There they sit, all by their lonesome, 18 planet-size gas balls drifting free of the influence of any star.
Their existence, if they indeed are planets and not some sort of star-wannabe, throws into some chaos assumptions about how things organize in the cosmos.
"The formation of young, free-floating, planetary-mass objects like these are difficult to explain by our current models of how planets form," said Maria Rosa Zapatero Osorio, a Spanish astronomer working at the California Institute of Technology in Pasadena and lead author of a report describing the objects published Friday in the journal Science.
The new objects, which range in size from five to 13 times bigger than Jupiter, the biggest planet in our solar system, were found in a star cluster field called Sigma Orionis, within the constellation Orion, about 900 light years from Earth. This particular cluster is only about 5 million years old, considered too young to have had time for any planets to form under prevailing theory.
Planets are thought to form over tens of millions of years from a disk of gas and dust swirling around a star. Our sun is thought to be 5 million to 6 billion years old, a billion or so years older than its planets.
Zapatero Osorio and colleagues at observatories in Spain, the Canary Islands and Hawaii found the objects using visible and infrared light detecting cameras mounted on telescopes.
Their relatively dim, reddish light suggests that the objects are relatively cool blobs like Jupiter rather than brown dwarf stars that have enough mass to sustain nuclear reactions that take place inside stars.
Astronomers have made numerous discoveries of planet-like objects outside our solar system in the past several years, but they've all been associated with stars. Most, in fact, have been "spotted" indirectly because of the wobble their presence produces in the flicker of their stars.
The new observations are the first to detect light emitted directly from the planets and to collect spectrographic information about the planet's temperature and composition. But their calculation of the planet's mass was done based on models and assumptions about the age of the cluster, and so could be subject to change if more is learned about Orionis.
Still, most astronomers classify planets as bodies that form within a planetary system and orbit one or more stars, notes theorist Alan Boss of the Carnegie Institution in Washington.
"They should call them planetary mass brown dwarfs," Boss said, adding that clouds of hydrogen can condense into full fledged stars or fragment into smaller objects.
For now Zapatero Osorio prefers to call the balls "planetary mass objects" because many astronomers only consider objects more than 13 times the mass of Jupiter to be a brown dwarf.
"If planets can only exist around stars, then our candidates are very low-mass brown dwarfs," she said. "But if planets must be (below) a certain mass, then these objects are planets."
Setting aside the debate on terminology, she said the real challenge is explaining how such objects could form at all without any apparent aid from nearby star leftovers. Zapatero Osorio speculates there may be many more such gassy worlds in our own Milky Way galaxy, since it contains many similar star clusters.
Web site:www.sciencemag.org