The distant exoplanet TrES-2b, shown here in an artist’s conception, is darker than the blackest coal. This Jupiter-sized world reflects less than one percent of the light that falls on it, making it blacker than any planet or moon in our solar system. Astronomers aren’t sure what vapors in the planet’s superheated atmosphere cloak it so effectively. Image: David A. Aguilar (CfA) |
Astronomers have discovered the darkest known exoplanet—a distant,
Jupiter-sized gas giant known as TrES-2b. Their measurements show that TrES-2b
reflects less than one percent of the sunlight falling on it, making it blacker
than coal or any planet or moon in our solar system.
“TrES-2b
is considerably less reflective than black acrylic paint, so it’s truly an
alien world,” says astronomer David Kipping of the Harvard-Smithsonian Center
for Astrophysics (CfA), lead author on the paper reporting the research.
In our solar
system, Jupiter is swathed in bright clouds of ammonia that reflect more than a
third of the sunlight reaching it. In contrast, TrES-2b (which was discovered
in 2006 by the Trans-Atlantic Exoplanet Survey, or TrES) lacks reflective
clouds due to its high temperature.
TrES-2b orbits
its star at a distance of only three million miles. The star’s intense light
heats TrES-2b to a temperature of more than 1,800 F—much too hot for ammonia
clouds. Instead, its exotic atmosphere contains light-absorbing chemicals like
vaporized sodium and potassium, or gaseous titanium oxide. Yet none of these
chemicals fully explain the extreme blackness of TrES-2b.
“It’s not
clear what is responsible for making this planet so extraordinarily dark,”
states coauthor 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 TrES-2b 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
believed to be tidally locked like our moon, so 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 6 parts per million,” says 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. A more reflective
world would have shown larger brightness variations as its phase changed.
Kepler has
located more than 1,200 planetary candidates in its field of view. Additional
analysis will reveal whether any other unusually dark planets lurk in that
data.
TrES-2b orbits the star GSC 03549-02811, which is located about 750
light-years away in the direction of the constellation Draco. (One light-year
is about 6 trillion miles.)
