The discovery, which the scientists called "groundbreaking," was made using a technique noted in 1912 by Albert Einstein through a network of telescopes positioned around the globe. Thetakes the nondescript name "OGLE-2005-BLG-390" after one such telescope, named OGLE, for Optical Gravitational Lensing Experiment.
OGLE, based in Poland's Warsaw University Observatory, was first used to spot the object on July 11, 2005.
"This planet is actually the first and only planet that has been discovered so far that is in agreement with the theories for how our solar system formed," said Uffe Grae Jorgensen, a scientist with the Niels Bohr Institute in Copenhagen, Denmark, and a member of the team who detected the planet.
The planet is five times the mass of Earth, but it's still considered the smallest and coolest ever detected outside of the solar system, according to the scientists. It circles a red parent star, which is five times smaller than the Sun, in about 10 years. Because the parent star is cool and within a large orbit, scientists believe OGLE's surface is icy and too cold for liquid water at an estimated 220 degrees Centigrade below zero.
Scientists also predict it has a thin atmosphere, like Earth, with a rocky surface buried deep under frozen oceans. These characteristics would make it a larger version of Pluto, rather than thelike Earth and Venus, they say.
The discovery was reported in this week's issue of Nature.
The method used to detect the planet is called "microlensing." Planetary team member Andrew Williams, of Australia's Perth Observatory, explained it as a method of allowing the gravity of a dim, intervening star to act as a giant natural telescope, magnifying a more distant star, which then temporarily looks brighter for about a month.
"A small 'defect' in the brightening reveals the existence of a planet around the lens star. We don't see the planet, or even the star that it's orbiting; we just see the effect of their gravity," he said.
Any other planets orbiting the star can produce an additional signal. Those signals can last for days if it's a giant planet or for hours if it's of a smaller mass, like Earth.
Microlensing involves nearly continuous monitoring of the stars through various telescopes around the world. What's called the Planet Network operates such a telescope network, with telescopes in Chile, Tasmania, Australia, South Africa, Spain and Hawaii, among other locations. Once the OGLE telescope spotted the planet, the network was triggered to begin taking continuous data.
Jean-Philippe Beaulieu, a scientist at the Institut d'Astrophysique de Paris, said that only three planets have been discovered through miscrolensing techniques. "While the other two microlensing planets have masses of a few times that of Jupiter, the discovery of a 5 Earth mass planet--though much harder to detect than more massive ones--is a strong hint that these lower-mass objects are very common," he said in a statement.