One of the largest alien worlds yet discovered may create a huge shock wave as it plows through the wind blowing off its host star, astronomers have found. That may mean the planet has a magnetic field that protects it in its perilously close orbit. The discovery may help astronomers understand the atmospheres, and ultimately the life-supporting potential, of worlds beyond our solar system.
Scientists identified the first alien world, or exoplanet, in 1992, and since then the number has ballooned to 531. In 2008, astronomers added a planet named WASP-12b to the roster. WASP-12b is a “hot Jupiter,” a gaseous planet similar in size to our own gas giant but with a blisteringly hot surface temperature of more than 2000°C. It whips around its host star at a distance of less than 1/16th the distance from the sun to Mercury, completing its orbit once every 26 hours. In 2010, scientists at the Open University in the United Kingdom used the Hubble Space Telescope to view WASP-12b’s transit and noticed that as it crossed its star, its "shadows" appeared first in the ultraviolet (UV) wavelengths before the rest of the planet blocked the sun.
Aline Vidotto and her team at the University of St. Andrews in the United Kingdom theorized that the out-of-sync dimming could be explained by a shock wave in the stellar wind, the stream of charged gas particles flowing off the star and immersing the planet. The shock wave is similar to that created by a supersonic jet aircraft as it travels through air, building up pressure waves around its nose that merge together. The planet’s shock wave would be pushed in front of it as it orbits at supersonic speeds, and the wave would absorb some of the UV light emitted from the star. Earth and Saturn exhibit similar “bow shocks,” but this is the first evidence of a shock surrounding an exoplanet.
Vidotto and her team calculated the distance from the planet’s surface to the front of the apparent bow shock and were surprised that it was more than four times the planet’s radius. The team reasoned that strong magnetic forces were repelling the stellar wind because gaseous pressures alone would not be enough to hold it off at such a distance, says Vidotto, who presented the findings at this week’s meeting of the Royal Astronomical Society in Llandudno in the United Kingdom.
Earth’s magnetic field shields our atmosphere from the destructive powers of the solar wind, and many scientists believe a magnetic field may be a prerequisite for a habitable planet. “While WASP-12b is far too hot to support life, being able to detect planetary magnetic fields will help with our understanding of and identifying the habitable zones around exoplanets," says Joseph Llama, a Ph.D. student on Vidotto’s team.
To further explore exoplanet magnetism, the team has put together a list of potential planets that they believe offer the most promising chances to glimpse similar bow shocks. “It’s very competitive to get [Hubble] time, so we cannot ask to look at all the transiting planets,” Vidotto says.
Alan Aylward, an astrophysicist at University College London, cautions that a magnetic field may not be the only plausible explanation for the observed bow shock, noting that normal assumptions about atmospheric dynamics may not apply in such exotic environments. Still, he says, the possibility of such a field leads to many interesting lines of inquiry, including questions about how planets produce magnetic fields.
Heather Knutson, an astronomer at the University of California, Berkeley, says the findings could help scientists know where to concentrate their observing efforts to spot new astronomical phenomena. “A consistent story with exoplanets is that they are almost never what we expect them to be,” she says.
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