Earth-like planets are very common in the Milky Way galaxy

Earth-like exoplanets, including those with features such as continents and liquid water on the surface, may be numerous in the Milky Way galaxy.

Previous models of planetary formation have indicated that the abundance of water needed for life on Earth could have come from extremely lucky comet strikes, but new models seem to indicate that rocky, Earth-like planets can attract enough water during their developmental stage, to make them as favorable to life as the Earth.

On Earth, most life depends on things like oxygen and sunlight. Some extreme life forms, known as extremophiles, have shown the ability to live without sunlight, and some anaerobic life forms without oxygen. However, all life on Earth needs water in one form or another.

New research into planet formation shows that Earth’s oceans are not the result of an accidental cosmic collision, but were created during the normal development of the planet. Moreover, such a process means that Earth-like exoplanets with water on the surface and active continents are likely to be distributed throughout the galaxy.

The results of the latest study, conducted by scientists from the GLOBE Institute at the University of Copenhagen, suggest that water appeared on Earth during the natural process of planet formation – a model of planetary development known as “pebble accretion”.

“All of our evidence suggests that water has been part of the Earth’s building blocks from the very beginning,” Professor Anders Johansen of the Center for Star and Planet Formation said in a press release announcing the new study. “And since the water molecule is so common, there’s a reasonable chance that this applies to all the planets in the Milky Way.”

As to whether this water could exist in liquid form on the surface of such a planet, Professor Johansen said: “The decisive factor for the presence of liquid water is the planet’s distance from its star.” This orbital distance is most commonly referred to as the star’s habitable zone.

To reach this conclusion, Johansen and the rest of the GLOBE team used updated computer models of planetary formation. Using the latest data, these models showed that millimeter-sized particles of carbon and ice, “which are known to orbit all young stars in the Milky Way,” accreted together during the first 5 million years of Earth’s formation, resulting in so much water which is observed today.

Until the moment when the Earth grew to one percent of its current mass, our planet grew by capturing a mass of pebbles filled with ice and carbon,” explains Johansen. “Then the Earth grew faster and faster until after five million years it became as big as we know it today.”

During this phase, the temperature on the Earth’s surface must have risen sharply, causing the ice in the pebbles to evaporate on its way to the surface. As a result, says Johansen, “only 0.1 percent of the planet is made up of water, even though 70 percent of the Earth’s surface is covered in water.”

“All the planets in the Milky Way could be formed from the same building blocks,” Johansen added, “meaning that planets with the same amount of water and carbon as on Earth are potential places where life, are often found around other stars in our galaxy.”

According to a study published in the journal Science Advances, Venus and Mars could have gone through a similar stage of accretion that brought water to the surface. Of course, today none of the planets have water on their surface, but this is most likely due to individual planetary conditions, and not a lack of available water molecules at the time of their formation.

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