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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The mineral clouds on the extrasolar giant gas planets HD 209458b and HD 189733b

The giant gas planets HD 189733b and HD 209458b are the two most studied extrasolar planets today. Both have been observed by several research groups with varies telescopes including the Hubble and the Spitzer Space Telescopes, and the super-high-precision HARPS spectrograph mounted on the 3.6 m telescope in La Silla in Chile. These extensive observational programs have reviled details about the atmospheres of these planets, like for example the presence of CO and CH4 in HD 189733b and CO and H2O in HD 209458b. Observations have further established that both gas giants form clouds inside their atmospheres (e.g. Sing et al. 2016). Are these clouds similar to clouds on Earth? What are they made of? Why does HD 209458b seem to have more water vapour than HD 189733b? How different are clouds between the two planets?

We use results from 3D radiative-hydrodynamics simulations of the atmospheres of HD 189733b and HD 209458b to answer the above questions and to derive cloud characteristics. We apply the same ideas about cloud formation as described in our Drift-Phoenix post. First, condensation seeds form with a certain efficiency. Once they are present, many solid materials (e.g. MgSiO3[s], Fe[s], SiO[s], TiO2[s], [s] meaning solid phase) can condense on these numerous but small surfaces. As these cloud particles grow, they fall into the atmosphere (gravitational settling). These raining cloud particles will encounter changing ambient conditions because the gas temperature and the gas pressure increase inwards the atmosphere. On their way, the cloud particles change in size but also in composition.

We now probe the atmospheric cloud formation in HD 189733b and HD 209458b by calculating the cloud structure for the vertical atmosphere at different longitudes and latitudes shown on Fig. 1.

Figure1. Points in the atmosphere where cloud formation was probed

Figure 1. Points in the atmosphere where cloud formation was probed

We find that both planets have the smallest cloud particles near the top of the cloud and the largest cloud particles at the bottom of the cloud, which is far inside the atmosphere beyond observable heights. This can be seen from the black solid line in all the panels in Fig 2.

Figure 2 demonstrates the vertical cloud structures for the daysides of the giant gas planets HD 189733b (top) and HD 209458b (bottom). We show how the material composition changes with height for different latitudes (Φ=270°, 315°, 0°, 45°, 90°) along the equator. The material composition is visualized by the lines of different colours, representing one material each: TiO2[s] – solid dark blue, Al2O3[s] – solid blue, CaTiO2[s] – solid purple, Fe2O3[s] – dashed light green, Fe[s] – dotted green, SiO[s] – dashed brown, SiO2[s] – solid brown, MgO[s] – dashed dirty orange, MgSiO3[s] – dashed orange, Mg2SiO4[s] – solid orange. The contribution of the different materials to the volume of the cloud particles, Vs/Vtot, is shown in percentage. For example, VMg2SiO4/Vtot=0.3 means 30% of the cloud particle is made of Mg2SiO4.

Figure 2. Dayside cloud particle material composition (colour coded, left axis) and mean grain sizes (black, right axis) for both exoplanets. For colour codes refer to the original paper at Helling et al. 2016, fig. 7 , or to the text above

Figure 2. Dayside cloud particle material composition (colour coded, left axis) and mean grain sizes (black, right axis) for both exoplanets. For colour codes refer to the original paper at Helling et al. 2016, fig. 7, or to the text above

To the left in Fig. 2, where the gas pressure is low, is the upper part of the atmosphere and the cloud. Here, the cloud particles are small (10-2 μm) and made of a rich mix of materials indicated by many coloured lines appearing in the plots of Fig 2. Letting your eyes wonder more to the right shows that most of the lines disappear, because these materials evaporate (like Fe2O3[s], MgO[s]). Most of the cloud particles are now made of MgSiO3[s], Mg2SiO4[s], SiO2[s] and a bit of Fe[s]. When moving further inwards the atmosphere where the temperature increases beyond thermal stability of the silicate materials, a larger fraction of cloud particles will be made of Fe[s].

Inspecting Fig 2 a bit closer by comparing the results for HD 189733b and HD 209458b shows that the cloud particles at the inner rim of HD 189733b are more Fe[s] rich than for HD 209458b. The cloud particles in the upper atmospheric regions appear rather similar in material composition: they are made of silicates and oxides with only very small contribution form iron.

A major result of our work is that all cloud properties are interlinked and that it is extremely difficult to guess correct combinations of cloud particle-sizes and their material composition that will occur at a certain place inside the atmosphere of an extrasolar planet.

Why would HD 209458b show more water absorption than HD 189733b? Hence, why does HD 209458b seem to have more water vapour than HD 189733b according to observations? Water is the most abundant absorbing species in the gas phase and maybe one would not expect any differences between two relatively similar planets like HD 189733b and HD 209458b. However, our research shows that a considerably larger portion of the atmosphere of HD 209458b is affected by the cloud than for HD 189733b. The clouds in HD 209458b reach into regions of lower atmospheric pressure, hence lower gas densities, compared to HD 189733b. It should therefore be more difficult to observe water on HD 209458b than on HD 189733b. Therefore, the more a cloud layer extends into the low-density upper part of the atmosphere, the shallower the gas-absorption features will be in the observed spectrum (Fig. 3).

Figure 3. More water molecules accumulate above clouds at higher atmospheric pressure. Clouds on HD 209458b form lower in the atmosphere (right), therefore have more water molecules above them resulting in a deeper water absorption feature in the spectrum, than on HD 189733b (left)

Figure 3. More water molecules accumulate above clouds at higher atmospheric pressure. Clouds on HD 209458b form lower in the atmosphere (right), therefore have more water molecules above them resulting in a deeper water absorption feature in the spectrum, than on HD 189733b (left)

 

For more details check out the original paper on ADS:

 Ch. Helling, G. Lee, I. Dobbs-Dixon, et al. 2016, MNRAS, 460, 855

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