As children, we are taught that our sun is the centre of the solar system and the planets of the solar system all orbit around it (in the same direction) with periods that depend on the distance from the sun and planetary mass. We subsequently learn that our sun itself is in orbit around our galaxy, the Milky Way; so far so good.
Since 1995, astronomers have discovered more than 500 extra-solar planets which orbit other suns (stars). Recently, it has been shown that some of these systems have planets which orbit their sun in the opposite direction, a so-called flipped hot Jupiter. A ''hot Jupiter'' is a planet larger than the Jupiter of our own solar system which occupies an orbit close into its own star - hence ''hot Jupiter''. The conundrum was why should these giants be travelling in the wrong direction?
In a paper published in Nature, American researchers at Northwestern University have come up with an explanation for this apparently aberrant behaviour. The first author is Smadar Naoz, a postdoctoral fellow at Northwestern and the senior author is Frederic Rasio, professor of physics and astronomy at Northwestern's Weinberg College of Arts and Sciences is the senior author of the paper. The research team adapted the basic orbital mechanics approach used by NASA for sending satellites around the solar system.The model the team used involved a star of similar mass to our sun, an inner planet of a mass similar to Jupiter (a gas giant) and a second large planet further out from the star. The inner planet starts quite distant from the sun (which is where gas giants such as Jupiter are thought to form). The outer planet interacts with the inner planet disturbing the system through weak forces which gradually build up over very long time periods.
The influence of the outer planet on the “hot Jupiter” causes it to orbit very close to the sun and to adopt an orbit in the opposite direction to it. The model suggests that the changes induced by the outer planet are due to an exchange of angular momentum and energy loss at the inner planet through strong tides via the phenomenon of gravitational coupling.
The inner planet at first adopts an eccentric needle-shaped (tight elliptical) orbit due to gravitational coupling. It loses angular momentum to the outer planet, causing its orbit to shrink (as energy is dissipated by tides on the hot Jupiter), moving it closer to the star and heating it up. The flipping may occur during this process. Essentially, the process is one of evolving gravitational forces which move planets around in the heavens – it is quite possible that the outer planet may move away after the hot Jupiter has formed since the forces involved will have changed.
Astronomers have observed that about a quarter of hot Jupiters have flipped orbits and the Northwestern University model accommodates both flipped and normal hot Jupiters in its outcomes, depending upon the input parameters.
In explaining the peculiar configuration of an extrasolar system, the researchers also have added to our general understanding of planetary system formation and evolution and reflected on what their findings mean for the solar system.
''We had thought our solar system was typical in the universe, but from day one everything has looked weird in the extrasolar planetary systems,'' Rasio said. ''That makes us the odd ball really. Learning about these other systems provides a context for how special our system is. We certainly seem to live in a special place.''