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A Stratospheric Temperature Inversion on the Extra-Solar Planet XO-3b

Most of us are familiar with the fact that, at least near the surface of the Earth, the temperature of the air goes down as you go higher in the atmosphere.  This is why it is generally cooler in the mountains than it is down near sea level.  Our experience of the temperature variations as we move around from one place to another on the Earth is controlled by the lowest layer of the Earth's atmosphere: the troposphere.  In this layer, which extends up about 12 kilometers from the surface, the air temperature steadily decreases with altitude, going from about 20 degrees Celsius (293 degrees Kelvin) at the Earth's surface to about -55 degrees Celsius (218 degrees Kelvin) at the top of the troposphere. 

At the top of the troposphere, this steady decline in temperature halts, and the temperature begins to rise again as we move into the next layer of the Earth's atmosphere, the stratosphere.  This reversal in the temperature decline is called the stratospheric temperature inversion, and the reason for it is the specific way the different layers in the Earth's atmosphere are heated.  The troposphere is heated by the Earth itself which is warmed by the sunlight it absorbs.  As you move up in the troposphere away from the warming of the Earth, the temperature falls.  In the stratosphere, the oxygen that is in the Earth's atmosphere is in the form of ozone, a molecule with three oxygen atoms.  This molecule absorbs ultraviolet radiation from the Sun very efficiently and directly heats the stratospheric layers of the Earth's atmosphere, causing the temperature to start to rise again.

While we can study the structure of the Earth's atmosphere in great detail, we are only recently able to begin to perform these types of studies on planets orbiting other stars.  One such planet is XO-3b, a very massive planet (approximately 12 times the mass of Jupiter) orbiting a star about 850 light years away.  XO-3b was discovered in 2007 by Rice astronomer Christopher Johns-Krull and a team of both professional and amateur astronomers from around the world.  This planet is very different from the planets of our own solar system.  It is a gas giant, like Jupiter and Saturn, and while its mass is about 12 times that of Jupiter, its diameter is only about 20% larger than that of Jupiter.  But while Jupiter takes approximately 12 years to orbit the Sun, XO-3b zips around its star every 3.2 days because it is extremely close to that star.  Another oddity of XO-3b is that its orbit is noticeably eccentric: instead of moving around its star in a near perfect circle, the path it takes is a bit oblong. 

The odd nature of XO-3b, particularly its proximity to its parent star and the eccentricity of its orbit, make the conditions in its atmosphere very different from that of either the Earth or Jupiter.  Astronomers at Rice and at other institutions around the US have recently used the Spitzer Space Telescope  to study the conditions on XO-3b.  The Spitzer telescope studies infrared radiation, which is primarily heat radiation.  Studies of planets like XO-3b at visible wavelengths just see the light from the star and the effect that planet can have on that, primarily blocking some of the star light as the planet moves in front of the star.  Spitzer permits astronomers to study some of the radiation produced by the planet itself.  The analysis of these recent observations shows that XO-3b has a typical temperature of about 2000 degrees Kelvin and that it also has a stratospheric temperature inversion.  In addition, while it XO-3b is very compact, it is actually bigger than would be expected given its mass and how much radiation it is receiving from its star.  The recent observations suggest that the larger size of XO-3b is caused by its eccentric orbit.  As it moves closer to and further from its star, it experiences a different gravitational pull from the star.  This produces tidal forces on the planet which actually distort its shape just a bit.  This constant reshaping of the planet causes heat to be produced inside as the different parts of the planet rub against each other, and it is this extra source of heat that puffs up the planet a little bit bigger than it otherwise is expected to be.


Spitzer    xo3

Left: The Spitzer Space Telescope, launched by NASA in 2003.  The Spitzer Space Telescope is the final mission in NASA's Great Observatories Program - a family of four space-based observatories, each observing the Universe in a different kind of light.  Spitzer is designed to detect infrared radiation, which is primarily heat radiation.  Spitzer is the largest infrared telescope ever launched into space.

Right: An artist's rendition of the planet XO-3b passing in front of its parent star.  The planet orbits its star every 3.2 days at a distance that is closer to its star than Mercury is to the Sun.  At such a close position, XO-3b is heated to temperatures of approximately 2000 degrees Kelvin.


P. Machalek1, T. Greene, P. R. McCullough, A. Burrows, C. J. Burke, J. L. Hora, C. M. Johns-Krull, & D. L. Deming, "Thermal Emission and Tidal Heating of the Heavy and Eccentric Planet XO-3b", The Astrophysical Journal, 2010, Volume 711, p. 111. arXiv:1001.2319