The smallest Earth-like planet yet has been discovered outside our solar system, say astronomers who used a new method to find it.

The planet is 5.5 five times more massive than Earth. It orbits a star 28,000 light years away in the constellation Sagitarrius, near the centre of the Milky Way galaxy. (A light year is about 10 trillion kilometres, the distance light travels in a year.)

"This is an important breakthrough in the quest to answer the question, 'Are we alone?'" Michael Turner of the National Science Foundation said in a release.

Artist's impression of the newly discovered extrasolar planet. (Courtesy: European Southern Observatory)

Like Earth, planet OGLE-2005-BLG-390Lb has a rocky core. Its atmosphere is likely thin, and the surface temperature is estimated at -220 C, probably too cold to support life.

Researchers used a method called gravitational microlensing to detect the planet, which may be a more sensitive way of finding habitable planets that aren't too hot or cold, Turner said.

In 1912, Albert Einstein identified the microlensing technique.

"With this method, we let the gravity of a dim, intervening star act as a giant natural telescope for us, magnifying a more distant star, which then temporarily looks brighter," said team member Andrew Williams of the Perth Observatory in Australia.

Since July 2005, the researchers used three networks of telescopes on Earth to look for the telltale magnifying effect and confirm the existence of the planet. It is the third planet to be uncovered using microlensing.

Astronomers have found more than 160 planets orbiting outside the solar system using an older technique that detects the wobble in stars caused by the planet orbiting them.

But planets need to be relatively large or close to their stars to be detected using the wobble technique.

Researchers want to study such "exoplanets" to try to understand how planets form.