Layer of Sunscreen Discovered on Boiling-Hot Exoplanet

WASP-33b’s stratosphere was detected by measuring the drop in light as the planet passed behind its star (top). Low stratosphere temps rise as radiation is absorbed (right). Without a stratosphere, temps would cool at higher altitudes (left) / NASA/Goddar
Janet Fang 15 Jun 2015, 18:29

The blazing-hot exoplanet WASP-33b is more than four times the mass of Jupiter, and its close orbit to its star heats the surface to over 3,000oC (over 5432oF). Now, astronomers using NASA’s Hubble Space Telescope have detected a stratosphere around the planet. This primary atmospheric layer contains molecules that absorb ultraviolet and visible light from its host star -- protecting the planet like a sunscreen of sorts.  

“Some of these planets are so hot in their upper atmospheres, they’re essentially boiling off into space,” Avi Mandell of NASA’s Goddard Space Flight Center says in a news release. “At these temperatures, we don’t necessarily expect to find an atmosphere that has molecules that can lead to these multilayered structures.”

On Earth, the stratosphere sits atop the turbulent troposphere where weather happens. This layer stretches from the ground (where it’s warmer) up to where most clouds top out (where it’s cooler). Temperature in the stratosphere, on the other hand, increases with altitude. This so-called temperature inversion happens when ozone in the stratosphere absorbs UV radiation from the sun and prevents it from reaching us on the surface. That UV is then converted into heat that warms the stratosphere. Similar inversions are found on Jupiter and Saturn, though theirs are due to hydrocarbons instead of ozone. Well, neither of these can survive at the crazy hot temperatures of most exoplanets we know about, so researchers weren’t sure if stratospheres do exist for planets outside our solar system. 

But now, Mandell and colleagues report the discovery of a temperature inversion in the atmosphere of WASP-33b, and they think it’s caused by titanium oxide. The findings were published in The Astrophysical Journal this week.

The researchers analyzed observations from Hubble’s Wide Field Camera 3, which captures in the near-infrared region. When light from the exoplanet dipped as it passed behind its parent star (pictured above), the team was able to compare emissions from water and gases at different altitudes. Higher up, the temperature is about 3,300oC (6,000oF), and lower down, the temperature is about 1,650oC (3,000oF). Without a stratosphere, temperatures would cool at higher altitudes.

Titanium oxide is one of a few compounds that can strongly absorb visible and UV radiation while remaining in a gaseous form. That’s probably how it stays in an atmosphere that hot. 

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