Date: August 23, 2011
Title: Detection of Visible Light from the Darkest World
Podcaster: David Kipping
Description: Two weeks ago, it was announced that Kepler had discovered the darkest exoplanet; a world which reflects less than 1% of the incident light. In this podcast, lead author David Kipping will discuss how Kepler managed to accomplish this and what the detection means. The planet in question, TrES-2b, reflects less light than black acrylic paint and so the chemistry of this alien world’s atmosphere is surely very different from that of Jupiter of Saturn. Tune in to hear some of the ideas which could shed light upon this world of darkness.
Bio: David Kipping obtained a PhD in Astrophysics from University College London earlier this year. His thesis was entitled ‘The Transits of Extrasolar Planets with Moons’ and David’s main research interest revolves around exomoons. He is just starting a Carl Sagan Fellowship at the Harvard-Smithsonian Center for Astrophysics.
Sponsor: This episode of “365 Days of Astronomy” is sponsored anonymously.
Hi, my name is Dr. David Kipping and I’m a Carl Sagan Fellow for exoplanetary research at the Harvard-Smithsonian Center for Astrophysics.
Today, I’d like to talk with you about a recent discovery by myself and Dr. David Spiegel of Princeton: the detection of visible light from the darkest world. The planet in question is a gas giant called TrES-2b, which was the second exoplanet discovered by the Trans Atlantic Exoplanetary Survey, for which the corresponding acronym yields the name TrES.
The discovery was made in 2006 using the so-called transit method. This is where astronomers carefully monitor a star’s brightness of the course of many months looking for periodic decrements in the observed flux, caused by an orbiting planet eclipsing the host star. Now in order for an Earth-bound observer to see such eclipses, the geometry of the planet and the star must be almost perfectly aligned to our line-of-sight. Because we require such fortuitous alignment, only a small fraction of all planetary systems can be discovered in this way, but those that do are like Rosetta stones to an astronomer.
Transits allow us to measure the radius and orbital inclination of distant worlds. Combined with radial velocity measurements, we can infer the mass too meaning we now know the bulk density of the exoplanet. In recent years, it has been shown that transits allow one to search for all kinds of interesting things, from exomoons to rings, from molecules in the atmosphere to planetary oblateness.
The transit method is such a powerful tool for finding and characterizing exoplanets that a dedicated mission was launched by NASA in March 2009. The Kepler Mission, named in honour of the 16th century German astronomer Johannes Kepler, was tasked to determine the frequency of Earth-like planets in the cosmos. To achieve this ambitious goal, the 0.95 metre telescope was gifted with an exquisitely powerful camera, capable of detecting a star changing in brightness at the parts per million level. Kepler will stare at the same small patch of the sky for over 3 years in its quest for exoplanets.
Although the search for Earths is a work in progress, such a precise telescope is useful for other science too, for example detecting the pulsations of stars in an effort to understand their interiors. Another example, is the ability to detect the difference in brightness between the dayside and nightside of an exoplanet. Imagine watching a planet transit across its star from a nearby spaceship, as the planet moves across the face of the star, we find ourselves looking at the nightside of this alien world. The eclipse ends after a few hours and the planet continues on its orbit. As it moves round, the planet appears increasingly illuminated going from a slim crescent until fully bathed in light just before it disappears behind the back of the star. From our spaceship, we can watch the changing phases of this exoplanet, just like the changing phases of the Moon can be seen from our small terraqueous globe.
Kepler does not have the luxury of visiting other star systems but it can still see the planet and star, albeit mashed together as a single blob of light. Nevertheless, we should expect the combined light to vary in brightness as we see the changing phases of the exoplanet.
This task is not an easy one though. Planets do not produce any energy themselves, unlike the nuclear engine of a star. This means even hot-Jupiters like TrES-2b are orders of magnitude dimmer than the fiery star which incinerates the planet. To make things worse, recent searches for reflected light from hot-Jupiters have generally come up negative, hinting that such planets may be quite dark reflecting certainly less than 15% of the incident light. But these null-detections can only offer upper limits, so our goal in this research was to wield the awesome power of the Kepler telescope in a bid for these dark worlds to finally betray their hidden nature.
Coincidentally, three previously known exoplanets, including TrES-2b, happened to lie within Kepler’s field of view and so we used the telescope to look for changes in brightness for a period of four months, in which time this alien world whizzes round her star over fifty times given this planet’s very short period of 60 hours. We were thrilled to detect a faint whisper of light emanating from this world. The light, which takes around 750 years to cross the vacuum of space and reach Kepler, was measured to be just 6 millionths the brightness of the host star, the smallest change in brightness ever detected from an exoplanet. The signal was there, but it was so small that it meant that the difference in brightness between the dayside and the nightside was tiny. What could this mean?
The light we detected could have come from two sources: thermal emission or reflection. Light from thermal emission is the same light you would see after removing a piece of iron from a fire; it glows red with heat just like hot-Jupiters do. Since this planet is so close to its star, we expect temperatures of around 1500K. In turns out, that for all feasible atmosphere models of this world, the 6 parts per million signal we detected must be almost totally thermal emission. This means only a tiny fraction of the signal is due to reflected light. Quantitatively, the reflectivity of this world must be less than 1%, and in our best-fit models it is more like 0.1%.
To put that into perspective, TrES-2b is similar in mass and radius to Jupiter but Jupiter reflects some 50% of the incident light. TrES-2b has a reflectivity less than that of any other planet or moon in the Solar System or beyond. The reflectivity is significantly less than even black acrylic paint, which makes the mind boggle as to what a clump of this planet would look like in your hand. Perhaps an appropriate nickname for the world would be Erebus, the Greek God of Darkness and Shadow. But what really is causing this planet to be so dark?
Jupiter’s 50% reflectivity is due to the presence of ammonia clouds in the atmosphere, scattering sunlight back into space. So clearly TrES-2b does not have reflective clouds in its atmosphere. But even without any clouds at all, you still expect a hydrogen-dominated atmosphere to scatter light for the same reason that the Earth’s sky appears blue, namely atmospheric molecules are good at scattering light. So this means that there is the presence of some actively light absorbing chemicals in TrES-2b’s atmosphere. We know things like gaseous sodium and potassium are good at absorbing light, but there would have to be a large overabundance of these molecules to explain how dark this planet is. It seems more likely, that there may be some exotic chemistry occurring here that we just don’t know about yet. Either way, the mystery will not be truly solved until the next generation of telescopes arrive, namely the successor to the Hubble Telescope, a mission called the James Webb Space Telescope. Until then, I hope you don’t have any nightmares about this sinister world of darkness.
End of podcast:
365 Days of Astronomy
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