Finding life on distant exoplanets may be more difficult than scientists thought, researchers from China, the United States and Argentina said Monday.

Recent observations of several planet-hosting M dwarf stars -- the focus of current efforts to find Earth-like planets -- showed ultraviolet properties of the small stars are quite different from those of the sun, which could further complicate the search for alien life, the researchers told a meeting of the American Astronomical Society Division for Planetary Sciences in Denver.

That could complicate the search for chemical signs of life, they said.

Feng Tian, a professor at Tsinghua University, and his U.S. and Argentine colleagues have shown that the atmosphere of a hypothetical habitable planet around such a star could build up significant levels of oxygen -- one possible "signature" of alien life -- even in the absence of any such life.

"Before we can claim the discovery of life on exoplanets, we have to examine the stars harboring these planets more carefully," the researchers said.

Other scientist agreed.

"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," Alain Leger of the Institute d'Astrophysique Spatiale at University of Paris said.