Wednesday, October 9, 2013

Periastron Heating of HD 80606 b

HD 80606 b is a gas giant planet which swings around a Sun-like star on a wildly eccentric orbit and probably has the most eccentric orbit of any known planet. The planet’s orbit takes it as far out as 0.85 AU from its host star, and as close in as 0.03 AU (one AU is the average distance between Earth and the Sun). The mass of HD 80606 b is estimated to be about 4 times that of Jupiter.

HD 80606 b circles its host star every 111.4 days. While the planet spends most of its time at distances that would place it between Venus and Earth in our solar system, it zips through the closest part of its orbit in less than a day. At the maximum distance from its host star, HD 80606 b receives a similar amount of stellar flux as Earth receives from the Sun. However, at closest approach, the planet receives 825 times more stellar flux than it does when furthest from its host star.

Figure 1: Orbital geometry of HD 80606 b.

In November 2007, a team of astronomers led by Gregory Laughlin, a professor of astronomy and astrophysics at UCSC, used NASA’s Spitzer Space Telescope to observe what happens when HD 80606 b makes its closest approach to its host star (G. Laughlin et al., 2009). The team found that during closest approach, the planet’s temperature rose from 800 K to 1500 K over a mere 6-hour period. Such as extreme temperature swing indicates that the intense stellar flux is absorbed in a layer of the planet’s upper atmosphere and then rapidly radiated off as the planet moves away from its host star.

Figure 2: 30 hours of observations taken by NASA’s Spitzer Space Telescope in November 2007 as HD 80606 b swung through its closest approach to its host star. Just before closest approach, the planet was eclipsed by its host star as seen from Earth, allowing astronomers to determine the amount of energy coming from the planet in comparison to the amount coming from the star. Credit: NASA/JPL-Caltech/G. Laughlin (UCD/Lick Observatory).

Figure 3: Computer-generated images showing the evolution of severe weather patterns and how the night side of HD 80606 b radiates away heat after a scorching pass near its host star. The frames run from 4.4 days (upper left) to 8.9 days (lower right) after closest approach and are evenly spaced in time. Credit: NASA/JPL-Caltech/J. Langton (UC Santa Cruz).

The enormous pulse of heat delivered during the close swing of HD 80606 b past its host star scorches the planet’s day side. As the atmosphere expands, shock waves are unleashed with violent winds that flow away from the day side toward the night side. The planet’s rotation causes these winds to curl up into large-scale vortical storm systems that fade like swirling embers as the planet recedes from its host star after closest approach.

G. Laughlin et al., “Rapid heating of the atmosphere of an extrasolar planet”, Nature 457, 562-564 (29 January 2009)