NASA’s Kepler finds pitch black world

Astronomers using data from NASA’s Kepler spacecraft have identified a pitch black exoplanet that is the darkest yet discovered.

Artist's impression of the strange planet
Artist's impression of the dark planet TrES-2b orbiting its parent star. Credit: David A. Aguilar (CfA)

Every time the world, dubbed TrES-2b, passes in front of its parent star, it intercepts some of that starlight so that observers on Earth will see a reduction in light. This telltale signature of a transiting planet reveals a typical “hot Jupiter”, which is so close to its host star that its year only lasts a couple of days.

The close proximity to this solar-like star means that the planet has a temperature of more than 1,000 degrees Celsius. The high temperature prevents reflective clouds of ammonia from forming, like those which exist on Jupiter in our own Solar System.

TrES-2b is more likely to have an atmosphere composed of elements such as sodium and potassium, which absorb any incoming light. This means that while Jupiter reflects around a third of the light it receives from the Sun, TrES-2b reflects hardly any light at all, making it an extremely dark world.

However, such absorption doesn’t explain the blackness of this particular planet, so it seems that TrES-2b has some dark secrets it has yet to reveal. Spectroscopic analysis should help to resolve this mystery, but this won’t be easy due to the paucity of light from the planet. If the James Webb Space Telescope survives US budget cuts and is launched, it might be able to shed some light on this shady case.

Astronomers David Kipping and David Spiegel were able to gauge the light levels emanating from TrES-2b by studying data from Kepler. “We looked for changes in brightness caused by the changing phases of the exoplanet,” explains David Kipping of the Harvard-Smithsonian Center for Astrophysics. “When a planet transits in front of her host star, the observer is looking at the nightside of the exoworld. Half an orbit later, the planet moves round and disappears behind the star.

“Just before it does so, the planet appears fully illuminated since we are looking at the dayside. It is this day-night difference that we detected in this work. We measured a difference of 6.5 parts per million, which is the smallest ever change in brightness caused by an exoplanet. If the planet was very reflective, then we would expect the dayside to appear much brighter than the nightside but here the very low difference means that the planet must have an albedo of less than 1% – which is darker than coal.”

The ability to measure the albedo, which is the brightness of the exoplanets, is only a recent achievement due to the precision of space-based exoplanet hunters such as Kepler. While only a handful of other exoplanets have had their brightness measured, they all are much brighter than TrES-2b.

“It is too early to speculate as to whether TrES-2b is a ‘freak’ or a run of the mill ‘hot Jupiter’,” Kipping tells Skymania News. “The planet was a low-hanging fruit from the Kepler data because it is a fairly bright star and the exoplanet was observed intensively from the moment Kepler was launched because the planet had already been detected from a ground-based survey a few years before. So right now, TrES-2b is the only planet where we have really had the ability to measure such a tiny albedo.

“However, our work is based on just the first four months of Kepler data and with an extended mission (hopefully) Kepler should fly for 6+ years. With this much data we could measure the albedos of dark Neptunes and even bright Super-Earths. This would allow us to build up a deeper understanding of what general properties ‘hot Jupiters’ have.”


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By Paul Sutherland

Paul Sutherland has been a professional journalist for nearly 40 years. He writes regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy, plus he has authored three books on astronomy and contributed to others.

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