Astronomers from The University of Texas at Austin and WesleyanUniversity have used the Hobby-Eberly Telescope at UT Austin'sMcDonald Observatory to confirm that a Jupiter-size planet in anearby solar system is dissolving, albeit excruciatingly slowly,because of interactions with its parent star. Their findings couldhelp astronomers better understand star-planet interactions inother star systems that might involve life. The work will be published in the June 1 edition of TheAstrophysical Journal in a paper led by Wesleyan Universitypostdoctoral researcher Adam Jensen. The team includes Universityof Texas astronomers Michael Endl and Bill Cochran, as well asWesleyan professor Seth Redfield. The star, HD 189733, lies about 63 light-years away in theconstellation Vulpecula, the little fox. |
In 2010 another team studied this star in ultraviolet light withthe Hubble Space Telescope and discovered that its planet (calledHD 189733b) is discharging hydrogen into space. The Texas-Wesleyan study finds that this streaming hydrogen gas -studied in a different wavelength range by one of the world'slargest ground-based telescopes - is much hotter than anyone knew.This temperature is important: It indicates that the violent flaresthis star is throwing out are interacting with the planet'satmosphere. While this planet is not thought to be a home for life, suchstudies could help astronomers understand how interactions between"parent" stars and their "children" planets might affect life thatcould arise in other star systems. "One day we will use similar techniques to probe the atmosphere ofsmaller, Earth-like planets," The University of Texas' Endl said."I think the pace of progress is stunning, to say the least. Twentyyears ago we didn't really know of any exoplanets, and now we probeand study their atmospheres." The planet HD 189733b is not like Earth - it's a gas giant 20percent heavier than Jupiter that orbits 10 times as close to itsparent star as Mercury does to our sun, an exotic type of planetastronomers have dubbed a "hot Jupiter." To date, astronomers have discovered nearly 700 planets orbitingstars in our galaxy (with billions suspected), but they have probedthe atmospheres of only a handful,using space telescopes and thelargest ground-based telescopes such as the Hobby-Eberly Telescope(HET).
Studies of this planet's atmosphere are possible because it passesin front of its parent star as seen from Earth. "Each time the planet passes in front of the star," Redfield said,"the planet blocks some of the star's light. If the planet has noatmosphere, it will block the same amount of light at allwavelengths. However, if the planet has an atmosphere, gasses inits atmosphere will absorb some additional light." The passages arecalled transits. In 2007 as a postdoctoral researcher at the McDonald Observatory,Redfield announced he had found sodium in this planet's atmosphere.That announcement was based on hundreds of HET observations spreadout over a year, taken both with the planet in front of the star("in-transit") and when the planet was not.
Subtracting the latterfrom the former provided the planet's "transmission spectrum." Astronomers determine the spectrum of a star or planet whenspreading out the telescope-collected light into its componentwavelengths - a more sophisticated version of passing light througha prism to produce a rainbow. The spectrum is like a bar code thatastronomers can read to determine the object's chemicalcomposition, temperature, speed and direction of motion. Today, Redfield's postdoctoral fellow, Adam Jensen, is studyingthat same set of telescope observations and many more added by Endlin the intervening years. Just determining the spectrum of a transiting planet, let alonebeing able to decode it, is a difficult feat. As this planet passesin front of its parent star, it blocks only 2.5 percent of thestar's total light, plus another 0.3 percent for the planet'satmosphere.
Teasing out that 0.3 percent and decoding it is thegoal.
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