NASA Simulations Explore Habitability of Nearest Exoplanet
Since the discovery of the first planet outside our solar system in 1992 astronomers have confirmed more than 4,100 exoplanets. Empowered by the NASA Center for Climate Simulation (NCCS) Discover supercomputer, a team of NASA scientists ran the first climate simulations incorporating a dynamic ocean of the exoplanet nearest to Earth — Proxima Centauri b. As part of NASA's search for life on other planets, these simulations explored a broad variety of habitable climate scenarios.
Proxima b was discovered in 2016 orbiting the red dwarf star Proxima Centauri 4.2 light-years (25 trillion miles) away from Earth. A mass of at least 1.27 Earths makes it a possible rocky planet. Similarities to Earth end there, though, as Proxima b is 20 times closer to its star and thus has an orbit of only 11.2 days. The planet also might be gravitationally locked to the star with permanent day and night sides.
To determine whether this strange world could harbor life, the scientists ran 43 Proxima b simulations winnowed down to 18 published experiments. The simulations varied factors including the types and amounts of atmospheric greenhouse gases; the depth, size, and salinity of the oceans; the ratio of land to water; and the planet's spin and orbit.
The Proxima b simulations used the NASA Goddard Institute for Space Studies (GISS) ROCKE-3D model — an adaptation of the GISS Model E2 Earth general circulation model — coupled to a dynamic ocean. Model grid box resolution was 500 by 620 kilometers (311 by 385 miles).
Each simulation used 44 computing cores on Discover and took from 1 to 6 weeks to run depending on the length of the simulation (ranging from 5,000 to 20,000 orbits) and computing system availability. They produced up to 3 terabytes of data, which were held in Discover's online disk and then migrated to NCCS and GISS mass storage.
“The NCCS system is a unique resource that made our Proxima Centauri b project feasible within a 6-month time frame,” said GISS research scientist Michael Way. “Using a conventional cluster system, it could have taken years to produce these results. The IT support is also beyond reproach for their ability to quickly address issues that might take weeks or months to resolve in less supportive environments.”
Even with permanent day and night sides, the simulations show that Proxima b can host liquid water across much of its surface for a wide range of atmospheres, oceans, and land-sea masks. “Specifically, with a dynamic ocean, part of the nightside surface remains liquid despite never receiving any starlight,” Way noted. Another surprise was how much the differences in ocean salinity changed the amount of liquid water on the planet's surface.
In follow-up to the Proxima b simulations, the scientists are carrying out an intercomparison study with several planetary general circulation models. The study is focusing on the TRAPPIST-1 system, “some of whose worlds are not that different from Proxima Centauri b,” Way explained. “We hope to be able to produce transmission, reflection, and emission spectra from our models using the Planetary Spectrum Generator developed at Goddard Space Flight Center,” he said. “This will allow us to guide researchers to the most promising exoplanet candidates for James Webb Space Telescope observations.”
Del Genio, A.D., M.J. Way, D.S. Amundsen, I. Aleinov, M. Kelley, N.Y. Kiang, and T.L. Clune, 2019: Habitable climate scenarios for Proxima Centauri b with a dynamic ocean. Astrobiology, 19, no. 1, 99-125, doi:10.1089/ast.2017.1760.
NASA Web Feature: How Earth Climate Models Help Scientists Picture Life on Unimaginable Worlds, Jan. 24, 2020.
NASA Scientific Visualization Studio: Proxima Centauri b Climate Model Scenarios, Jan. 24, 2020.