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Now, researchers from the GLOBE Institute at the University of Copenhagen have published an eye-opening study, indicating that water may be present during the very formation of a planet. According to the study's calculations, this is true for both Earth, Venus and Mars.

"All our data suggest that water was part of Earth's building blocks, right from the beginning. And because the water molecule is frequently occurring, there is a reasonable probability that it applies to all planets in the Milky Way. The decisive point for whether liquid water is present is the distance of the planet from its star," says Professor Anders Johansen from the Centre for Star and Planet Formation who has led the study that is published in the journal Science Advances.

Using a computer model, Anders Johansen and his team have calculated how quickly planets are formed, and from which building blocks. The study indicates that it was millimetre-sized dust particles of ice and carbon -- which are known to orbit around all young stars in the Milky Way -- that 4.5 billion years ago accreted in the formation of what would later become Earth.

"Up to the point where Earth had grown to one percent of its current mass, our planet grew by capturing masses of pebbles filled with ice and carbon. Earth then grew faster and faster until, after five million years, it became as large as we know it today. Along the way, the temperature on the surface rose sharply, causing the ice in the pebbles to evaporate on the way down to the surface so that, today, only 0.1 percent of the planet is made up of water, even though 70 percent of Earth's surface is covered by water," says Anders Johansen, who together with his research team in Lund ten years ago put forward the theory that the new study now confirms.