NASA's Kepler spacecraft has discovered scores of planets between the sizes of Earth and Neptune (which is four times larger); but researchers are unsure if most of these planets have rocky terrains, a wet environment, or are composed of gas.
About three-quarters of the planet candidates recently discovered by Kepler are of this size, making them the majority of the Milky Way, a NASA's Jet Propulsion Laboratory news release reported.
Five newly-confirmed rocky planets are believed to be between 10 and 80 percent larger than our home planet. Two of them, named Kepler-99b and Kepler-406b, are 40 percent larger than Earth and are most likely of about the same density as lead.
These planets may seem promising for harboring life, but they orbit their host stars in less than five days; their proximity to the star means their environment is much too hot to support life.
The follow-up observations that confirmed the mass and composition of the planets relied heavily on Doppler measurements. In these measurements the research team looked at the "reflex wobble" of the host stars, which is caused by a gravitational pull imposed by orbiting planets. These measurements help researchers determine the mass of the planets; the higher the mass the greater the tug.
"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," Geoff Marcy, professor of astronomy at the University of California, Berkeley, said. "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."
The researchers used a ground-based telescope to confirm 41 percent of the planets discovered by Kepler and determine the mass of 16. Using the mass information the researchers could gauge the composition of the newly-discovered planets.
"The density measurements dictate the possible chemical composition of these strange, but ubiquitous planets," the news release reported.
The study suggests planets smaller than Neptune (mini-Neptunes) have a uniformly rocky core; but the composition of the planets' outer "envelope" varies greatly and is even absent in some cases.
"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," Natalie Batalha, Kepler mission scientist at NASA's Ames Research Center, said. "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 researchers used a second method of determining planet mass, called TTV, which looks at gravitational tug on neighboring planets as opposed to a host star. The results suggest a number of planets that are about 1.5 times smaller than Earth's radius are composed of iron, nickel and magnesium; all of which are present in our own solar system.
The team hopes they will be able to use this data to uncover additional rocky planets, which would bring them one further in the search for life in the universe.
