Worlds with the mass of Neptune are the most likely planet to form in the icy outer realms of planetary systems, scientists have found in a pioneering new study co-authored by Kiwi researchers.
The Nasa-led study, which drew on a technique called "gravitational microlensing", provides the first indication of the types of planets waiting to be found far from a host star, where scientists suspect planets form most efficiently.
Gravitational microlensing takes advantage of the light-bending effects of massive objects predicted by Einstein's general theory of relativity.
It occurs when a foreground star, the lens, randomly aligns with a distant background star, the source, as seen from Earth.
As the lensing star drifts along in its orbit around the galaxy, the alignment shifts over days to weeks, changing the apparent brightness of the source.
The precise pattern of these changes provides astronomers with clues about the nature of the lensing star, including any planets it may host.
The new paper, published today, came on the back of a collaboration between researchers in Japan and New Zealand called the Microlensing Observations in Astrophysics (MOA) group.
Creating alerts for the microlensing events is necessary because the alignments between stars are rare, and occur randomly, meaning astronomers must monitor millions of stars for the telltale brightness changes that signal a microlensing event.
Between 2007 and 2012, the MOA group issued 3300 alerts informing the astronomical community about ongoing microlensing events, and identified 1474 well-observed microlensing events, with 22 displaying clear planetary signals.
This included four planets that were never previously reported.
"Planets form in a disc of material circling their host star," explained Auckland University physicist Dr Nicholas Rattenbury, one of the scientists who work with data collected by the MOA group and others to hunt for signals arising from planetary systems.
"This work tells us what is the most likely mass of the planets formed in this disc where planet formation is most efficient.
"The technique we used here, gravitational microlensing, is the most sensitive technique for finding planets in this important region around a star."
The Japan/New Zealand collaboration MOA used its 1.8m telescope at the Mt John University Observatory, Lake Tekapo, to monitor millions of stars every night and announce microlensing events to the world.
Also involved was Massey University's Associate Professor Ian Bond, who was responsible for developing the techniques to find the events and create the alerts observed by the observatory.
This involved developing the astronomical imaging software designed to analyse the images from the telescope, in real-time and detect any object in the sky that is of a transient nature, which included microlensing as well as phenomena like flares stars and nova.
In the new study, the researchers said the technique had helped them find the apparent "sweet spot" in the sizes of cold planets.
"Contrary to some theoretical predictions, we infer from current detections that the most numerous have masses similar to Neptune, and there doesn't seem to be the expected increase in number at lower masses," said lead scientist Daisuke Suzuki, a post-doctoral researcher at Nasa's Goddard Space Flight Centre.
"We conclude that Neptune-mass planets in these outer orbits are about 10 times more common than Jupiter-mass planets in Jupiter-like orbits."
Co-author and Nasa astrophysicist David Bennett said the study was able to determine the mass ratio of identified planets to the host star and their separation.
"For about 40% of microlensing planets, we can determine the mass of the host star and therefore the mass of the planet."
More than 50 exoplanets had been discovered using microlensing, compared to thousands detected by other techniques, such as detecting the motion or dimming of a host star caused by the presence of planets.
But microlensing held great potential as it could detect planets hundreds of times more distant than most other methods, allowing astronomers to investigate a broad swath of our Milky Way galaxy.
The technique could locate exoplanets at smaller masses and greater distances from their host stars, and it was sensitive enough to find planets floating through the galaxy on their own, unbound to stars.
"Studies like this one are shedding light on what type of planetary systems exist in our galaxy," Dr Rattenbury said.
"We in the microlensing community are getting ready for Nasa's WFIRST telescope, which will observe microlensing events from space.
"This will provide observations of unprecedented sensitivity to planets down to the mass of Mars and will complete our understanding of what sort of planets are out there."
