Exoplanet Reflects Practically No Light—and Scientists Have No Idea Why

By Joseph Castro | August 12, 2011 3:16 pm

spacing is important

What’s the News: Using data from NASA’s Kepler spacecraft, astronomers from Princeton University and the Harvard-Smithsonian Center for Astrophysics have discovered the darkest known planet. The exoplanet, called TrES-2b, is located about 750 light-years away from Earth and reflects less than 1 percent of the incident light from its parent star, making it blacker than the blackest piece of coal. The discovery was published recently in the Monthly Notices of the Royal Astronomical Society (pdf).

What’s the Context:

  • For astronomers, albedo—the percentage of light that is reflected off an object’s surface—is a very useful concept that allows them to infer much about an object’s properties. For example, by comparing the albedo of an asteroid with the albedos of known materials here on Earth, astronomers can figure out how much of the body’s surface is covered with ice, dust, or metallic compounds. Albedo can also help pinpoint the temperature of a solar body.
  • For comparison, the Earth has an albedo of about 0.37, meaning that it reflects 37 percent of incoming light. Jupiter’s albedo, on the other hand, is 0.52 because of the large amount of reflecting clouds of ammonia and water ice in the planet’s upper atmosphere. The albedo of TrES-2b, as you could probably guess, is less than 0.01, while coal has an albedo of 0.05–0.1.

How Do They Know:

  • The Kepler spacecraft cannot see extrasolar planets directly—it detects them via the transit method. As a planet passes in front of its star, it causes a dip in the star’s brightness. Astronomers know that the star’s dimming is caused by a planet if it happens periodically (i.e. every time the planet revolves around the star).
  • Astronomers can also tease out a planet’s albedo with this method. Just as the Moon goes through phases as it revolves around the Earth, a planet goes through phases as it orbits its star, reflecting more or less light to an observer as it moves.
  • When the planet’s behind the star, we see only light from the star. As the planet crosses the front of its star—its transit—it blocks some of the starlight. When the planet’s to the side of the star (from our perspective), it reflects some starlight, allowing researchers to find its albedo. “By combining the impressive precision from Kepler with observations of over 50 orbits, we detected the smallest-ever change in brightness from an exoplanet,” lead author David Kipping said in a prepared statement.

Why Is the Planet So Dark:

  • The researchers can’t quite explain TrES-2b’s unusually low albedo. They have proposed that the planet’s atmosphere is composed of certain light-absorbing chemicals, such as vaporized sodium and potassium or gaseous titanium oxide, but even these molecules can’t account for the planet’s extreme blackness. “There’s a good chance it’s a chemical we haven’t even thought of yet,” Kipping told Space.com.

[via Harvard-Smithsonian Center for Astrophysics]

Image courtesy of David Aguilar, Center for Astrophysics


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