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Astronomers: planet is blacker than coal, but glows faintly

Aug. 11, 2011
Courtesy of the Royal Astronomical Society
and World Science staff

As­tro­no­mers say they have dis­cov­ered the dark­est known plan­et – a dis­tant, Ju­pi­ter-sized gas gi­ant known as TrES-2b.

Their mea­sure­ments in­di­cate the plan­et re­flects less than one pe­r­cent of the star­light fall­ing on it. That would make it blacker than coal or any plan­et or moon in our so­lar sys­tem, which it is not part of. 

"It's truly an al­ien world," said as­tron­o­mer Da­vid Kip­ping of the Har­vard-Smith­son­ian Cen­ter for As­t­ro­phys­ics, lead au­thor of a re­port on the ob­ject ap­pear­ing in the re­search jour­nal Monthly No­tices of the Roy­al As­tronomical So­ci­e­ty.

In our so­lar sys­tem, Ju­pi­ter is swathed in bright clouds of am­mo­nia that re­flect more than a third of the sun­light reach­ing it, ac­cord­ing to as­tron­o­mers. In con­trast, the new­found plan­et lacks re­flective clouds due to its high tem­pe­r­a­ture.

TrES-2b or­bits its star at a rel­a­tively small es­ti­mat­ed dis­tance of five mil­lion kilo­me­tres (three mil­lion miles). The star's in­tense light heats the plan­et to an es­ti­mat­ed 1000 more de­grees Cel­si­us – much too hot for am­mo­nia clouds. In­stead, its ex­ot­ic at­mos­phere con­tains light-absorbing chem­i­cals like va­por­ized so­di­um and po­tas­si­um, or gas­e­ous ti­ta­ni­um ox­ide, the as­tron­o­mers say. Yet none of these chem­i­cals fully ex­plain the ex­treme black­ness.

"It's not clear what is re­spon­si­ble," said study co-au­thor Da­vid Spie­gel of Prince­ton Uni­vers­ity. "How­ever, it's not com­pletely pitch black. It's so hot that it emits a faint red glow, much like a burn­ing em­ber or the coils on an elec­tric stove."

Kip­ping and Spie­gel de­ter­mined the re­flecti­vity of the plan­et us­ing da­ta from NASA's Kep­ler space­craft. Kep­ler is de­signed to meas­ure the bright­nesses of dis­tant stars with ex­treme pre­ci­sion. The team mon­i­tored the bright­ness of the TrES-2 sys­tem as the plan­et or­bited its star. They de­tected a sub­tle dim­ming and bright­en­ing due to the plan­et's chang­ing phase.

TrES-2b is thought to be tid­ally locked like our moon, mean­ing one side of the plan­et al­ways faces the star. And like our moon, the plan­et shows chang­ing phases as it or­bits its star. This causes the to­tal bright­ness of the star plus plan­et to vary slight­ly.

"By com­bin­ing the im­pres­sive pre­ci­sion from Kep­ler with ob­serva­t­ions of over 50 or­bits, we de­tected the smallest-ever change in bright­ness from an exoplan­et: just six parts per mil­lion," said Kip­ping. "In oth­er words, Kep­ler was able to di­rectly de­tect vis­i­ble light com­ing from the plan­et it­self."

The ex­tremely small fluctua­t­ions proved that TrES-2b is in­credibly dark, Kip­ping added; a more re­flective world would have shown larg­er bright­ness varia­t­ions as its phase changed. TrES-2b, dis­cov­ered in 2006 by the Trans-Atlan­tic Exoplan­et Sur­vey, or­bits the star GSC 03549-02811, which lies an es­ti­mat­ed 750 light-years away in the di­rection of the con­stella­t­ion Dra­co. One light-year is about 10 tril­lion kilo­me­tres, or 6 tril­lion miles.


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Astronomers say they have discovered the darkest known planet – a distant, Jupiter-sized gas giant known as TrES-2b. Their measurements indicate the planet reflects less than one percent of the starlight falling on it, making it blacker than coal or any planet or moon in our solar system, which it is not part of. "It's truly an alien world," said astronomer David Kipping of the Harvard-Smithsonian Center for Astrophysics, lead author of a report on the object appearing in the research journal Monthly Notices of the Royal Astronomical Society. In our solar system, Jupiter is swathed in bright clouds of ammonia that reflect more than a third of the sunlight reaching it, according to astronomers. In contrast, the newfound planet lacks reflective clouds due to its high temperature. TrES-2b orbits its star at a relatively small estimated distance of five million kilometres (three million miles). The star's intense light heats the planet to an estimated 1000 more degrees Celsius – much too hot for ammonia clouds. Instead, its exotic atmosphere contains light-absorbing chemicals like vaporized sodium and potassium, or gaseous titanium oxide, the astronomers say. Yet none of these chemicals fully explain the extreme blackness. "It's not clear what is responsible," said study co-author David Spiegel of Princeton University. "However, it's not completely pitch black. It's so hot that it emits a faint red glow, much like a burning ember or the coils on an electric stove." Kipping and Spiegel determined the reflectivity of the planet using data from NASA's Kepler spacecraft. Kepler is designed to measure the brightnesses of distant stars with extreme precision. The team monitored the brightness of the TrES-2 system as the planet orbited its star. They detected a subtle dimming and brightening due to the planet's changing phase. TrES-2b is thought to be tidally locked like our moon, meaning one side of the planet always faces the star. And like our moon, the planet shows changing phases as it orbits its star. This causes the total brightness of the star plus planet to vary slightly. "By combining the impressive precision from Kepler with observations of over 50 orbits, we detected the smallest-ever change in brightness from an exoplanet: just six parts per million," said Kipping. "In other words, Kepler was able to directly detect visible light coming from the planet itself." The extremely small fluctuations proved that TrES-2b is incredibly dark, Kipping added; a more reflective world would have shown larger brightness variations as its phase changed. TrES-2b, discovered in 2006 by the Trans-Atlantic Exoplanet Survey, orbits the star GSC 03549-02811, which lies an estimated 750 light-years away in the direction of the constellation Draco. One light-year is about 10 trillion kilometres, or 6 trillion miles.