Size Of Planets

Interstellar space 'full of Jupiter-size orphan planets'

A team of astronomers has identified a novel new kind of galactic wanderer - lone, Jupiter-sized planets expelled from forming solar systems and drifting in the empty void between the stars.

The researchers, led by Takahiro Sumi of Japan's Osaka University, spotted 10 such free-floating "orphan planets" in data from a 2006-7 microlensing survey of our galaxy's centre, which searched for the tell-tale sign of transiting bodies' gravitational fields distorting light from distant stars.

Team member David Bennett, of the University of Notre Dame in Indiana, explained that this first sighting in a small portion of the Milky Way points to enormous numbers of orphans. He said: "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."

In fact, the scientists reckon there are twice as many wandering Jupiters as there are stars – "hundreds of billions of lone planets" each ploughing a solitary furrow in the galactic ether. Takahiro Sumi said: "The existence of free-floating planets has been predicted by planetary formation theory, but nobody knew how many there are."

The microlensing technique used to nail the planets isn't sensitive to worlds smaller than Jupiter and Saturn, but since "lower-mass planets like Earth should be ejected from their stars more often" there are potentially even greater numbers of Earth-sized orphan planets out there.

The findings are published in today's edition of Nature , where astronomer Debra Fischer of Yale University said: "This is an amazing result, and if it's right, the implications for planet formation are profound."

NASA has more on the wandering Jupiters here .

Free-Floating Planets May be More Common Than Stars | Mini Physics ...

The discovery is based on a joint Japan-New Zealand survey that scanned the center of the Milky Way galaxy during 2006 and 2007, revealing evidence for up to 10 free-floating planets roughly the mass of Jupiter. The isolated orbs, also known as orphan planets, are difficult to spot, and had gone undetected until now. The newfound planets are located at an average approximate distance of 10,000 to 20,000 light-years from Earth. The discovery indicates there are many more free-floating Jupiter-mass planets that can't be seen. The team estimates there are about twice as many of them as stars. In addition, these worlds are thought to be at least as common as planets that orbit stars. This would add up to hundreds of billions of lone planets in our Milky Way galaxy alone. Previous observations spotted a handful of free-floating, planet-like objects within star-forming clusters, with masses three times that of Jupiter. But scientists suspect the gaseous bodies form more like stars than planets. These small, dim orbs, called brown dwarfs, grow from collapsing balls of gas and dust, but lack the mass to ignite their nuclear fuel and shine with starlight. It is thought the smallest brown dwarfs are approximately the size of large planets. On the other hand, it is likely that some planets are ejected from their early, turbulent solar systems, due to close gravitational encounters with other planets or stars. Without a star to circle, these planets would move through the galaxy as our sun and other stars do, in stable orbits around the galaxy's center. The discovery of 10 free-floating Jupiters supports the ejection scenario, though it's possible both mechanisms are at play. The survey, the Microlensing Observations in Astrophysics (MOA), is named in part after a giant wingless, extinct bird family from New Zealand called the moa. A 5.9-foot (1.8-meter) telescope at Mount John University Observatory in New Zealand is used to regularly scan the copious stars at the center of our galaxy for gravitational microlensing events. These occur when something, such as a star or planet, passes in front of another, more distant star. The passing body's gravity warps the light of the background star, causing it to magnify and brighten. Heftier passing bodies, like massive stars, will warp the light of the background star to a greater extent, resulting in brightening events that can last weeks.

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