Small Exoplanets May Abound
Friday, June 29, 2012
Science Update - A look at CfA discoveries from recent journals

Not all stars are created equal. Although all of them are born mostly with hydrogen, the trace amounts of other elements present can vary by at least a factor of ten, depending on the particular conditions of the star's birth cloud. Since planets condense out of the material in disks
around young stars, astronomers wonder whether stars with relatively large amounts of these other elements will make rocky planets more easily.

Spectral observations of the star's light can reveal its original elemental composition, while the dramatic new methods for detecting exoplanets can identify, at least approximately, which stars host rocky planets (like the Earth) and which ones host primarily gaseous planets (like Jupiter). Recent observations have concluded that heavy-element-rich
stars are much more likely to harbor gas giant planets like Jupiter, but the relationship for the smaller, Earth-sized planets has been unknown.

In the latest issue of the journal, Nature, CfA astronomers Dave Latham, Guillermo Torres, Gilbert Esquerdo, John Geary, Bob Stefanik, and Samuel Quinn, and their colleagues report on their spectroscopic studies of 152 stars hosting exoplanets, a subset of the 1235 candidate stars identified by the Kepler satellite as host stars.

The scientists find that planets with radii less than four Earth radii form around host stars with a wide range of elemental abundances (but on average, they look like the Sun), whereas large planets like Jupiter preferentially form around stars that have a higher abundances of elements than does the Sun. The wide variety of conditions in stars that host Earth-like planets prompts the team to conclude that terrestrial planets may be widespread in the disk of the Galaxy, with no special requirement of enhanced element abundances needed for their formation.