Scientists have identified two complex organic molecules, or building blocks of life, on a comet for the first time, shedding new light on the cosmic origins of planets like Earth.

Ethyl alcohol and a simple sugar known as glycolaldehyde were detected in Comet Lovejoy, according to the study in the journal Science Advances.

"These complex organic molecules may be part of the rocky material from which planets are formed," the study's authors said.

Other organic molecules have previously been discovered in comets, most recently in comet 67P/Churyumov-Gerasimenko, on which the European space agency's Philae found four that had never before been detected on a comet.

Since comets contain some of the oldest and most primitive material in the solar system, scientists regard them as time capsules, offering a peek at how it all started 4.6 billion years ago.

But while the latest study does not end the debate over whether falling comets indeed seeded Earth with the components necessary for life, it does add something to our knowledge, said study co-author Dominique Bockelee-Morvan, an astrophysicist at the French National Centre for Scientific Research.

"The presence of a major complex organic molecule in comet material is an essential step toward better understanding the conditions that prevailed at the moment when life emerged on our planet," she said.

"These observations show a possible explanation for its (life's) origin on our planet."

Other hypotheses exist too, including the gas from volcanic eruptions.

Comet Lovejoy is of particular interest to scientists because "it is one of the most active comets in Earth's orbital neighbourhood", the study read.

The research was done using a 30-metre long telescope at the Institut de Radioastronomie Millimetrique in Sierra Nevada, Spain in January 2015, when the comet was brightest and most productive.

AFP