Finding life on exoplanets, or planets outside the solar system, may be more difficult than people thought, researchers from China, the United States and Argentina said Monday.

Current efforts to find a second Earth focus on so-called M dwarfs or red dwarfs, stars that are smaller than the Sun but make up more than 75 percent of the stars in the solar neighborhood. High levels of atmospheric oxygen are considered the most promising indicator for life on exoplanets.

However, recent observations of several planet-hosting M dwarfs showed that the ultraviolet (UV) properties of these small stars are quite different from those of the Sun, which could further complicate the search for alien life, the researchers said at a meeting of the American Astronomical Society Division for Planetary Sciences in Denver.

Using the observed UV spectrum of the M dwarf star GJ 876, Feng Tian, professor at Tsinghua University, and his U.S. and Argentine colleagues have shown that the atmosphere of a hypothetical habitable planet around the star could build up significant levels of oxygen even in the absence of life.

"In this case, the atmosphere of a lifeless planet can be close to that of the Earth's 2.2 billion years ago, after the so called Great Oxidation Event in Earth's geological history," Tian said in a statement.

In his new report, Tian further studied Earth-mass planets using the UV spectra of four other M dwarfs, including GJ 667C which contains three potentially habitable planets.

These studies provided further support to their previous conclusion: "Before we can claim the discovery of life on exoplanets, we have to examine the stars harboring these planets more carefully."

"Prof. Feng Tian's research addresses one of the most important questions of contemporary astrophysics and indeed of great interest to the general public: Are there other habitable planets near Earth, and is there any evidence that they are indeed inhabited?" said Professor Jeffrey Linsky of the University of Colorado at Boulder.

"The authors of this paper make an important point regarding the confidence we could have in the detection of O2 simultaneously with H2O and CO2, as a biosignature in the spectrum of an Earth- like exoplanet around an M star," said Alain Leger of the Institute d'Astrophysique Spatiale at University Paris XI, France.

The researchers, however, said the work requires further confirmation by other scientists.

"This is somewhat sending the cat among the pigeons in our confidence in the O2, H2O, and CO2 biosignature, but in a limited way. It concerns only M stars and the presence of O2 in small amounts," Leger said.