HUMANS are not alone in struggling to stay slim. Some planets go through a "fat" stage that swells their waistlines temporarily, which possibly explains why some gas giants are unexpectedly large.

"Astronomers have power leveling found a lot of planets whose sizes cannot be explained by standard theory," says Laurent Ibgui of Princeton University. The difference between predicted and measured widths of so-called "hot Jupiters" can be 30 per cent or more.

Previously, astronomers assumed that, because cold gas takes up less volume than hot gas, hot Jupiters would shrink as they lost their initial heat.
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Now a computer simulation by Ibgui suggests this effect can be temporarily halted in hot Jupiters that begin life in highly elliptical orbits. These planets are alternately squeezed and stretched as they circle their stars, resulting in "tidal heating" that warms the gas inside the planet. This counteracts the cooling effect, inflating the planet - an effect that can last for a billion years or more. Eventually, though, the planet's orbit will become more circular, and the hot Jupiter resumes shrinking. Ibgui presented the research at an American Astronomical Society meeting in Pasadena, California, last week.
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The model doesn't quite explain the superpuffy appearance of all hot Jupiters, says Jonathan Fortney of the University of California, Santa Cruz. "It requires these planets to start their lives on very eccentric orbits, which is possible, but not currently the preferred scenario."