The next decades will see the first atmospheric characterization of potentially habitable planets around other stars, taking the first step in the search for biosignatures beyond the Solar System. The search for spectroscopic signatures of biogenic gases in the atmospheres of extrasolar planets is a pillar of astrobiology and one of the most promising avenues for finding evidence of life beyond the Solar System (NAS, Astro2020). While the road to living worlds stretches decades into the future, the decade in front of us will include discoveries from the James Webb Space Telescope (JWST) and other extremely large telescopes that will be distinguished by statistical studies of atmospheres over a broad range of planet and host star properties.

To prepare for the search for life and this epoch of atmospheric characterization, we have assembled an interdisciplinary team with a data-driven approach that focuses on the questions that can be hypothesis tested today to accelerate and support the search for biosignatures in the near future. Our interdisciplinary team proposes a synergistic program of observations, laboratory experiments, and modeling to understand the journey of volatiles, particularly but not limited to carbon and oxygen-containing species, from protoplanetary disks to exoplanet atmospheres.

We address four interrelated questions focused on discrete steps in that journey. Each is discussed in more detail below.

  1. What is the inventory of volatiles in planetary building blocks?
  2. What are a planet’s external sources and sinks of volatiles?
  3. How are volatiles distributed between a planet’s interior and atmosphere/surface?
  4. What can atmospheric observations tell us about the volatile inventories and chemistries of exoplanets?

Team: Natalie Batalha, Natasha Batalha, Bin Chen, Ian Crossfield, Elena Dobricā, Jonathan Fortney, Tom Greene, Daniel Huber, Gary Huss, Rebecca Jensen-Clem, Meredith MacGregor, Ruth Murray-Clay, Francis Nimmo, Andrew Skemer, Myriam Telus, Jonathan Williams, Xi Zhang

What is the inventory of volatiles in planetary building blocks?

We will measure the composition and structure of protoplanetary disks across all scales, as well as the “debris” disks that are left over from planet formation. We will investigate the composition of the gas and dust in disks, i.e. the abundance of volatiles; and the migration of volatiles and the extent to which this is controlled by the evolution of the disk and growth of protoplanets. To do this we will obtain millimeter and radio spectra of the outer (>5 AU) regions of disks at different evolutionary stages to study the disk-integrated dust and gas composition. We will probe, via the optical scattering properties of dust grains and the absorption lines of gas, the composition and structure of inner (<5 AU) regions of disks corresponding to the scale of the orbits of most known exoplanets, including within circumstellar habitable zones.

  • What is the composition of dust and gas that will form planets?
  • Does the formation of protoplanets affect the transport of volatiles in disks?

Team: Jonathan Williams, Meredith MacGregor, Ruth Murray-Clay, Eric Gaidos

What are a planet’s external sources and sinks of volatiles?

A planet can capture volatiles as gas from the protoplanetary disk and in condensed form via planetesimals. Some of these volatiles can escape back to space. These processes are manifest not only in individual planets but in the ensemble properties of exoplanets. To investigate the capture, retention, or loss of primordial gas from the protoplanetary disk we will use data from space missions to map the distribution of rocky planets with and without envelopes of H/He-rich gas, search for ongoing escape of such atmospheres, and improve our understanding of the stellar activity that drives it. To investigate the volatile content of planetesimals accreted by planets we will use laboratory experiments to determine the hydrothermal reaction pathways and kinetics by which volatiles such as H2O are incorporated into less volatile mineral phases, and we will model how the residual volatile abundance of planetesimals are controlled by radioactive-decay heating, growth rate, and the presence of ices.

  • Under what conditions and for how long do small planets retain primordial atmospheres of gas from the protoplanetary disk?
  • What is the volatile content of planetesimals accreted by planets?

Team: Dan Huber, Natalie Batalha, Ruth Murray-Clay, Eric Gaidos, Meredith MacGregor, Francis Nimmo, Elena Dobricā

How are volatiles distributed between a planet’s interior and atmosphere/surface?

The energy released by accreted planetesimals will drive volatiles into a primordial atmosphere, but can also maintain a magma ocean into which some of those volatiles will dissolve. These volatiles will then segregate into the atmosphere, silicate mantle, or metallic core of the planet. We will conduct heating experiments on meteorites to measure the release of gases during planetary accretion. We will perform high-pressure experiments to determine the partitioning of volatiles, particularly C, and the fractionation of their isotopes between the mantle and core during their formation. And we will model how differences in the abundance and chemistry of mantle volatiles could affect the composition and evolution of a rocky planet’s atmosphere and produce diverse outcomes that could be observed by future observatories.

  • We will conduct meteoritic heating experiments to emulate the release of gases during planetary accretion
  • We will perform high-pressure experiments to determine the partitioning of volatile elements and the fractionation of their isotopes between mantle and core during the magma ocean and core formation phases of a protoplanet
  • We will explore how the redistribution of volatiles between surface, mantle and core affects the composition and evolution of rocky planet atmospheres

Team: Myriam Telus, Jonathan Fortney, Bin Chen, Eric Gaidos, Gary Huss, Francis Nimmo

What can atmospheric observations tell us about the volatile inventories and chemistries of exoplanets?

We will determine the composition of numerous exoplanet atmospheres, interpret these in terms of the framework of planetary volatiles developed in response to Questions 1–3, and use these to investigate aspects relevant to the detection and interpretation of biosignatures, in particular the presence of chemical disequilibrium. The structure and composition of an exoplanet atmosphere can be inferred by measuring absorption features in a transmission or emission spectrum. However, there are large but poorly understood uncertainties and systematic errors in translating observations into atmospheric properties. We will select an optimal set of planets for characterization by JWST in order to investigate how atmospheric composition depends on planet and host star properties. We will use models and advanced retrieval techniques to determine the fidelity and precision with which atmospheric composition can be inferred from observations. Finally, as a synthesis of our work, we will investigate how the abundances of C and O can be linked to the composition of the host star, the natal protoplanetary disk, and the processing of that material during planet formation.

  • How does a planet’s complement of currently-observable atmospheric constituents depend on planet and host star properties?
  • How can C/O and other atomic or isotopic abundance ratios bridge the gaps between observational studies of protoplanetary disk material, laboratory investigation of primitive solar system solids, and measurements of the spectra of exoplanet atmospheres?
  • How reliably can observations of exoplanet atmospheres be translated into compositions?

Team: Ian Crossfield, Jonathan Fortney, Natasha Batlha, Xi Zhang, Dan Huber, Andy Skemer, Tom Greene, Natalie Batalha, Eric Gaidos,

 

Interdisciplinary Consortia for Astrobiology Research (ICAR) | ICAR Research | ICAR Team