Earth is the only inhabited planet known to us. Therefore, in search of life on other planets, we are guided by the parameters of our own. But the conditions for life as we know it may not be limited to celestial bodies like Earth.. Scientists have now identified a type of exoplanet that could be habitable for billions of years, Science Alert reports..

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The key parameter for the search is the presence of liquid water, which can exist for a long time.. On Earth, liquid water was an essential ingredient for the origin of life.. Exoplanets that have water have a higher chance of developing life as we know it..

Now a team of scientists led by Marit Mol Luz at the University of Zurich in Switzerland has concluded that a dense atmosphere of hydrogen and helium can maintain the temperature and conditions necessary for life to exist for long enough..

“One of the reasons that water on Earth can be liquid is its atmosphere.. Thanks to its natural greenhouse effect, it traps just enough heat to create the right conditions for oceans, rivers and rain,” said theoretical astrophysicist Rawit Helled of the University of Zurich..

But Earth's atmosphere wasn't always the way we know it today.. Now most of it is nitrogen, followed by oxygen and only small amounts of hydrogen and helium.. But when a planet first forms, it is surrounded by a primordial atmosphere, which consists mainly of hydrogen and helium, the main components of the cloud of gas and dust from which the Sun and our system formed..

The Earth lost its primary atmosphere fairly quickly, probably through several processes, including radiation from the young Sun and bombardment by meteorites.. But it's likely that a super-Earth exoplanet — more massive than Earth but less massive than Neptune — could retain a primordial atmosphere for much longer..

“Such massive primordial atmospheres can also cause a greenhouse effect — much like Earth’s atmosphere today.. So we wanted to find out if these atmospheres could help create the right conditions for liquid water,” Helled explains..

The researchers conducted simulations of planets with different masses of the core, atmosphere and different distances to stars.. It turned out that planets with dense primary atmospheres can be warm enough to support the presence of liquid water for up to 10 billion years..

But there are some limitations. To avoid intense radiation, the planet must be at a sufficient distance from the star. About twice as far as the Earth is from the Sun. For the solar system, this is so far away that any water should freeze..

The sun is not the only source of heat available to the planet. Some worlds, including Earth, can generate their own heat. This can occur for a number of reasons, such as geothermal processes and the presence of radioactive elements that release heat as they decay..

Thus, if a super-Earth exoplanet at this distance from its parent star had both a primordial atmosphere and sufficient internal heat to keep warm, then the conditions for liquid water on the surface would be met, the researchers say..

This internal heat model could hypothetically support life on worlds with a thick icy shell, such as Saturn's moon Enceladus and Jupiter's moon Europa.. However, for the model of scientists it is necessary that several conditions are met in a particular place.. It's not impossible, scientists say, but it could take a long time..