The darkest planet outside our solar system—an exoplanet—has been discovered about 750 million light-years away towards the constellation of Draco.
Known as TrES-2b, it is a gas giant—a huge planet mainly composed of non-solid matter. The exoplanet appears very dark because it reflects less than one percent of the sunlight that hits it.
“TrES-2b is considerably less reflective than black acrylic paint, so it’s truly an alien world,” said study lead author David Kipping from the Harvard-Smithsonian Center for Astrophysics (CfA) in a press release.
Yet the reason it absorbs so much light is not clear. Jupiter, also a gas giant, has ammonia clouds that reflect more than a third of the sunlight it receives.
However, TrES-2b is orbiting a star, GSC 03549-02811, which is only 3 million miles way. Its intense light heats up the planet to temperatures higher than 1,800° Fahrenheit, which is too hot for ammonia clouds to exist.
TrEs-2b contains light-absorbing chemicals in its atmosphere, such as vaporized sodium, potassium, and titanium oxide gas. But these alone cannot explain the extreme darkness of the planet, which remains a mystery.
“It’s not clear what is responsible for making this planet so extraordinarily dark,” said co-author David Spiegel of Princeton University in the release.
“However, it’s not completely pitch black,” he added. “It’s so hot that it emits a faint red glow, much like a burning ember or the coils on an electric stove.”
The planet may also be tidally locked, meaning it faces its star with the same side during its orbit.
The researchers carried out their investigation using NASA’s Kepler spacecraft, which is extremely accurate at measuring the brightness of distant stars.
“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,” said Kipping.
“In other words, Kepler was able to directly detect visible light coming from the planet itself.”
It was these tiny fluctuations in brightness that revealed the planet’s extreme darkness, since planets that reflect light more effectively have larger brightness variations as their phase changes.
The findings was published in the Monthly Notices of the Royal Astronomical Society.
Read the research paper here.
