What a scorcher: ‘Hot Jupiter’ puzzle explained

Scientists said they could explain why heat-seared giant planets in other solar systems can mysteriously orbit their star the “wrong” way.

These space oddities belong to a class of planets known as hot Jupiters – gassy giants that are the size of our own Jupiter but encircle their sun at sometimes scorchingly close distances.

What has bedevilled astronomers is the discovery that in some star systems, the host star spins one way while a hot Jupiter can orbit in the opposite direction.

This throws down a challenge to the yardstick of celestial mechanics, which is our solar system.

In our neighbourhood, all the planets trot obediently around the Sun in the same direction as its own spin. (The solar “day” – the amount of time it takes for the Sun to complete one revolution at its equator – is 26 days.)

“We had thought our solar system was typical in the Universe, but from day one, everything has looked weird in the extrasolar planetary systems. That makes us the oddball really,” said Frederic Rasio, an astrophysicist at Northwestern University in Illinois.

In a study published by the science journal Nature, Dr Rasio’s team devised a model that, they believe, explains this contra-rotation conundrum.

The researchers start with the theoretical basis of a star that is similar in size to the Sun.

It has a system comprising two large planets that lie relatively close to each other and, initially, are located at a long range from the star. Each orbit the star in the “right” direction.

The idea for the model is that a combination of forces – a gravitational coupling between the two planets as they near each other, and powerful tides on the innermost planet – causes this cosy planetary system to break up.

Over a very long time, the inner planet starts to shift into an eccentric needle-shaped orbit, which gradually shrinks.