While over 100 gaseous Jupiter-sized planets have been discovered in the last decade, and four medium Neptune-sized planets, until now there have been no discoveries of Earth-sized terrestrial planets that could support life.
Dr Philip Yock from The University of Auckland's Faculty of Science says the latest find brings the goal of locating an Earth-like planet in the Milky Way a step closer.
The astronomers used the gravitational fields of stars as huge, naturally occurring lenses, as originally proposed by Einstein. This technique is called "gravitational microlensing".
"The new planet is Neptune-sized and icy, but unlikely to be covered with a layer of gas like Neptune. Instead it may be more akin to a large, chilly version of our own Earth. The researchers were able to deduce this because they showed that the new planet has no Jupiter-like companion.
"Ten years ago, such a finding would have been unthinkable. At the time only a handful of Jupiter-like planets had been found, and medium Neptune-sized planets weren't even on the horizon. Competition between the various groups involved in the hunt for an Earth-twin spurs development onwards."
In 2002, Dr Yock of The University of Auckland, Dr Ian Bond of Massey University and Dr Nicholas Rattenbury of Manchester University published a variant of Einstein's microlensing method by which they thought earth-sized planets might be found. At the time, all were working at The University of Auckland.
Their method focused on lenses that produce very high magnification, in the order of 100 times or more. Not surprisingly, these lenses have greater sensitivity to small planets like Earth. Bond, Rattenbury and Yock were able to demonstrate that there should be enough of these lenses to make practicable measurements. The demonstration used data from the Japan/NZ microlensing group called MOA, based at Mt John in New Zealand.
Bond, Rattenbury and Yock are co-authors on a new paper reporting the discovery of this planet - which utilised their strategy. This has been submitted to The Astrophysical Journal for publication. Co-authors include Dr Christine Botzler, Dr Grant Christie, Ms Jennie McCormick and Mr Stephen Swaving of Auckland who supplied data and assisted with its interpretation.
Several pioneers of gravitational microlensing are also co-authors, including Dr Andrzej Udalski of Poland, and Drs David Bennett, Andrew Gould and Bohdan Paczynski of the US. They are members of Polish-based and US-based microlensing groups known as OGLE and MicroFUN. The gravitational lens that was used to locate the new planet, named OGLE-2005-BLG-169Lb, was found by the OGLE group.
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