New Zealand researchers have helped discover a collection of Jupiter-sized free-floating planets that do not orbit stars.
The discovery of these "exoplanets" -- published in tomorrow's edition of scientific journal Nature -- provides new knowledge about "lonely" planets wandering through distant solar systems, apparently with no star of their own toorbit.
The scientists in the Japan/NZ MOA (Microlensing Observations in Astrophysics) collaboration used a 1.8m telescope at Mt John University Observatory near Lake Tekapo, with support from researchers at Auckland, Massey, Victoria and Canterbury universities.
They suggested today that the planets may have formed in dense gas clouds around newly formed stars and were then scattered into unbound or very distant orbits, so that there are twice as many of these as planets tied to stars.
"The detected Jupiter-mass objects may have actually been formed in protoplanetary disks formed around new stars and were subsequently scattered out of the multi-planet system by collisions with other planets," said Victoria University physicist Professor Denis Sullivan, a co-author of the paper.
Over a decade ago, astronomers found wandering planets using "microlensing" to detect how the planets' gravitational fields bent and amplified light from distant background stars.
Kailash Sahu, of the Space Telescope Science Institute, and colleagues monitored 83,000 stars and found a normal dwarf star -- about one tenth the mass of our sun -- which focused the light of the background star, causing it to appear 10 times brighter before it returned to normal over a period of 18 days.
But those researchers also found half a dozen smaller objects about 8500 light-years from Earth -- where a background star doubled in brightness for less than 20 hours -- and these appeared to be wandering planets plying their own course in the distant sea of stars.
Massey University astrophysicist Dr Ian Bond, a co-author of the Nature paper, said MOA typically found 500-600 microlensing events each year. Similar work is done by the Polish OGLE telescope that operates from Las Campanas in Chile.
He said the timescale of less than two days over which wandering planets bent the light of a background star made them hard to detect.
"Our results point to a population of free floating planets of around Jupiter mass," he said.
Jupiter is nearly 11 times the diameter of the earth, and about a tenth the size of the Earth's sun.
