While four other terrestrial planets have been discovered in binary systems that have similar separations using different techniques, this is the first discovery within a binary system of a planet that is both Earth-like in size and follows an Earth-like orbit.
It has given astronomers the first evidence that terrestrial planets can form in orbits similar to Earth's - even in a binary star system where the stars are not very far apart.
"This discovery is exciting because we weren't certain that terrestrial planets could form around one star of a binary star system," University of Auckland Department of Physics senior lecturer Nick Rattenbury said.
"This tells us there are many more stars in our Galaxy that could potentially be the host star to habitable planets."
In this case, the planet orbits its host star at almost exactly the same distance from which Earth orbits the sun - almost 135 million kilometres.
However, because the planet's host star shines 400 times less brightly than the Sun, the temperatures on the planet are -210C, a colder even than Saturn's icy moon Titan.
Four international research teams, led by Professor Andrew Gould of The Ohio State University, published their discovery today in the latest issue of the prestigious international journal, Science.
New Zealand astronomers, both professional and amateur, who were members of these research teams made significant contributions to the discovery using a powerful technique called "gravitational microlensing", which allows scientists to detect planetary bodies no matter how little light they emit.
The planet was spotted among combined microlensing data sets taken with a worldwide network of sophisticated telescopes.
Four amateur observatories in New Zealand contributed observations covering four nights in April 2012, while working as part of the international MicroFUN collaboration (Microlensing Follow-up Network).
In particular, Ian Porritt of Palmerston North, worked through gaps in clouds to obtain the first few critical measurements that revealed the planet was in a binary star system.
Stardome astronomer Dr Grant Christie said the give-away was "a very slight little dip" in the change in brightness of the binary system.
"The analysts knew that this was a sure sign of the existence of a planet in that system."
Although this planet itself was too cold to be habitable, the same planet orbiting a sun-like star in such a binary system would be in the so-called "habitable zone" - the region where conditions might be right for life.
"Small dim stars are the most common in our galaxy and the majority of these are found in binary systems," Dr Christie said.
"They have much longer lives than our Sun and could potentially provide a stable habitable environment over very large time spans."
Planets such as this were likely to be volcanically active, so potential habitats for life could exist beneath the surface.
