Mysterious Rogue Exoplanet’s Weather Map Revealed

An artist's impression of PSO J318.5-22. MPIA/V.Ch.Quetz
Robin Andrews 04 Nov 2015, 17:06

Most people don’t trust weather forecasts, but to be fair to meteorologists, they are notoriously difficult predictions to make. A team of researchers has upped the ante somewhat by analyzing the weather patterns not on our own planet, but on a young gas giant 75 light-years from home. Their findings, which reveal an exoplanet’s weather for the first time, have been submitted for publication in The Astrophysical Journal.

PSO J318.5-22 is somewhat similar in size to Jupiter but eight times more massive. Exoplanets – planets outside our own Solar System – are regularly discovered these days, but this one is particularly strange as it does not orbit a star. In what is termed a free-floating, or “rogue,” planet, this 20-million-year-old object is zooming through deep space. It may have been set loose in much the same way that Jupiter, with its vast gravitational pull, threw another planet out of our own Solar System.

As this planetary object isn’t orbiting a star, the team was able to accurately measure changes in its brightness as light from distant stars reflected off its atmosphere and back into space. Much of their work looked at the changes in the object’s infrared spectrum – commonly used to detect changes in heat flow – over its speedy five-hour rotation period.

Using a specialized telescope called SOFI based in Chile, the University of Edinburgh researchers were able to build up a detailed picture of the comings and goings on the distant world. They also had another observational advantage: The object has planetary neighbors and, by comparing their brightness levels to each other, additional details about their physical characteristics could be ascertained.

Any changes in cloud cover would show up as dips and peaks in brightness; if PSO J318.5-22 had a more turbulent atmosphere, it would also show greater variations in its brightness. As it turns out, its highly fluctuating brightness means that this relatively dense gas giant has complex cloud cover patterns, with overlapping layers of thin and thick clouds concealing a broiling, 800°C (1,472°F) internal temperature.

Headlines declaring that the planet has molten iron rain may be somewhat overstated, however. Dr. Beth Biller, who led the research, told IFLScience: “There’s a 2011 study that [computationally] models another similar planetary object, which looks at silicate clouds and iron-rich clouds – for PSO J318.5-22, I would bank on hot silicates, not hot iron droplets, but you could have both.” The chemistry of the clouds was not a direct part of the paper. “We don’t know, really,” Biller added.

The team noted that the massive variability in the brightness of the object could be explained by other means: The object could be an extremely hot object, including a star or a “failed star” called a brown dwarf. Starspots, the extrasolar equivalent to our sunspots, could hypothetically explain the brightness changes. However, compared to stars or brown dwarfs, PSO J318.5-22 is relatively cool, and the most likely mechanism for the observed brightness variability is very changeable cloud cover. Though what these clouds consist of is currently unknown.

“We now have Hubble and Spitzer time allocated to observe this object,” Biller said. These two powerful telescopes will no doubt reveal more about this mysterious, distant world far beyond our Solar System.

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