The hottest known planet in the Milky Way galaxy may also be itsshortest-lived world. The doomed planet is being eaten by itsparent star, according to observations made by a new instrument onNASA’s Hubble Space Telescope, the Cosmic Origins Spectrograph(COS). The planet may only have another 10 million years leftbefore it is completely devoured.
The planet, called WASP-12b, is so close to its sunlike starthat it is superheated to nearly 2,800 degrees Fahrenheit andstretched into a football shape by enormous tidal forces. Theatmosphere has ballooned to nearly three times Jupiter’s radius andis spilling material onto the star. The planet is 40 percent moremassive than Jupiter.
This effect of matter exchange between two stellar objects iscommonly seen in close binary star systems, but this is the firsttime it has been seen so clearly for a planet. “We see a huge cloudof material around the planet, which is escaping and will becaptured by the star. We have identified chemical elements neverbefore seen on planets outside our own solar system,” says teamleader Carole Haswell of The Open University in Great Britain.
Haswell and her science team’s results were published in the May10, 2010 issue of The Astrophysical Journal Letters. A theoreticalpaper published in the science journal Nature last February byShu-lin Li of the Department of Astronomy at the Peking University,Beijing, first predicted that the planet’s surface would bedistorted by the star’s gravity, and that gravitational tidalforces make the interior so hot that it greatly expands theplanet’s outer atmosphere. Now Hubble has confirmed thisprediction.
WASP-12 is a yellow dwarf star located approximately 600light-years away in the winter constellation Auriga. The exoplanetwas discovered by the United Kingdom’s Wide Area Search for Planets(WASP) in 2008. The automated survey looks for the periodic dimmingof stars from planets passing in front of them, an effect calledtransiting. The hot planet is so close to the star it completes anorbit in 1.1 days.
The unprecedented ultraviolet (UV) sensitivity of COS enabledmeasurements of the dimming of the parent star’s light as theplanet passed in front of the star. These UV spectral observationsshowed that absorption lines from aluminum, tin, manganese, amongother elements, became more pronounced as the planet transited thestar, meaning that these elements exist in the planet’s atmosphereas well as the star’s. The fact the COS could detect these featureson a planet offers strong evidence that the planet’s atmosphere isgreatly extended because it is so hot.
The UV spectroscopy was also used to calculate a light curve toprecisely show just how much of the star’s light is blocked outduring transit. The depth of the light curve allowed the COS teamto accurately calculate the planet’s radius. They found that theUV-absorbing exosphere is much more extended than that of a normalplanet that is 1.4 times Jupiter’s mass. It is so extended that theplanet’s radius exceeds its Roche lobe, the gravitational boundarybeyond which material would be lost forever from the planet’satmosphere.
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