It may be hard to imagine a Planet Blacker than Coal, but that is what astronomers say they’ve discovered in our home galaxy with NASA’s Kepler Space Telescope.
Orbiting only about 3 million miles out from its star, the Jupiter-size Jovian-planet, dubbed TrES-2b, is heated to 1,800 degrees Fahrenheit (980 degrees C). Yet the apparently inky world appears to reflect almost none of the starlight that shines thereon , consistent with a recent study.
“Being less reflective than coal or maybe the blackest acrylic paint—this makes it far & away the darkest planet ever discovered,” lead study author David Kipping said.
“If we could see it up close it might appear as if a near-black ball of gas, with a small glowing red tinge to it—a true exotic amongst exoplanets,” added Kipping, an astronomer at Harvard Smithsonian Center for Astrophysics in Cambridge, Massachusetts.
NASA’s Planet Detector
The Earth-orbiting Kepler spacecraft was specifically designed to seek out planets outside our system . But at such distances—TrES-2b, as an example , is 750 light-years from us—it’s not as simple as snapping pictures of alien worlds.
Instead, Kepler—using light sensors called photometers that continuously monitor tens of thousands of stars—looks for the regular dimming of stars.
Such dips in stellar brightness may indicate that a planet is transiting, or passing ahead of a star, relative to Earth, blocking a number of the star’s light—in the case of the coal-black planet, blocking surprisingly little of that light.
Black Planet Spurs Dimmest of Dimming
When a planet passes ahead of its star, the world’s shaded side faces Kepler. But because the planet begins orbiting to the side of and “behind” its star, its star-facing side come to face the viewer. the number of starlight grows until the earth , becoming invisible to Kepler, passes fully in back of its star.
Watching TrES-2b & its star, Kepler detected only the slightest such dimming & brightening, though enough to determine that a Jupiter-size gas giant planet was the cause.
The light reflected by the newfound extrasolar planet, or exoplanet, changed by only about 6.5 parts per million, relative to brightness of the host star.
“This represents the littlest photometric signal we’ve ever detected from an exoplanet,” Kipping said.
What’s more, because the coal-black planet passed ahead of its star, the starlight’s dimming was “so small that it’s just like the dip in brightness we might see with a fruit fly getting into front of a car headlight.”
The Dark Mystery of TrES-2b
Current computer models predict that hot-Jupiter planets—gas giants that orbit very on the brink of their stars—could be only as dark as Mercury, which reflects about 10% of the sun light that hits it.
But TrES-2b is so dark that it reflects just 1% of the starlight that strikes it, suggesting that the present models may have tweaking, Kipping said.
Assuming the new study’s measurements are sound, what exactly is making the new planet’s atmosphere so dark?
“Some have proposed that this darkness could also be caused by an enormous abundance of gaseous sodium & titanium dioxide ,” Kipping said. “But more likely there’s something exotic there that we’ve not thought of before.
“It’s this mystery that I find so exciting about this discovery.”
TrES-2b may even represent an entire new class of exoplanet—a possibility Kipping & company hope to place to the test with Kepler, which has thus far detected many planets outside our Solar system.
“As Kepler discovers more & more planets by the day, we will hopefully scan through those & compute if this is often unique or if all hot Jupiters are very dark,” Kipping said.
Meanwhile, the very darkness of the new exoplanet suggests perhaps a catchier moniker for TrES-2b, Kipping said. “Maybe an appropriate nickname would be Erebus“—Ancient Greece’s god of darkness.
The coal-black planet study has been accepted for publication in Monthly Notices of the Royal Astronomical Society.