Megan's Blog

In the news this month: a planet darker than coal

Stars are bright because they generate heat and light through nuclear fusion processes in their cores, planets are visible because they reflect some of that light.  The percentage of light that is reflected, a quantity known as a planet's albedo, varies depending on the nature of the planet's surface and its atmosphere.  Jupiter, with its thick bands of highly reflective cloud, has an albedo of 52%, while the Earth's is somewhat lower, only reflecting around 37% of the sunlight which falls on the surface.  But now, a duo of astronomers have discovered a planet with an exceptionally low albedo, reflecting just 1% of its host star's light, making it less reflective than coal.

The planet, discovered by the Trans-Atlantic Exoplanet Survey, is known as TrES-2b and lies about 750 light years away in the constellation of Draco.  David Kipping (of the Harvard-Smithsonian Center for Astrophysics) and David Spiegel (of Princeton University) used data from the Kepler space telescope to investigate the planet's nature.  To calculate its albedo, the astronomers measured its brightness at two different points in its orbit around the host star, once when it was located directly between us and the star, and again when it was on the far side, just before it went into eclipse.  The difference between the two measurements is therefore the difference in brightness between the day and night sides of the planet, and tells us how much of the star's light is reflected by the surface of TrES-2b.

The planet orbits its star at a distance of just five million kilometres, far closer than Mercury is to the Sun.  Mercury is a dense rocky planet with a large iron core and a rocky, silicate surface which has an albedo of 12%.  Surface temperatures on the closest planet to the Sun range between 90 and 700 degrees Kelvin.  In contrast, TrES-2b, an exoplanet in the category of "hot-Jupiters" shows brightness variations of just 6.5 parts per million, corresponding to an albedo of less than 1%.  But, this assumes that the only cause of the brightness variations is due to the geometry of the planet.  The authors calculate that there is significant emission coming from the day side of the planet.  Its proximity to its host star means that its surface temperature is likely to be around 1000 degrees and any atmosphere it has will likely be composed of chemicals such as vaporised sodium and potassium, or gaseous titanium oxide.  Such a hot temperature also means that the planet actually emits some of its own light, possibly glowing dimly red like an electric bar heater.

The Kepler satellite is designed to search for planets using the transit technique, observing one densely-packed star field for its entire operational lifetime, searching for the tiny fluctuations in brightness of a star due to a transiting planet. The exceptional sensitivity of Kepler's instruments has led to this particular discovery in just four months of data acquisition.  In their paper, accepted for publication in the Monthly Notices of the Royal Astronomical Society, the researchers suggest that, over six years of continuous observation, the telescope may be capable of detecting planets with albedos as low as 0.1%.

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This blog post is a news story from the Jodcast, aired in the September 2011 edition.

David M. Kipping, & David S. Spiegel (2011). Detection of visible light from the darkest world Monthly Notices of the Royal Astronomical Society arXiv: 1108.2297v2