New Class of Planets

The Science Teacher, Summer 2011 | Go to article overview
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New Class of Planets


University of Notre Dame astronomer David Bennett is coauthor of a paper describing the discovery of a new class of planets--dark, isolated, Jupiter-mass bodies floating alone in space, far from any host star. Bennett and the team of astronomers involved in the discovery believe that the planets were most likely ejected from developing planetary systems. The study is described in the journal Nature.

The discovery stems from an analysis of observations of the central bulge of the Milky Way galaxy taken in 2006 and 2007 by a joint Japan--New Zealand survey. This analysis provides evidence of what appear to be 10 free-floating planets that are roughly the mass of Jupiter.

"Our results suggest that planetary systems often become unstable, with planets being kicked out from their places of birth by close encounters with other planets," Bennett explains.

This discovery not only confirms that free-floating planets exist in space, but also indicates that they are quite common. Free-floating planets are hard to detect, so the fact that the survey found up to 10 implies that there are many more that are undetected. The team of scientists who made the discovery estimates that there are about twice as many free-floating, Jupiter-mass planets as stars. This implies that free-floating planets are likely to be at least as common as planets, such as ours, that orbit stars.

"Our survey is like a population census--we sampled a portion of the galaxy and, based on these data, can estimate overall numbers in the galaxy," Bennett says. "The survey is not sensitive to planets less massive than Jupiter and Saturn, but theories suggest that lower-mass planets like Earth should be ejected from their stars more often and are thus more common than free-floating Jupiters."

Some scientists have even suggested that free-floating, Earth-mass planets could be warm enough to host life, due to the greenhouse effect of a large amount of hydrogen in their atmospheres. NASA's planned Wide-Field Infrared Survey Telescope (WFIRST) mission will use the microlensing method to reveal how many free-floating, Earth-mass planets inhabit the Milky Way galaxy.

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