Climate of an Earth-Like World with Changing Eccentricity

Having a giant planet like Jupiter next door can really wreak havoc on your orbit! A new study examines what such a bad neighbor might mean for the long-term climate of an Earth-like planet.

Influence of a Bad Neighbor

The presence of a Jupiter-like giant planet in a nearby orbit can significantly affect how terrestrial planets evolve dynamically, causing elements like the planets’ orbital eccentricities and axial tilts to change over time. Earth is saved this inconvenience — Jupiter isn’t close enough to significantly influence us, and our large moon stabilizes our orbit against Jupiter’s tugs.

outcomes

Top panels: Authors’ simulation outcomes for Case 1, in which the planet’s eccentricity varies from 0 to 0.283 over 6500 years. Bottom panels: Outcomes for Case 2, in which the planet’s eccentricity varies from 0 to 0.066 over 4500 years. The higher eccentricities reached in Case 1 causes the climate parameters to vary more widely. Click for a better look! [Way & Georgakarakos 2017]

Mars, on the other hand, isn’t as lucky: it’s possible that Jupiter’s gravitational pull causes Mars’s axial tilt, for instance, to evolve through a range as large as 0 to 60 degrees on timescales of millions of years! Mars’s orbital eccentricity is similarly thought to vary due to Jupiter’s influence, and both of these factors play a major role in determining Mars’s climate.

As exoplanet missions discover more planets — many of which are Earth-like — we must carefully consider which among these are most likely to be capable of sustaining life. If having a nearby neighbor like a Jupiter can tug an Earth-like world into an orbit with varying eccentricity, how does this affect the planet’s climate? Will the planet remain temperate? Or will it develop a runaway heating or cooling effect as it orbits, rendering it uninhabitable?

Oceans and Orbits

To examine these questions, two scientists have built the first ever 3D global climate model simulations of an Earth-like world using a fully coupled ocean (necessary for understanding the transport of heat across the planet) with a planetary orbit that evolves over time.

surface air temperature

The surface air temperature variation of a planet with orbital eccentricity of 0.283. The top panel shows the surface temperature when the planet is closest to the star in its orbit (periastron); the bottom when the planet is furthest from the star in its orbit (apoastron). [Way & Georgakarakos 2017]

The scientists, Michael Way (NASA Goddard and Uppsala University, Sweden) and Nikolaos Georgakarakos (New York University Abu Dhabi), focus in this study on the specific effects of a varying orbital eccentricity on an Earth-like planet’s climate, holding the planet’s axial tilt steady at Earth’s 23.5°. They explore two scenarios: one in which the planet’s eccentricity evolves from 0 to 0.283 over 6500 years, and the other in which it evolves from 0 to 0.066 over 4500 years.

Temperate Outcomes

Way and Georgakarakos find that the planet with the more widely varying eccentricity has a greater increase rainfall and humidity as the planet approaches its host star in its orbit. Nonetheless, this effect is not enough to cause a runaway greenhouse scenario in which the planet becomes too warm for habitability. Similarly, the ocean ice fraction remains low enough even at apoastron in high-eccentricity scenarios for the planet to remain temperate.

What does these results imply? Having a changing eccentricity — caused by the gravitational pull of a nearby Jupiter-like neighbor — may make a planet’s climate more variable, but not to the extent where the planet is no longer able to support life. Therefore, as we discover more such planets with current and upcoming exoplanet missions, we know that we needn’t necessarily assume that they aren’t interest for habitability.

Citation

M. J. Way and Nikolaos Georgakarakos 2017 ApJL 835 L1. doi:10.3847/2041-8213/835/1/L1

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