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Friday, 6 July, 2001, 10:15 GMT 11:15 UK
Hubble detects floating planets
Hubble Stsci
Planets might be floating between the stars of cluster M22
By BBC News Online science editor Dr David Whitehouse

Looking into the heart of a star cluster, the Hubble Space Telescope (HST) has seen what could be a strange and unexpected population of planet-sized objects drifting through space.


Hubble's excellent sharpness has allowed us to make this remarkable new type of observation

Kailash Sahu
Observations of the star cluster M22 have revealed six unusual so-called microlensing events caused by passing planets some 80 times larger than the Earth, a small object by cosmic standards.

The results are so surprising that they will have to be confirmed by follow-up HST observations.

But scientists say if they are verified, the discovery could yield new insights about how stars and planets form.

Smallest ever detected

"Hubble's excellent sharpness has allowed us to make this remarkable new type of observation, successfully demonstrating our ability to see very small objects," said Kailash Sahu of the Space Telescope Science Institute.

Microlensing occurs when a background star brightens for a while as a foreground object drifts in between it and us.

The objects thought to cause these events are far too dim to be seen directly, but they can be detected by the way their gravity deflects and amplifies light from a distant background star.

Between February and June 1999, astronomers monitored the 83,000 stars of M22 to look for any brightening. They detected just one clear microlensing event caused by a normal star in the cluster. It appeared to grow ten times brighter before it returned to its normal brightness after 18 days.

Early Universe

But in addition to the "normal" microlensing event, astronomers recorded six unexpectedly brief events where a background star jumped in brightness by as much as a factor of two for less than 20 hours. This means that the microlensing object must have been much smaller than a normal star.

These short durations can be explained if the mass of the intervening object is about 80 times that of Earth. Objects this small have never before been definitely detected by the microlensing technique.

If these results are confirmed by follow-up HST observations, the bodies would be the smallest celestial objects ever seen that are not orbiting any star.

Researchers say they might be planets that were gravitationally torn away from parent stars in the cluster.

"Since we know that globular clusters like M22 are very old, this result opens new and exciting opportunities for the discovery and study of planet-like objects that formed in the early Universe," said Nino Panagia of the European Space Agency.

The research is reported in the journal Nature.

See also:

01 Apr 98 | Science/Nature
13 Jul 00 | Science/Nature
07 Aug 00 | Science/Nature
27 Apr 01 | Science/Nature
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