More than three-quarters of the exoplanet candidates discovered by NASA’s Kepler spacecraft have sizes ranging from that of Earth to that of Neptune, which is nearly four times as big as Earth. Such planets dominate the galactic census but are not represented in our solar system. Astronomers don’t know how they form or if they are made of rock, water, or gas.
The Kepler team reports on four years of ground-based follow-up observations targeting Kepler’s exoplanet systems. These observations confirm that the numerous Kepler discoveries are indeed planets and yield mass measurements of these enigmatic worlds that vary between Earth and Neptune in size.
Included in the findings are five new rocky planets ranging in size from 10 to 80 percent larger than Earth. Two of the new rocky worlds, dubbed Kepler-99b and Kepler-406b, are both 40 percent larger in size than Earth and have a density similar to lead. The planets orbit their host stars in less than five and three days, respectively, making these worlds too hot for life as we know it.
A major component of these follow-up observations were Doppler measurements of the planets’ host stars. The team measured the reflex wobble of the host star, caused by the gravitational tug on the star exerted by the orbiting planet. That measured wobble reveals the mass of the planet: The higher the mass of the planet, the greater the gravitational tug on the star, and hence, the greater the wobble.
“This marvelous avalanche of information about the mini-Neptune planets is telling us about their core-envelope structure, not unlike a peach with its pit and fruit,” said Geoff Marcy from the University of California, Berkeley, who led the summary analysis of the high-precision Doppler study. “We now face daunting questions about how these enigmas formed and why our solar system is devoid of the most populous residents in the galaxy.”
Using one of the world’s largest ground-based telescopes at the W. M. Keck Observatory in Hawaii, scientists confirmed 41 of the exoplanets discovered by Kepler and determined the masses of 16. With the mass and diameter in hand, scientists could immediately determine the density of the planets, characterizing them as rocky or gaseous, or mixtures of the two.
These density measurements dictate the possible chemical composition of these strange but ubiquitous planets. The density measurements suggest that the planets smaller than Neptune — or mini-Neptunes — have a rocky core, but the proportions of hydrogen, helium, and hydrogen-rich molecules in the envelope surrounding that core vary dramatically, with some having no envelope at all.
The ground-based observation research validates 38 new planets, six of which are non-transiting planets only seen in the Doppler data.
A complementary technique used to determine mass, and in turn density of a planet, is by measuring the transit timing variations (TTV). Much like the gravitational force of a planet on its star, neighboring planets can tug on one another, causing one planet to accelerate and another planet to decelerate along its orbit.
Ji-Wei Xie of the University of Toronto used TTV to validate 15 pairs of Kepler planets ranging from Earth-sized to a little larger than Neptune. Xie measured masses of the 30 planets, thereby adding to the compendium of planetary characteristics for this new class of planets.
“Kepler’s primary objective is to determine the prevalence of planets of varying sizes and orbits. Of particular interest to the search for life is the prevalence of Earth-sized planets in the habitable zone,” said Natalie Batalha from NASA’s Ames Research Center in Moffett Field, California. “But the question in the back of our minds is: Are all planets the size of Earth rocky? Might some be scaled-down versions of icy Neptunes or steamy water worlds? What fraction are recognizable as kin of our rocky terrestrial globe?”
The dynamical mass measurements produced by Doppler and TTV analyses will help answer these questions. The results hint that a large fraction of planets smaller than 1.5 times the radius of Earth may be comprised of the silicates, iron, nickel, and magnesium that are found in the terrestrial planets here in the solar system.
Armed with this type of information, scientists will be able to turn the fraction of stars harboring Earth-sized planets into the fraction of stars harboring bona-fide rocky planets. And that’s a step closer to finding a habitable environment beyond the solar system.