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Lonely planets floating freely near galaxy’s centre

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free-floating planetA 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.

Two years of data analysed

For two years, a Japan-New Zealand research team, known as Microlensing Observations in Astrophysics, monitored more than 50 million stars in the direction of the centre of the galaxy, each star at least once every hour.

From this data, researchers were able to pinpoint 474 individual microlensing events, 10 of which lasted for periods less than two days. This timescale suggests they are planets with a Jupiter-mass and are likely free-floating, said Sumi.

Additional data was then used to show that the number of these short-term microlensing events was far greater than expected given the documented number of stars and brown dwarfs in the Milky Way. The implication is that these stand-alone Jupiter-mass objects could exist in greater number than stars.

Free-floating Earth-sized planets?

Bailey said the find reveals new information about how planets form and interact in their early history, and is interesting because of the nature of detecting such planets.

“These are very hard objects to find because a planet sitting alone in space isn’t going to be very bright,” he said. “You need to do a lot of observing … a planet in one of its random motions needs to pass in front of a star.”

“The study tells us that there could be a huge number of these planets floating around in space that are not orbiting any star… presumably there will also be Earth-sized planets – these would be more easily thrown out of a traditional orbit.”

Could they sustain life?

There has been speculation about whether an Earth-like planet without a host star could possibly sustain life, drawing heat and energy from radioactive elements in its core rather than an external energy source. A dense atmosphere and a layer of ice might theoretically trap enough of the planet’s internal heat to allow water to remain liquid, said Bailey.

“The discovery of these systems adds weight to these possibilities. The problem remains that these planets would be incredibly hard to detect.”

Data from NASA’s Wide-Field Infrared Survey Telescope, a space-based observatory that employed similar techniques to search for free-floating Earth-mass objects, could help researchers as they move forward.

Via CosmoMag

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