Astrobiology, Exoplanets and ROCKE-3D

The discovery of exoplanets (planets outside our Solar System) has occurred at a rapid pace. Some of these planets may be habitable at their surfaces, in the sense that they are warm enough to sustain liquid water, a necessary ingredient for life as we know it. Knowing which ones are most likely to be habitable would facilitate the search for life. However, to date exoplanets that might sustain life have only been observed indirectly — through detection of the gravitational wobble due to the tug between planet and parent star, or through the dimming of the star's light as the planet passes in front of it. Until next-generation space observatories are able to directly measure spectral details, we must rely on theoretical understanding and on planets closer to home to ascertain the possibility of water and life for diverse planets elsewhere.

Circular logo for the ROCKE-3D model

The Solar System is home to the only known inhabited planet: Earth. Our Solar System's history, especially Earth, Mars, and Venus, has driven early thinking about concepts such as the “habitable zone” that have traditionally been applied to evaluate the habitability of planets discovered orbiting other stars. Three-dimensional (3D) planetary general circulation models (GCMs) derived from the models that we use to project 21st Century changes in Earth's climate can now be used to address outstanding questions about how Earth became and remained habitable despite wide swings in solar radiation, atmospheric chemistry, and other climate forcings; whether these different eras of habitability manifest themselves in signals that might be detected from a great distance; whether and how planets such as Mars and Venus were habitable in the past; how common habitable exoplanets might be; and how we might best answer this question with future observations.

Map of simulated Proxima Centauri aquaplanet surface temperature

The discovery of the planet Proxima Centauri b orbiting the star closest to Earth generated much inquiry whether it might be habitable. With ROCKE-3D we studied Proxima Centauri b as an “aquaplanet” covered by water. Because the planet is close to its star, it may show the same face to the star all the time, as the Moon does to the Earth. If so, the dayside remains a few degrees above freezing (yellow colors). Elsewhere, the ocean is perpetually covered by ice (dark blue colors), except near the equator where winds and ocean currents push sea ice eastward onto the dayside where it breaks up and melts (pale blue to light yellow colors).

ROCKE-3D

ROCKE-3D (Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments with Dynamics) is a three-dimensional coupled atmosphere, land, ocean general circulation model. It is used to model terrestrial planets like those in our inner solar system. The ROCKE-3D team, a multi-institution collaborative project, is addressing questions like those described above. GISS's role in this project is to perform 3D GCM simulations of past climates of Earth, other rocky Solar System planets, and theorized exoplanets (e.g., Proxima Centauri b) to broaden our understanding of planetary habitability, to use similar simulations to assess the habitability of rocky exoplanets, and to produce synthetic disk-integrated spectra and phase curves of these planets.

The Planet_2.0 version of ROCKE-3D was released in May 2024. It can be downloaded from the GISS ModelE resources website. Planet_1.0 (our workhorse model) is still available and is still the most used model internally. We will soon submit a publication that details Planet_2.0, but the meantime you can read the Planet_1.0 paper if you would like to understand more about our model in Way et al. (2017).

Our project uses solar radiation patterns and planetary rotation rates from simulations of spin-orbit dynamical evolution of planets over Solar System history provided by our colleagues at the Columbia Astrobiology Center and at other institutions that were part of NExSS. In turn, the synthetic disk-integrated spectra we produce from the GCM will be used as input to a whole planetary system spectral model that emulates observations that candidate future direct imaging exoplanet missions might obtain (see the NASA Goddard Space Flight Center Haystacks project).

ROCKE-3D Visualizations

Maps of climate parameters from ROCKE-3D simulations of Proxima Centauri b, ancient Earth, ancient Venus, and modern Earth can be created here.

Map of simulated ancient Venus radiative forcing

A ROCKE-3D simulation of a possible scenario for how an ancient Venus that began with a shallow ocean may have remained habitable for several billion years. The figure shows the cooling by sunlight reflected back to space by clouds. Shielding of sunlight by cloud is strong on the dayside (blue colors). Near the subsolar point (dark blue colors), where the Sun is directly overhead, clouds reflect more than 1000 W of energy per square meter of Venus's surface area, an amount almost as large as the energy that Earth receives from the Sun.

Photosynthesis and Astrobiology

Photosynthesis produces signs of life we can see from space: atmospheric oxygen and the vegetation “red-edge", a spectral reflectance signature of vegetation on land. Oxygen is produced through the splitting of water molecules during photosynthesis. The vegetation “red-edge” is so-called because healthy foliage absorbs light in the red with chlorophyll a (Chl a), which contrasts with strong scattering in the near-infrared. However, the Earth harbors a greater diversity of photosynthetic organisms than vascular plants, and includes algae, cyanobacteria, and anoxygenic photosynthetic bacteria, all of which occur in a wide array of colors, due to adaptation and acclimation to different light and chemical environments. This diversity raises questions about how photosynthesis developed on Earth, and provides clues as to what might dominate over a planet's history and yield alternative “biosignatures” on a planet orbiting a different kind of star from our Sun.

Support

Support for Astrobiology and ROCKE-3D research at NASA GISS is provided by the NASA Planetary Atmospheres, Exobiology, and Habitable Worlds Programs, and by the Sellers Exoplanet Environments Collaboration (SEEC). GISS is part of the NASA Nexus for Exoplanet System Science (NExSS). SEEC and NExSS are cross-discipline efforts that bring astrophysicists, heliophysicists, planetary scientists, and Earth scientists together to understand all the factors that determine exoplanet habitability, from stars, to their protoplanetary disks, to the rocky planets that form from them, to the atmospheres that determine their climates, to the planet-star interactions that determine which atmospheres remain stable over geologic time.

Publications

A list of papers resulting from GISS Astrobiology — ROCKE-3D research is available separately.

Source Code

The ROCKE-3D GCM can be downloaded from the GISS Model E resources website.

Contacts

Please address inquiries about astrobiology research at NASA GISS and ROCKE-3D to Dr. Michael Way. Inquiries about