The Earth as a habitable planet may be rarer than we thought. Scientists have used computer models to show that the reason our planet and has a moderate climate and isn't an ocean world is because there was a massive star in the vicinity of the primordial solar system. This star provided the growing inner planets with radioactive elements that evaporated some of the water that otherwise would have been delivered to them.

One of the great scientific mysteries during the Victorian era was why the Earth wasn't a frozen wilderness? As geologist learned more about how to calculate the age of the planet accurately, the numbers produced a real poser because they found that the Earth shouldn't be anywhere near as warm as it is.

The accepted theory at that time was that the Earth had started out as a giant molten mass that gradually cooled off. This was the reason why the age of the dinosaurs was hotter than it is today and suggested that the planet would be cooler centuries from now. The problem was that geologists had discovered that the Earth was billions of years old, which meant that the world should have long ago been reduced to its ambient base temperature with the only heat coming from the Sun to warm it.

However, that wasn't the case. The Earth is very warm and the interior is molten and very active. Why? The answer came with the discovery of radioactivity. It was found that the presence of radioactive elements like uranium provide the interior of the Earth with an energy source that keeps it hot even after many epochs of cooling.

Today, scientists have learned much more about how planets form and one result of the latest computer simulations is that a planet like the Earth is very odd. We think of it as being watery – comparing it to planets like Mercury or Mars, the oceans that cover 75 percent of the Earth's surface do make it seem like a water world. But the models say that in a planetary system like ours, the Earth should be a giant ball of ice with a frozen ocean many miles thick covering the entire surface from pole to pole.

According to University of Michigan astronomer Michael Meyer, the computer simulations provided part of the answer if it's assumed that there was a massive star in the area of the solar system as it formed. When such stars reach the end of their lives, they go nova and expel enormous amounts of matter with some of it made up of radioactive elements like aluminium-26.

The simulations suggest that having these elements present in the planetesimals or building blocks of the new planet (and its neighbors), provided added heat that helps to evaporate most of the water, preventing a global ocean with an impenetrable layer of ice on the ocean floor from forming. This allows the carbon cycle to begin, which helps stabilize the climate and results in surface conditions favorable to life.

The important thing about this discovery isn't just that it sheds new light on how the Earth forms, but it will also help space scientists predict which exoplanets beyond our solar system are worth looking at for signs of life. By looking for the proper radioactive isotopes, it may be possible to predict whether a candidate planet is Earth-like or a giant ice world. In addition, a better understanding of the mechanism will help in calculating how many Earth-like worlds there are in the galaxy.

"It is great to know that radioactive elements can help make a wet system drier and to have an explanation as to why planets within the same system would share similar properties," says Meyer. "But radioactive heating may not be enough. How can we explain our Earth, which is very dry, indeed, compared to planets formed in our models? Perhaps having Jupiter where it is was also important in keeping most icy bodies out of the inner solar system."

The research was published in Nature Astronomy.