ST.
LOUIS — In their quest to find some place just like home, astronomers are
exploring a new family of planets beyond the solar system — orbs only a few
times heavier than Earth. Over the past three years, researchers have
discovered five superEarths, ranging from five to about 10 times the mass of
the Earth.
With
exoplanet-seeking missions like the European Space Agency’s COROT, launched in
2006, and NASA’s Kepler, set for launch later this year, astronomers expect to
find many more — and much smaller — superEarths.
But
because the bodies are too small and faint for state-of-the-art telescopes to
image, researchers have only limited information — typically only
mass and radius.
So Diana Valencia of HarvardUniversity
and her colleagues set out to develop criteria, using information only on mass
and radius, that could distinguish a primarily wet superEarth from a dry, rocky
one. (Although covered by a thin layer of water over much of its surface, Earth
is considered a rocky planet by volume.) Determining which superEarths are
oceanic or icy — like a scaled-up version of Jupiter’s moon Europa — and which
are rocky could be crucial because life is more likely to gain a foothold on
rocky orbs like ours, theory suggests. If astronomers could differentiate, they
might choose to focus more of their initial studies on planets they know to be
rocky.
But
making the distinction isn’t easy, Valencia
noted in St. Louis
at the April meeting of the American Physical Society.
Planets of the same mass
can be roughly the same size even if they have different mixtures of icy and
rocky material in the core and mantle and on the surface. Despite this difficulty, Valencia
and her colleagues say they can crudely differentiate oceanic planets from
rocky superEarths — if the radius of the exoplanet is measured to an accuracy
of better than 5 percent and its mass to an accuracy better than 10 percent.
The only way it could be any larger than that maximum threshold is if at least 10 percent of the body were made of water rather than rock, and that would make the body at least partly oceanic. Theorist Sara Seager of MIT says she admires the work but notes that the model would have difficulty telling apart a rocky planet with even a small atmosphere from an oceanic planet, if astronomers only have information on mass and radius. Valencia also reported the results of another study, which indicates that bigger is better when it comes to the habitability of rocky planets. Her team found that rocky superEarths are more likely to undergo tectonic plate activity than lighter-weight ones.
The larger internal heat source of a heavier planet would more easily drive such activity, in which thin plates of material at or near the planet’s surface collide to build mountains and move continents. Plate tectonics is essential for life as we know it, because such activity recycles carbon dioxide between rock and the atmosphere, Valencia says. The carbon recycling acts as a thermostat, stabilizing the planet’s temperature.
Therefore, a heavier superEarth, with enhanced plate tectonic activity, may offer an even more hospitable environment for life than our planet, says Valencia.
