Celestial discovery rocks planet theory

Astronomers have found a cluster of planet-like objects drifting outside our solar system but not held in orbit by a central star.

Astronomers have found a cluster of planet-like objects drifting outside our solar system but not held in orbit by a central star.

The unusual nature of the find could upset theories that planets form around a star aftaer massive clouds of gas and dust clump together over millions of years. The finds are described as "young, cool bodies" because they are estimated to be just five million years old - compared with the 4.5 billion years of the Earth - and emit hardly any light.

They were discovered in a star cluster known as sigma Orionis, in the Orion constellation, by an international team led by Maria Rosa Zapatero Osorio. "The formation of young, free-floating, planetary mass objects like these are difficult to explain by our current models of how planets form," Dr Zapatero Osorio, of the Instituto de Astrofisica de Canarias, in Tenerife, told the journal Science.

One theory is that the 18 objects are failed stars called brown dwarfs, but the team says they look too small for this. The find was made by detecting the low-intensity light from the 18 objects.

Until now, planets outside the solar system have been found by the gravitational "wobble" of a star as a planet orbits it. "How can we explain the formation and evolution of planetary-mass objects outside the solar system?" Dr Zapatero Osorio said.