How Hot are Super-Earths?

 

Artist’s impression of a trio of super-Earths discovered by an European team using the HARPS spectrograph on ESO’s 3.6-m telescope at La Silla, Chile. The three planets, having 4.2, 6.7, and 9.4 times the mass of the Earth, orbit the star HD 40307 with periods of 4.3, 9.6, and 20.4 days, respectively. Credit: ESO

Rocky super-Earths might be hotter than previously thought. New results show that they may cool down very slowly after their formation, making them less Earth-like than other studies have reported.

Exoplanet hunters have detected hundreds of planets in our galaxy, from gas-giant planets like Jupiter to more rocky, Earth-like worlds. Many super-Earths — planets with a mass of up to ten times that of Earth — have been found recently by telescopes like NASA’s Kepler Space Telescope. But super-Earths are something of a mystery to us since we have no such worlds in our own solar system.

"We are discovering planets orbiting distant stars that are similar to Earth in composition but more massive than Earth," says Vlada Stamenkovic, a researcher at the Massachusetts Institute of Technology. "The major question is: are they just scaled-up versions of Earth, or are they fundamentally different?"

Stamenkovic’s study approaches this question by looking at how rocky super-Earths cool down over time.

"We especially want to know if rocky super-Earths have thick atmospheres, volcanic activity, magnetic fields or plate tectonics, and the cooling behavior has an impact on all of these," says Stamenkovic, who presented the results at the European Planetary Science Congress on Wednesday 26th September. "Some of these features are crucial for determining if a planet might be capable of supporting surface life."

On Earth, plate tectonics and volcanic activity help regulate the climate, and release and recycle nutrients for life. The thinking among most astrobiologists is that any rocky planet that strays too far from Earth’s formula of volcanics-plus-tectonics may not have the ingredients necessary for sustaining life.

Boiling From Below

Earth’s mantle is composed of rock layers sandwiched between the outer crust and the inner metallic core. Thanks to high temperatures and pressures, viscous and solid rocks in the mantle slowly churn over millions of years, with hot material rising and cooler and heavier material sinking. This convective circulation is what drives the crashing and banging of Earth’s tectonic plates over millennia.

Beneath Earth’s solid crust are the mantle, the outer core, and the inner core. Credit: World Book illustration by Raymond Perlman and Steven Brayfield, Artisan-Chicag0

The viscosity of solid mantle rock describes how easily convection takes place. Rocks with a high viscosity do not deform easily.

For massive super-Earths, internal pressures are tens of times greater than in the interior of our own planet. Stamenkovic and his colleagues find that this higher pressure raises the viscosity of solid rock. A planet with high internal pressure will have a more viscous mantle. This stiffer mixture could mean that massive super-Earths experience less convection — the circular roiling of sub-surface material that keeps Earth’s tectonic plates sliding around.

“However, if there is convection or not depends on how hot those planets are. The lower temperatures used in previous scientific studies would result in large stagnant zones, where material does not convect,” says Stamenkovic. “But if those planets start initially molten, they would convect, although sluggishly, which would keep their lower mantles and cores much hotter than previous studies assumed.”

The team found that super-Earths should be less likely to have plate tectonics if they had the same amount of water in the lithosphere as Earth does. However, if they had a more water-rich lithosphere, then plate tectonics would be possible.

Stamenkovic says our telescopes will never be able to determine the amount of water a super-Earth’s lithosphere contains, so short of visiting these worlds someday, we’ll always question whether they have plate tectonics.

The Key to Climate

Artist impression of ExoPlanetSat. Credit: MIT

Earth’s climate depends on the amount of carbon dioxide in its atmosphere. Our planet becomes a hothouse when we have high amounts of this greenhouse gas. Scientists think that volcanoes spewing out carbon dioxide even may have helped kick Earth out of “Snowball” Ice Ages in the past.

The team found that when planetary mass increases, the duration of volcanic outgassing decreases. This could have severe impacts on climate regulation, especially during global Ice Ages on super-Earths.

"Our work shows that super-Earths are more diverse than expected," says Stamenkovic. "Moreover, it highlights the importance of understanding the time-dependent thermal evolution of planets. Theory shows the possibilities, which are far larger than previously thought, but remains full of uncertainties. What we need to fully answer these questions is to gather more data from high-pressure experiments in the lab.”

Spectroscopic observations of super-Earth atmospheres orbiting close-by bright stars also could provide insight, says Stamenkovic. Researchers at MIT, under the lead of Sara Seager, are now developing small low-cost satellites called ExoplanetSat that will pave the way to detect such super-Earth candidates.