Hubble spies puzzling pariah 'planets'

larger   Planet-sized objects could be wandering through globular cluster M22. The main image is a close-up of the entire cluster shown in the insert

(CNN) -- Astronomers have spotted what could be a small population of objects the size of planets, wandering through the galaxy without the gravitational fetters of parent stars.

Using the Hubble Space Telescope, the scientists detected what could be six bodies, perhaps only 80 times the mass of Earth, which would make them the smallest celestial objects ever seen outside the orbit of a star.

The international researchers, who used the magnifying effects of background stars to calculate the size of the objects, cautioned that the preliminary observations needed confirmation.

But the scientific team was enthusiastic about using the experimental method, which takes advantage of a natural phenomenon known as microlensing, to search for diminutive objects in the universe.

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"Hubble's sharpness allowed us to make this remarkable new type of observation, successfully demonstrating our ability to see very small objects," said Hubble scientist Kailash Sahu, lead author of a report this week in the journal Nature.

"This holds tremendous potential for further searches for dark, low-mass objects," Kailash said in a statement.

Microlensing takes place when a background star briefly brightens as a foreground object drifts in front of it; the gravity of the closer body amplifies the light of the distant star.

Astronomers can estimate the mass of the closer objects, depending on the duration and strength of the magnification.

These particular bodies, which are far too dim to be observed directly, were detected when they passed in front of stars in the massive central bulge of the Milky Way. Some are thought to be no larger than one quarter the mass of Jupiter.

If the wandering objects turn out as small as billed, what would they be? Orphan planets gravitationally ripped away from parent stars?

There may be too many for that explanation. Astronomers estimate that such bodies comprise as much as 10 percent of the mass of the small star cluster in which they reside, known as M22, which is about 8,500 light years away.

Microlensing has been used before to hunt for low-mass bodies in the disk and halo of our galaxy. But the extraordinary vision of Hubble was needed to attempt such a search mission in the small globular cluster.

In a normal lensing event, a background star brightens and dims for a length of time depending on the mass of the lensing body. The short, "spurious" events seen by the team are shorter than the interval between the Hubble observations, leading to an upper estimate for the mass of an object of one quarter Jupiter's mass.

To double-check the findings, Sahu and colleagues plan to observe the heart of the globular cluster continuously over a period of seven days. They expect to detect enough short-duration microlensing events to measure more accurately the masses of the small bodies.

If confirmed, the diminutive wanderers could provide unexpected insights into the formation of planets and stars, the astronomers said.