Planetary Sciences Editors' Vox

Imagining a Different Earth

Data will be critical to start to answer whether the newly found TRAPPIST-1 worlds are truly Earth-like.

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Planetary Science is, in many ways, a study of the Earth. The same physical and chemical processes that operate on our home planet also operate on other worlds, simply with different boundary conditions and materials. Thus, study of other places provides a means to test theories that have been proposed for Earth. With the recently published discoveries of Gillon et al. [2017], the pantheon of these natural laboratories has expanded. See also Wendel’s news article from 22 February.

Gillon et al. [2017] reported the presence of 7 planets in tight orbits around the star TRAPPIST-1, an M-type dwarf star only 39 light years away.  Notably, their presence has only been inferred, with no direct observations. By examining dips in the light intensity as the planets transit between the star and us, the authors constrained the orbital properties, sizes, masses, and bulk densities of the worlds. These latter measurements show that they are remarkably Earth-like.

Truly Earth-like?

Conferring true Earth-like status is a challenge, however, because these planets aren’t even points of light in the sky, but absences of starlight. Without direct observations that resolve the planets beyond a handful of pixels, they are not fully realized geophysical bodies. They do, however, invite speculation.

Take the case of planet TRAPPIST-1e (clearly, better names are needed).  Within error, here is a world identical to Earth with an equilibrium surface temperature about equal to that of the Moon (and hence the Earth without its blanketing atmosphere). What would the Earth look like if it were placed in this orbit around this star?

It would probably be a much more geologically vigorous place. In addition to its radiogenic heating, it would plausibly have significant tidal heating. The near integer correspondences between the orbital periods of these planets suggest tidally evolved orbits, and the mutual gravitational interactions of the planets would likely pump their orbits away from circular. Indeed, Gillon et al. [2017] found that the orbital eccentricities could be up to nearly 9%.  Tidal dissipation—like what powers Io, the most volcanically active body in our solar system—could be pronounced. Perhaps this transplanted Earth is a volcanic wasteland.

But what about our oceans? Again, tides likely play a role. Tidal forces are much more sensitive to distance than gravitational forces, so even though TRAPPIST-1 is a less massive star than our Sun, tidal forces would be large because of the closer orbital distances (TRAPPIST-1e has a semi-major axis less than 3% of the Earth’s orbit around the Sun). This likely means that all the planets are synchronously locked and only show one face towards the star (like our Moon to us). Whether oceans can endure when there is a persistent hot sub-stellar point and a frigid antipode is subject to debate.

The Question of Life

And then there is the question of life. The multicellular life that pervades our surface is based upon photosynthesis. A cooler star with photospheric temperatures less than half that of our Sun, TRAPPIST-1’s emissions peak in the infrared. Photosynthesis using chlorophyll won’t work, but other chemical pathways might be available. Without a doubt, any plant life would not look like ours.

The picture is more optimistic for chemotrophy. A transplanted Earth would still have redox gradients between the interior and the hydrosphere (assuming liquid water can exist). Life thus could closely resemble chemosynthetic microorganisms on Earth, and the denizens of this world could be strictly unicellular.

All of this is, of course, speculation, but that may change soon. NASA’s James Webb Space Telescope, which launches next year, should be able to analyze starlight filtering through any potential atmosphere. These data will be critical to start to answer whether these newly found worlds are truly Earth-like.

—Andrew J. Dombard, Editor, Geophysical Research Letters; email: [email protected]