look rather dull and dark-grey.

Figure 1: The top panels show the material composition that we predict from our cloud models in per cent: 1.0 means that the cloud particles are made to 100% of this material (e.g. Al2O3 – light blue, carbon – grey) and 0.2 means that the cloud particles contain 20% of that material (e.g. Mg2SiO4 or MgSiO3 – yellow/orange). Different lines represent different materials. The lower pictures visualize how the cloud would look like if we were able to observe it more closely: An oxygen-rich giant planet (left) would sparkle in different colours while a carbon-rich planet would look rather dull and dark-grey (Helling et al. 2016).

Here are three ideas how this could work:

1) Extremely carbon-rich stars formed in very early universe (Mashian & Loeb 2016). These carbon-rich stars were young at some point and may therefore have formed carbon-rich planets. The problem with that hypothesis is that our Sun is oxygen-rich and not that old. The Sun is a young star compared to the stars we know of in our present universe. It is therefore highly unlikely that any presently observable carbon-rich planet has formed in the early universe.

2) Carbon-rich stars in our present-day universe bath their originally oxygen- rich planets in a strong wind that carries large amounts of carbon-rich gas and dust. As this wind continues to hit the planets atmosphere, it increases slowly but steady its carbon content and appear as carbon-rich at some point. The obvious conclusion is that all carbon-rich stars with strong mass losses should have carbon-rich planets. This has not been observed so far.

What are we left with? We need to take a closer look at the star and planet formation processes and see whether chemical niches appear that allow us to explain the possible emergence of carbon-rich planets or episodes of carbon- enrichment. In other words, we need to study how the local chemistry changes in planet-forming disks that emerge around young stars in order to conserve the angular momentum of the collapsing gas-cloud that will form a young star. This local chemistry will play a large role in determining what kind of planets will form in such disks. Hence, the beauty lies in the detail.

Knowing about the problem, we have merged our expertise in atmosphere modelling (Christiane Helling) and planet-forming disk modelling (Peter Woitke). Through this work, published in the Life journal in 2014, we realized that we need to understand cloud formation a lot better in order to be able to say if the chemical composition of a planetary atmosphere would be carbon-rich or oxygen-rich.

In Helling, Tootill, Woitke & Lee 2016 we worked out a model that would allow us to predict how clouds form in carbon- rich atmospheres. The oxygen-rich case was well studied already and we could build on our experiences with it.

Figure 1 visualises our main results. The left-hand panel shows the results for the cloud layer that forms in an oxygen-rich atmosphere of a planet, the right-hand side shows the cloud forming in the carbon-rich planet. Here are the three main observations from Figure 1:

a) Clouds in oxygen-rich atmospheres are made of 40% silicates like Mg2SiO4 and MgSiO3, 20% SiO, 10% iron, with the remaining 30% being a mix of other materials.

b) Clouds in carbon-rich planets would be made of a minimum of 60% carbon with the remaining 40% (or less!) being a mix of other carbon-binding materials and iron.

c) While silicate clouds in oxygen-rich atmospheres are semi-transparent at the top and dark further inside, carbon-clouds are graphite-black at the top and diamond-clear further inside.

It appears that an oxygen-rich planet with strong winds should be much more beautiful to observe because many mineral cloud particles could appear in many colours. Carbon-rich planets would look rather dull and grey in comparison.

For more details check out the original paper on ADS:

Helling, Tootill, Woitke & Lee 2016, eprint arXiv:1612.01863

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Cosmic Rays enhance Prebiotic Chemistry on Sunless Worlds

In the International Journal of Astrobiology, Camille Bilger, Christiane Helling and Paul Rimmer (2014) presented a proof-of-concept on the potential effect of cosmic rays in the upper atmospheres of exoplanets (atmospheric pressures between 0.000001 bar and 0.000000001 bar, where 1 bar is the pressure of Earth’s atmosphere at sea level). We model the cosmic ray transport through the atmosphere of a planet with elemental composition and surface gravity similar to (but not the same as) Jupiter, if Jupiter were much farther away from the sun. This model atmosphere was produced using the Drift-Phoenix code (an upcoming post will be available soon on the Drift-Phoenix code).

Cosmic rays are charged particles (electrons, protons, bare nuclei) hurled through our galaxy at relativistic speeds by supernovae. When they strike the upper atmosphere of a planet, found to change its chemistry.

Cosmic Ray air shower

Figure 1. Cosmic Ray air shower

The combination of ultraviolet light from a star and cosmic ray ionization involves a delicate interplay between physics and chemistry, and is a hard problem to solve. It is simpler to consider cosmic ray chemistry on planets without daylight. These planets are often gas giants far from their host star, or rogue planets without a host star at all. These gas giants, like Jupiter, have an atmosphere made up not of mostly oxygen and nitrogen, but of mostly hydrogen, with a significant amount of nacient atomic hydrogen.

This is therefore a reducing atmosphere, and provides an ideal environment for making molecules believed to be important for the origin of life. Cosmic rays would ionize the molecular hydrogen, and this would make the atmosphere even more reducing.

Artistic impression of cosmic rays entering Earth's atmosphere. (Credit: Asimmetrie/Infn via CERN).

Figure 2. Artistic impression of cosmic rays entering Earth’s atmosphere. (Credit: Asimmetrie/Infn via CERN).

A reducing atmosphere is the standard initial chemical environment used in “origin-of-life” experiments, such as the Urey-Miller experiment. In the Urey-Miller experiment, an ionizing source in the form of an electrical discharge is initiated in a molecular gas, and so long as the atmosphere is reducing, prebiotic molecules are formed, including the twenty common amino acids found in living systems. If a reducing atmosphere, such as one dominated by oxygen and nitrogen, is used, the experiment produces no organic compounds.

The ion-neutral reactions made possible by cosmic ray ionization allow more of the hydrogen to be liberated from its molecular form, and increases the rate of reducing reactions. These reactions are found to be responsible for much of the prebiotic chemistry. Specifically, cosmic rays help to make biologically important molecules such as ammonia and acetylene. How much do cosmic rays help? They increase the abundance of some of these species by 10x or 100x in some cases.

Figure 3. Volume fraction of various species as a function of the gas pressure, p [bar] for the model atmosphere of a free-floating giant gas planet. The results assuming chemical quenching of C2H2 and C2H4 at height at ∼10−3 (dashed), and the results with cosmic ray ionization (solid) are all presented in this plot. A thick black horizontal line indicates the pressure above which termolecular (??) reactions may dominate.

Figure 3. Volume fraction of various species as a function of the gas pressure, p [bar] for the model atmosphere of a free-floating giant gas planet. The results assuming chemical quenching of C2H2 and C2H4 at height at ∼10−3 (dashed), and the results with cosmic ray ionization (solid) are all presented in this plot. A thick black horizontal line indicates the pressure above which termolecular reactions may dominate. (Rimmer et al. 2014, Fig. 4.)

The third picture in this article is from our paper, showing how much various molecules are enhanced or reduced because of cosmic rays. Some of the molecules that are enhanced, like ammonia, are key ingredients in the formation of the amino acid Glycine. These ingredients often must overcome a reactive barrier in order to form the amino acid, and overcoming this barrier may be made possible by electrostatic activation of ammonia (see our previous post here).

An image of the HR8799 planetary system from the December 2010 press release.

Figure 4. An image of the HR8799 planetary system from the December 2010 press release. (Credit: NRC-HIA, C. Marois, and Keck Observatory)

Our work may be relevant for directly imaged exoplanets orbiting HR 8799. Since these planets are ten times farther from their host star than Jupiter is from the sun, they are forever shrouded in the dark of night. It will be good to see how a star would change the chemistry, but this is a difficult problem. In order to find out what a star does in the upper atmosphere, it will be necessary to consider how the starlight passes through the atmosphere, and how that will affect thetemperature. The temperature will change the chemistry, and the chemistry will in turn affect how the starlight passes through theatmosphere. Much work still needs to be done to solve the problem for planets closer to their sun.

For more details check out the original paper on arXiv:

P. B. Rimmer, Ch. Helling and C. Bilger 2014, IntJAstrobio, 13, 173

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