Artist's concept showing free-floating planet that has roughly the mass of Jupiter. Researchers have show that these lone worlds, perhaps ejected from the planetary systems of their birth, are probably more common in our galaxy than stars

Credit: NASA/JPL-Caltech/R. Hurt

Far-infrared rendition of the Jovian-mass planet MOA-ip-10. It is either free-floating or extremely distant from its host star, and thousands of light-years away towards the galactic centre. The planet’s gravity creates Einstein arcs of a background star.

Credit: Artwork by Jon Lomberg

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SYDNEY: A population of 10 Jupiter-mass exoplanets have been identified floating freely in the Milky Way galaxy, apparently unbound to any host star, a new study has found.

At a distance of more than 10 times that of the Earth to the Sun from their nearest star, the objects are either in a far-flung orbit or not gravitationally linked to a parent star, researchers said. They also conclude that these Jupiter-mass objects are nearly twice as abundant as stars.

The finding, published in this week's issue of Nature, could transform current ideas about how planets are formed and raises an interesting question about how planets end up in distant orbits or detached entirely.

"It is the first time to find that such free-floating planets are as common as stars," said co-author Takahiro Sumi, an astronomer from Osaka University in Japan. "Previous observations only tell us about planets that are surviving in orbits now. This finding informs us how many gas giants have formed and scattered out."

Standing alone: How did they get there?

Astronomers have observed many exoplanets that don't exist in circular orbits, but instead move along an eccentric pathway. They get there because giant planets are formed in unstable configurations, said Jeremy Bailey, a planetary scientist at the University of New South Wales, who was not involved in the research.

The gravity of these planets affects one another, and in some cases, a close encounter event can result in a planet being dislodged from its normal path and cast off into an elliptical orbit or out of orbit altogether, he said.

Sumi said there are two scenarios for how the planets ended up all alone: the first is the unlikely possibility that they formed like stars, albeit on a smaller scale. The second theory, considered more probable by researchers, is that they formed like traditional planets, inside proto-planetary disks, and were subsequently ejected into unbound or very distant orbits.

Gravitational microlensing determines mass

Since 1995, more than 500 exoplanets have been discovered. Twelve of these planets have been found using gravitational microlensing. Originally proposed as a way of searching for dark matter, this technique can also be used to uncover distant planetary systems.

Based on Einstein's theory of general relativity, when a foreground star passes in front of another more distant star the latter becomes magnified.

Light rays from the background star are bent, causing them to brighten and form a curve. The height of this curve and the duration of the event are dictated, in part, by the mass of the foreground object.

If the foreground star - called a lens star - harbours a planetary system, then those planets can also act as magnifying lenses, creating additional deviations in the background light that can be measured to determine a planet's mass. "It is about 20 days for a star but it is about one day for a Jupiter-mass lens," said Sumi.