Workshop on the origin of volatiles in habitable planets

I had the opportunity to help organize a workshop held here at University of Michigan earlier this week. The workshop was sponsored by the Michigan Institute for Research in Astrophysics, which aims to foster interdisciplinary collaborations. The meeting was organized by both Astronomy and Earth & Environmental Sciences faculty to put together a diverse set of speakers. We heard talks from cosmo-chemists,  geo-physicists, exoplanet demographers, planetary scientists, and everything in between.

This was a great learning experience for me and I felt the workshop did a great job bringing out the big picture — what questions each sub-field is trying to answer. Of course the *right* answer has to agree between all disciplines. I’ll share some of my favorite highlights from the meeting, in no particular order.

  • From exoplanet observations, we see that planets of a given mass have a range of bulk densities, even within the same system. This implies widely varying envelope size, that can’t just be explained by the initial composition of a disk. This is especially notable in Kepler multi-planet systems that show a range of bulk densities. A popular theme during the talks was that impacts throughout the solar system evolution tune the atmospheres of planets post-formation.
  • When it comes to super-earths we really want to know if they have water. The range of bulk densities could means there is a degeneracy between rocky worlds and water worlds. Knowing the mass is very important. But even on top of this, we can come up with a range of Hydrogen-Helium-Rock mixtures that mimc the density of water, so bulk density doesn’t tell the whole story.
  • In our own solar systems, we can look at the abundance of volatiles and noble gases and how these vary between the sun, terrestrial planets, solar system moons, and various classes of chondrites. These abundances also seem to vary among the terrestrial planets, which could support a similar scenario that post-formation events lead to the eventual atmospheres. This also highlights that we should proceed with caution assuming solar abundance for exoplanets.
  • The presence of giant planets could be very important for getting the volatile elements to the inner terrestrial planets, by scattering small bodies as they grow and/or migrate.
  • It is unclear how the volatile delivery/transport scenario trends with the host star mass. More massive stars tend to have more massive disks, but less massive stars have disks that live longer. The demographics also change with stellar type. These different variables could all affect transport of materials.
  • Impacts are very likely a big part of the story, but giant impacts don’t necessary deplete all the volatiles! Hilke Schlichting showed us that small impacts are more efficient at removing atmospheres. And Sarah Stewart convinced us through many awesome videos that a hot, even turbulent, rotating liquid body does not mix well. Volatiles can remain trapped even after a large impact event.
  • I also learned the origin of the term “synestia” — Syn: co-, Hestia: Goddess of architecture — which describes an object whose outer material is not co-rotating with its core, what could result after a giant impact.
  • Laboratory and modeling suggest that tiny particles could efficiently stick together and grow to about ~mm-cm size pebbles before collisions and fragmentation start to be a problem, which is a problem for having the larger planetary embryos form through this same mechanism. Instead the larger planetesimals could form from some kind of gravitational instability. The observation of binary asteroids in the Kuiper Belt supports this. Once we have both planetesimals and pebbles, numerical models show we can have pebble accretion to form the cores of giant planets.
  • Solar system chemistry starts in the ISM, where we see different kind of chemistry in different states of material (gases, ices, etc). Gas-grain interaction is the core of interstellar chemistry that leads to more complex molecules that could not form in the gas alone. Photolysis of ices can also lead to complex species. Protoplanetary disks process ISM material into the building blocks of planets. Measuring molecules in disks  is key to learn about the processes that differentiate disk composition from ISM composition.
  • The solids in meteorites reflect integrated effects of every environment they saw in solar nebula/ protoplanetary disk. Rather than a simple model of disk chemistry with fixed zones, we must think of the disk as dynamic. The former cannot explain meteorite content — a mixture of things that had to form in different environments.
  • “Choose your geochemist friends wisely.” Without being in the field it’s hard to understand the agreement/controversy surround reported composition values for Earth. There is often a lot of disagreement.

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