super-Earth
This artist's depiction shows a gas giant planet rising over the horizon of an alien waterworld. New research shows that oceans on super-Earths, once established, can last for billions of years.David A. Aguilar/CfA

Super-Earths can have oceans that last over 10 billion years and have the potential to host life after 5.5 billion years, scientists have said.

These planets are up to five times the mass, or 1.5 times the size of our own Earth. Scientists from the Harvard-Smithsonian Center for Astrophysics (CfA) was looking to find out if super-Earths could recycle water in the same way Earth does, and if so, how long it would take for them to form.

Laura Schaefer, lead author of the study, said: "When people consider whether a planet is in the habitable zone, they think about its distance from the star and its temperature. However, they should also think about oceans, and look at super-Earths to find a good sailing or surfing destination."

Presenting her findings at the meeting of the American Astronomical Society, Schaefer said super-Earth's can not only have oceans, but that they can maintain them for billions of years.

In our current understanding, planets need liquid water for life to develop. On Earth, oceans have existed for almost as long as the planet has.

The ocean covers about 70% of Earth's surface but makes up only a very small fraction of the planet's overall bulk: "Earth's oceans are a very thin film, like fog on a bathroom mirror," study co-author Dimitar Sasselov said.

Earth from space
About 70% of Earth's surface is made up of water.Nasa

However, research also shows that Earth's mantle holds vast quantities of water – far more than is on the surface – which was dragged underground by plate tectonics then returned through volcanic activity.

Using simulations to see if this recycling process would take place on super-Earths, Schaefer found that planets between two and four times the mass of Earth are best at establishing and maintaining oceans – better than Earth, in fact.

Oceans on these super-Earths would last for at least 10 billion years, the authors found.

The study also showed that the largest planet studied – five times the mass of Earth – took longer for an ocean to develop, suggesting that if scientists want to search for life it is better to look to older super-Earths.

Optimally - and assuming evolution takes place at a similar rate to Earth – complex life should start appearing after about five and a half billion years after the planet's birth.

Sasselov said: "It takes time to develop the chemical processes for life on a global scale, and time for life to change a planet's atmosphere. So, it takes time for life to become detectable."