Astronomers have long been looking out into the vast universe in hopes of discovering alien civilizations. But for a planet to have life, liquid water must be present. The probability of this scenario seemed impossible to calculate, as was the assumption that planets like Earth got their water by chance when a large ice asteroid hit the planet.
Now, researchers from the GLOBE Institute at the University of Copenhagen have published a study indicating that water may be present during the formation of a planet itself. According to the study's calculations, this is true for Earth, Venus and Mars alike.
"All our data suggests that water was part of the building blocks of Earth from the very beginning. And since the water molecule is occurring frequently, there is a reasonable probability that it applies to all planets in the Milky Way. The decisive point for the presence of liquid water is the distance of the planet from its star," says Professor Anders Johansen of the Center for FormingStars and Planet, who led the study published in the journal Science Advances.
Using a computer model, Anders Johansen and his team calculated how quickly planets are formed, and from what building blocks. The study indicates that it was millimeter-sized dust particles of ice and carbon - known to orbit around all the young stars in the Milky Way - that 4.5 billion years ago accumulated in the formation of what later becamewould become the Earth.
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"Up to the point where the Earth had grown to one percent of its current mass, our planet grew by capturing masses of pebbles filled with ice and carbon. The 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 at the surface rose sharply, causing the ice on the pebbles to evaporateon the way to the surface, so that today only 0.1 percent of the planet consists of water, although 70 percent of the 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.
The theory, called 'pebble accretion', says that planets are formed by pebbles that clump together, and that the planets then grow bigger and bigger.
Anders Johansen explains that the water molecule H2O is found everywhere in our galaxy, and that the theory therefore opens up the possibility that other planets may have been formed in the same way as Earth, Mars and Venus.
"All planets in the Milky Way may be formed by the same building blocks, which means that planets with the same amount of water and carbon as Earth - and therefore potential places where life may be present - often occur around other stars in our galaxy, provided the temperature is right," he says.
If the planets in our galaxy had the same building blocks and temperature conditions as Earth, there is also a good chance that they could have about the same amount of water and continents as our planet.
Professor Martin Bizzarro, co-author of the study, says:
"With our model, all planets get the same amount of water, and this suggests that other planets may have not only the same amount of water and oceans, but also the same amount of continents as here on Earth. This provides good opportunities for the emergence of life," he says.
If, on the other hand, it was random how much water was present on the planets, the planets could look very different. Some planets would be too dry to develop life, while others would be completely covered by water.
"A water-covered planet would of course be good for sea creatures, but would offer less than ideal conditions for the formation of civilizations that can observe the universe," says Anders Johansen.
Anders Johansen and his research team are looking forward to the next generation of space telescopes, which will offer much better opportunities to observe exoplanets orbiting a star other than the Sun.
"The new telescopes are powerful. They use spectroscopy, which means that by looking at what kind of light is being blocked from the planets orbit around its star, you can see how much water vapor there is. It can tell us something about the number of oceans on that planet," he says.
With information from University of Copenhagen.
