An established method used for detecting biosignatures on distant planets - one of the ways scientists search for extraterrestrial life - is flawed and can produce false-positive results, according to new research.

Astronomers are able to detect the presence of chemicals on exoplanets, and the presence of methane and oxygen, for example, is considered evidence of past or present life. However, research led by University of Toronto Scarborough scientist Hanno Rein suggests that such biosignatures cannot be taken as gospel.

Rein's team discovered that a lifeless exoplanet with a lifeless moon can still give off the appearance of having the methane-oxygen biosignature.

Rein contends that the technical limitations of current telescopes prevent them from properly identifying a genuine biosignature from a false-positive.

"You wouldn't be able to distinguish between them because they are so far away that you would see both in one spectrum," Rein said of the chemicals commonly associated with life.

"A telescope would need to be unrealistically large, something one hundred meters in size and it would have to be built in space," he said in a statement. "This telescope does not exist, and there are no plans to build one any time soon."

Current methods allow scientists to estimate an exoplanet's size and temperature, which can be used to determine whether liquid water could exist on the planet's surface. However, collecting accurate data on the atmosphere of exoplanets remains elusive.

"We can't get an idea of what the atmosphere is actually like, not with the methods we have at our disposal," Rein said.

However, the odds of life existing elsewhere in the universe are good. There are 1,774 confirmed exoplanets - planets outside our solar system - but some estimates suggest there could be more than 100 billion planets in the Milky Way galaxy alone.

"There are plenty of reasons to be optimistic that we will find hints of extraterrestrial life within the next few decades, just maybe not on an Earth-like planet around a Sun-like star," Rein said.

Rein contends that the priority for the search for extraterrestrial life should remain within our solar system, noting the recent discovery of a liquid ocean on Saturn's moon Enceladus.

"We should make sure we are looking at the right objects," Rein said. "As for exoplanets we want to broaden the search and study planets around stars that are cooler and fainter than our own Sun. One example is the recently discovered planet Kepler-186f, which is orbiting an M-dwarf star."

Rein and his collaborators published their work in the Proceedings of the National Academy of Sciences.