Planets Unleashed: More common than stars?

“Although free-floating planets have been predicted, they finally have been detected, holding major implications for planetary formation and evolution models,” said Mario Perez, exoplanet scientist at NASA headquarters in Washington, D.C.

In fact, astronomers, including one NASA-funded team member, haven’t just detected free-floating planets, they’ve discovered a whole new class of Jupiter-sized planets floating alone in space without an accompanying star. Despite their size, the planets, unleashed from a star’s gravitational pull, are difficult to spot, allowing them to have gone previously undetected. The discovery also indicates there are many more free-floating Jupiter-mass planets out there that can’t be seen. An exciting, if not exactly reassuring thought.

Alone in Space
This artist's conception, provided by NASA, illustrates a Jupiter-like planet alone in the dark of space, floating freely without a parent star.

The discovery of orphan planets is based on a joint Japan-New Zealand survey that scanned the center of the Milky Way galaxy during 2006 and 2007. Evidence of 10 free-floating planets, at an average approximate distance 10,000 to 20,000 light years away from Earth, was revealed. The team believes these lonely giants were probably ejected from developing planetary systems.

The study, led by Takahiro Sumi from Osaka University in Japan appears in today’s issue of the journal Nature. 

The survey was not sensitive to planets smaller than gas-giants such as Jupiter or Saturn. Even so, the team estimates that there are about twice as many unleashed orphan planets as there are stars. They are thought to be at least as common as the planets orbiting stars, meaning there could be billions of lone planets in the Milky Way galaxy alone. Theories also suggest that lower-mass planets like Earth should be ejected from their stars more often indicating they are may be more common that the free-floating Jupiters. The observations cannot rule out some of these orphan planets having very distant orbits around stars, however, in the case of the Jupiter-sized planets such distant orbits are rare.

“Our survey is like a population census,” said David Bennett, a NASA and National Science Foundation-funded co-author of the study from the University of Notre Dame in South  Bend, IN. “We sampled a portion of the galaxy, and based on these data, can estimate overall numbers in the galaxy.”

Previous observations identified a handful of free-floating planet-like objects within star-forming clusters, with masses three times that of Jupiter. Some scientists suspect gaseous bodies form more like stars than planets. These small, dim orbs, known as brown dwarfs, are born of collapsing balls of dust and gases, however they lack the mass to ignite their nuclear fuel and shine with starlight. Brown dwarfs are thought to be approximately the size of large planets.

“If free-floating planets formed like stars, then we would have expected to see only one or two of them in our survey instead of 10,” explains Bennett. “Our results suggest that planetary systems often become unstable with planets being kicked out of their planets being kicked out from their places of birth.”

To account for the number of orphan planets found, it is likely that some planets are ejected from their early, turbulent solar systems probably as a result of 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 center of the galaxy.

The survey, the Microlensing Observations in Astrophysics (MOA), is named after a giant wingless bird family from New Zealand called the moa that is now extinct. A 5.9-foot (1.8-meter) telescope at Mount John University Observatory in New Zealand regularly scans the copious stars at the center of our galaxy for gravitational microlensing events.

Microlensing events occur when something, a star or a planet or other similar celestial body, passes in front of another more distant star. The gravity of the passing object warps the light of the background star, causing it to magnify and brighten. The heftier the passing body, the more the light from the background star will be warped. The resulting brightening events can last for weeks.

A second microlensing survey group, the Optical Gravitational Lensing Experiment (OGLE), contributed to this discovery using a 4.2-foot (1.3-meter) telescope in Chile. The OGLE group also observed many of the same events, and their observations independently confirmed the analysis of the MOA group.

We, as sentient beings, may or may not be alone in the Universe. These free-floating planets, despite their numbers, definitely are.

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