Planets as big as Earth but smaller than Neptune are likely even more common in our galaxy than previously believed, according to a statistical analysis of recent results from NASA's Kepler mission. The results help bolster the case for future missions aiming to discover and characterize Earth-like planets.

The study, co-authored by University of Florida associate professor of astronomy Eric Ford, is being published in the journal Nature. The study was led by Lars Buchhave of the University of Copenhagen.

"While many of the Sun's nearest neighbors are not likely to harbor Jupiter-mass planets, we now know that they are excellent hunting ground for smaller planets, ranging in size from Earth to Neptune," Ford said.

The Kepler Mission is NASA's 10th Discovery mission. It uses a 1-meter space telescope, specifically designed to survey a region of the Milky Way, registering the small decreases in the light from stars caused when a planet crosses in front of them. Its main goal is to determine how many Earth-size planets in or near the habitable zone are present in the billions of stars in our galaxy.

Previously, the Kepler mission had identified more than 2,300 planet candidates, most with sizes between that of Earth and Neptune. This study added observations made by ground-based telescopes to analyze the chemical composition of more than 200 host stars and compared the frequency of planets around stars with different chemical abundances.

Stars like the sun are primarily composed of hydrogen and helium, the lightest elements in the universe. Nearly all the heavier elements were produced via nuclear fusion in stars. While these heavy elements make up less than 2 percent of the sun's mass, they make up 90 percent of the mass of the Earth and the life on it.

Planets form by the merger of smaller rocky bodies called planetessimals. Thus, astronomers worried that if only stars born rich with heavy elements were able to form these planetessimals and hence rocky planets, then they might have to search a greater number of more distant stars to find planetary systems with rocky planets like Earth, Venus and Mars.

Previous research had shown that stars with abundance of heavy elements are much more likely to harbor a gas giant planet, similar to Jupiter, orbiting close to the star.

Most stars in the Milky Way have fewer heavy elements than the sun, limiting the number of stars with such planets. Recent ground-based surveys suggested that this correlation was not as strong for Neptune-sized planets. The new results find no correlation between the composition of the star and the frequency of planets with sizes between Earth and Neptune that orbit close to their host star.

"Stars that formed in an environment with less heavy elements would have had fewer iron, carbon, oxygen and nitrogen atoms available to form planets. This could prevent the formation of large rocky planetary cores that seed the formation of giant gas planets like Jupiter and Saturn," Ford said.

"Alternatively, it could just take so long that there isn't enough hydrogen gas left for the planet to grow into a giant planet.

In either case, this study shows that small planets form close to their host star at a similar rate, regardless of whether their host star has half or twice as many heavy elements as our sun."

Read the paper in detail here.