An international team of astronomers claimed Thursday that they may have spotted the first "exomoon ", or a moon orbiting a planet that lies outside the solar system.
But the object could also be a huge planet orbiting a small, faint star, Xinhua quoted the team as stating in the Astrophysical Journal.
"We won't have a chance to observe the exomoon candidate again, " lead author David Bennett of the University of Notre Dame said in a statement released by the US space agency NASA. "But we can expect more unexpected finds like this."
The possible exomoon was discovered during a study led by the joint Japan-New Zealand-American Microlensing Observations in Astrophysics (MOA) and the Probing Lensing Anomalies NETwork ( PLANET) programmes, using telescopes in New Zealand and Australia.
The astronomers used a technique called gravitational microlensing, in which one star passes in front of another as seen from Earth. The closer star can act like a magnifying glass to focus and brighten the light of the more distant one for a period of days to weeks during the event.
If the foreground star has a planet circling around it, the planet will act as a second lens to brighten or dim the light even more. By carefully scrutinising these brightening events, astronomers can figure out the mass of the foreground star relative to its planet.
In some cases, however, the foreground object could be a free- floating planet, not a star. Researchers might then be able to measure the mass of the planet relative to its orbiting companion.
In the new study funded by NASA, the astronomers found the ratio of the larger body to its smaller companion is 2,000 to 1, although the nature of the foreground, lensing object is not clear.
That means the pair could be either a small, faint star circled by a planet about 18 times the mass of Earth, or a planet more massive than Jupiter coupled with a moon weighing less than Earth, the astronomers said.
But they cannot tell which of these two scenarios is correct, since the encounter can be witnessed only once.
"One possibility is for the lensing system to be a planet and its moon, which if true, would be a spectacular discovery of a totally new type of system," said Wes Traub, chief scientist for NASA's Exoplanet Exploration Programme office at NASA's Jet Propulsion Laboratory, who was not involved in the study.
"The researchers' models point to the moon solution, but if you simply look at what scenario is more likely in nature, the star solution wins," Traub added.