In the last two decades, the study of extrasolar planets - those that lie outside our own solar system - has become one of the most important fields of astrophysics.

Now a National Aeronautics and Space Administration (NASA) team that includes Prof. Tsevi Mazeh of Tel Aviv University's Department of Astronomy and Astrophysics and the Director of the Wise Observatory has discovered two new planets, named Kepler-34 and Kepler-35, each of which revolves around its own double suns. Together with Kepler-16, discovered a few months ago, there are now three such known systems in the galaxy.

According to Prof. Mazeh, these discoveries indicate that planets revolving around binary suns (suns that are formed as a pair) are a common phenomenon. Double stars or suns are typical in the universe, and now we know that planets can orbit around these intriguing phenomena, he says.

The team discovered the planets, which are 5,000 light years from Earth in the Cygnus constellation, by measuring the light emitted by the double suns. The data was collected by NASA's Kepler satellite, and the results recently published in the journal Nature.

It takes two
Most suns in the universe exist in pairs, explains Prof. Mazeh. These partnerships closely mimic human relationships - if two suns are formed together, they stay together, unless a third star comes too close to the pair and breaks the bond between the two.

Our solar system, which revolves around one sun, is more unusual, though we can't dismiss the possibility that our sun has an undiscovered distant companion, he says. And while the phenomenon of binary stars has been well known for centuries, the recent discoveries prove that binary suns can also support planets.

Each sun in these systems revolves around its mate in a regular, cyclical pattern. During sunsets on Kepler-34 and Kepler-35, one sun will descend first, followed by a twilight period. Afterwards, the second sun will set and night will fall.

In Hebrew, the word for twilight means "between the suns," explains Prof. Mazeh, saying that the translation is an accurate description of what twilight is like on these newly discovered planets. Kepler-34 revolves around its double sun every 289 days, Kepler-35 every 131 days.

This discovery provides a unique opportunity to learn about solar systems that are very different from our own, says Prof. Mazeh. In the future, more research will be done on the planets themselves, including their possible atmospheres and the rotation of the planets.

A limitless universe
An expert in extrasolar planets and recent recipient of the Weizmann Prize for Excellence in Science, Prof. Mazeh is grateful to be working with the Kepler team. When he began his work in the early 1980s, it was widely believed that all planets and suns must be similar to the ones within our own solar system. And this simply isn't the case, he says.

"We shouldn't limit our search by assuming that all the planets are like those in our solar system. Some of them are very different from what we have here, and every time we find a new planet, we're explorers landing on unknown territory.

"The sky is not the limit," he smiles.