CfA & RAS: Alien World Is Blacker than Coal

[bystander]

Alien World Is Blacker than Coal
Center for Astrophysics | Royal Astronomical Society | Kepler | 2011 Aug 11

[i]The distant exoplanet TrES-2b, shown here in an artist's conception, is darker than the blackest coal. Astronomers aren't sure what vapors in the planet's superheated atmosphere cloak it so effectively. (Credit: David A. Aguilar (CfA))[/i]

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Astronomers have discovered the darkest known exoplanet - a distant, Jupiter-sized gas giant known as TrES-2b. Their measurements show that TrES-2b reflects less than one percent of the sunlight falling on it, making it blacker than coal or any planet or moon in our solar system.

"TrES-2b is considerably less reflective than black acrylic paint, so it's truly an alien world," said astronomer David Kipping of the Harvard-Smithsonian Center for Astrophysics (CfA), lead author on the paper reporting the research.

In our solar system, Jupiter is swathed in bright clouds of ammonia that reflect more than a third of the sunlight reaching it. In contrast, TrES-2b (which was discovered in 2006 by the Trans-Atlantic Exoplanet Survey, or TrES) lacks reflective clouds due to its high temperature.

TrES-2b orbits its star at a distance of only three million miles. The star's intense light heats TrES-2b to a temperature of more than 1,800° Fahrenheit - much too hot for ammonia clouds. Instead, its exotic atmosphere contains light-absorbing chemicals like vaporized sodium and potassium, or gaseous titanium oxide. Yet none of these chemicals fully explain the extreme blackness of TrES-2b.

"It's not clear what is responsible for making this planet so extraordinarily dark," stated co-author David Spiegel of Princeton University. "However, it's not completely pitch black. It's so hot that it emits a faint red glow, much like a burning ember or the coils on an electric stove."

Kipping and Spiegel determined the reflectivity of TrES-2b using data from NASA's Kepler spacecraft. Kepler is designed to measure the brightnesses of distant stars with extreme precision.

The team monitored the brightness of the TrES-2 system as the planet orbited its star. They detected a subtle dimming and brightening due to the planet's changing phase.

TrES-2b is believed to be tidally locked like our moon, so one side of the planet always faces the star. And like our moon, the planet shows changing phases as it orbits its star. This causes the total brightness of the star plus planet to vary slightly.

"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."

The extremely small fluctuations proved that TrES-2b is incredibly dark. A more reflective world would have shown larger brightness variations as its phase changed.

Kepler has located more than 1,200 planetary candidates in its field of view. Additional analysis will reveal whether any other unusually dark planets lurk in that data.

TrES-2b orbits the star GSC 03549-02811, which is located about 750 light-years away in the direction of the constellation Draco. (One light-year is about 6 trillion miles.)

Detection of visible light from the darkest world - DM Kipping, DS Spiegel

• Monthly Notices of the Royal Astronomical Society (online 2011 Sep 02) DOI: 10.1111/j.1745-3933.2011.01127.x
  arXiv.org > astro-ph > arXiv:1108.2297 > 10 Aug 2011 (v1), 16 Aug 2011 (v2)

Coal-Black Alien Planet Is Darkest Ever Seen
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[neufer]

Alien World Is Blacker than Coal
Center for Astrophysics | Royal Astronomical Society | Kepler | 2011 Aug 11

TrES-2b orbits its star at a distance of only three million miles. The star's intense light heats TrES-2b to a temperature of more than 1,800° Fahrenheit - much too hot for ammonia clouds. Instead, its exotic atmosphere contains light-absorbing chemicals like vaporized sodium and potassium, or gaseous titanium oxide. Yet none of these chemicals fully explain the extreme blackness of TrES-2b.

"It's not clear what is responsible for making this planet so extraordinarily dark," stated co-author David Spiegel of Princeton University. "However, it's not completely pitch black. It's so hot that it emits a faint red glow, much like a burning ember or the coils on an electric stove."

So...would TrES-2b then be said to lie in the Cinderella Zone rather than the Goldilocks Zone

[Beyond]

So...would TrES-2b then be said to lie in the Cinderella Zone rather than the Goldilocks Zone

I should think-yes, unfortunately. The Goldilocks Zone would definately be more Bearable

[Ann]

Nice post, Art! Yes, think of all those poor hot Jupiters in the Cinderella zone. There are a lot of angry suns out there, and to make it all worse they're all their Cinderella babies' real mother, or at least their very big siblings!!

But a more bear-able zone - I don't know....

Think I prefer the cat zone. Here you are also sure to have something to eat.















Ann

[Beyond]

In the Cinderella Zone, you would need asbestos slippers, so Ann's cat zone sounds just about purrfect

[bystander]

Hubble to Target 'Hot Jupiters'
University of Arizona | Daniel Stolte | 2011 Aug 19

[img3="Artist's impression of a scorching "Hot Jupiter" orbiting its parent star at close range. (Illustration: ESA - C.Carreau)"]http://www.nasa.gov/images/content/1819 ... 1-516a.jpg[/img3]

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An international team of astronomers led by a former UA graduate student has set out on the largest program to date exploring the alien atmospheres of "Hot Jupiters" - massive planets in solar systems far away from our own.

An international team of scientists has secured a large program of nearly 200 hours of observing time with NASA's Hubble Space Telescope to explore the atmospheric conditions of planets outside our solar system, known as exoplanets.

The research will focus on "Hot Jupiters," exoplanets similar in size to Jupiter, but with temperatures of 1,000 degrees Kelvin (1,340 degrees Fahrenheit) or more because they orbit so closely to their respective stars.

Large programs using the Hubble Space Telescope historically have led to data sets with a lasting legacy. The team is led by David Sing, who obtained his doctorate at the University of Arizona working in the departments of physics, astronomy and planetary sciences and is now at the University of Exeter in the U.K.

Gilda Ballester, an associate staff scientist at the UA's Lunar and Planetary Laboratory who is one of the researchers participating in the project, said securing an observation program of this magnitude is an outstanding feat for a researcher at the beginning of his career.

"Data gathered using the Hubble Space Telescope will tell us more than ever before about the atmospheric composition of these planets outside our solar system," Ballester said. "So far, only two extrasolar planets have been observed in some detail in the visible light spectrum with Hubble."

"We are going to study eight, exploring for the first time the full diversity of Hot Jupiter atmospheres and identify important physical processes common to the entire class of these planets."

The planets to which the team will point Hubble vary in mass (from one-fifth to one and one-half times the mass of Jupiter), radius (one to twice that of Jupiter) and temperature (1,300 to 4,600 degrees Fahrenheit).

According to Ballester, Hot Jupiters orbit their star at very close range, and their rotation is synchronous to their orbit, meaning the same side always faces the star, much like the moon always points the same side at the Earth.

This makes for a lot of heating and for interesting dynamics, for example how the intense energy streaming onto the planet's exposed side is distributed around the planet.

If the Earth orbited the sun as closely as a typical Hot Jupiter, the star would dominate the sky to an imaginary observer looking up from the planet's point closest to the star. The opposite side would be plunged in eternal darkness.

Ballester said the typical distance of a Hot Jupiter to its star is a mere 2 to 5 one-hundredths of the distance between the Earth and the sun. And unlike Earth, which takes a year to complete its orbit around the sun, Hot Jupiters complete the journey in only one to five days.

By understanding the chemical make-up of exoplanet atmospheres around eight Hot Jupiters and perfecting the difficult techniques needed to make these precise measurements, the researchers will also help prepare for future searches for life on exoplanets.

Lead researcher Sing said: "Astronomers have now detected hundreds of exoplanets, and we now know that some of these planets have extreme environments, unlike anything in our own solar system. Everything we have discovered so far about these planets has been puzzling so I am expecting the unexpected."

The team aims to use the Hubble Space Telescope to detect and understand a mysterious gas in the stratospheres around these planets, causing a similar effect to the ozone layer on Earth. This gas is detectable as it absorbs light from the parent star when the planet passes in front of the star.

"When we observe an exoplanet passing in front of its star, we see it obscure part of the star light, not only with its disc, but also with its surrounding atmosphere," Ballester explained. "The way the atmosphere influences the light passing through tells us about its chemical composition, temperature and other characteristics."

Ballester added that the UA's Lunar and Planetary Laboratory has a large role in the program, analyzing about 40 percent of the large amounts of data Hubble will send to Earth from its vantage point in space.

Adam Showman, an associate professor at LPL, will model the atmospheric dynamics and thermal profiles. Together, they will mentor a post-doctoral fellow to work on this program.

Extrasolar planets still are shrouded in mystery, Ballester said.

"Some are bigger, some are heavier, some are closer to their stars. And some have what we call a thermal inversion zone, better known as a stratosphere, where temperatures increase with altitude and where the weather tends to be calm. Others don't have a stratosphere and we don't know why."

"Observing and modeling the atmospheres of those planets is a key to unraveling some of their mysteries."

Pitch Black: The (almost) dark truth about hot Jupiters
Scientific American | Caleb A. Scharf | 2011 Aug 22

Not an exoplanet - but the same principle: Venus transiting the Sun as seen in optical and ultraviolet wavelengths by the TRACE spacecraft. (Credit: NASA/LMSAL)

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The first exoplanet discovered around a normal star in 1995 was anything but normal in comparison to our own solar system. 55 Pegasi b is a gas giant world orbiting every 4.23 days – placing it some eight times closer to its stellar parent than the planet Mercury is around the Sun. At least half as massive as Jupiter, this new planet wasn’t just one of the first exoplanets, it was the first of a whole new class of objects – the hot Jupiters. How such a giant, gas laden world could exist orbiting so close to a star was a huge mystery. We still don’t fully understand all the details, but it seems that planets like this must form much further away from their stars and then orbital evolution transports them inwards – through mechanisms like disk-migration, and the gravitational perturbations of other planets in a system.

These are extraordinary planets to us, but in a cosmic context they are far from unusual. Perhaps 3% of stars harbor giant objects like these on close stellar orbits; an enormous total in this galaxy of 200 billion suns. For our still fledgling efforts to find exoplanets they have been a boon, far easier (relatively speaking) to detect through their gravitational pull on their parent stars, and far more likely to be seen transiting across those stellar disks than smaller or more distantly orbiting objects. Hot Jupiters have also provided us with the first glimpses of extraterrestrial atmospheres, from the bright infrared glow of star-heated hot spots and storms, to some of the basic constituents – sodium atoms, hydrogen, carbon, and oxygen. They have also stunned us with the unexpected; some with vastly inflated atmospheres, some in retrograde orbits.

But one of their most counter-intuitive properties has generally escaped popular notice. Hot Jupiters tend to be extremely dark. This was brought into the spotlight (ahem) recently with a paper by Kipping and Spiegel in the Monthly Notices of the Royal Astronomical Society. The planet TrES2-b was discovered in 2006 as it transited its Sun-like star. TrES2-b is 1.2 times the mass of Jupiter and orbits at a mere 0.04 astronomical units, racing around with a year-length of 2.5 Earth-days. This system is also within the field of view of NASA’s Kepler mission, allowing Kipping and Spiegel to dig into the extremely high-fidelity Kepler data to sniff for the variations in visible light as TrES2-b whizzes through its orbit.

Using this data it’s possible to measure the difference in brightness between the day and night sides of the planet – from our point of view the night side is when it passes between us and the star, and the day side is glimpsed just before and after the planet zips around the backside of the star. Remarkably the Kepler data indicates a variation in the total light reaching us from this star-planet system between the day-night passage of only about seven parts per million. This translates into the discovery that the fully illuminated day side of TrES2-b is barely reflecting any fraction of visible starlight towards us at all. Not only that, but by modeling the possible composition and structure of the planet’s atmosphere Kipping and Spiegel find that at least half of the light seen from the day side is actually coming from the planet’s own warmth. This close to the star the outer atmosphere of TrES2-b is heated to over 1000 Celsius, more than enough for it to glow like an ember in the fireplace.

The conclusion is that seen head-on this planet reflects less than 1% of the visible light falling on it, perhaps only 0.04% in their best models. It is more efficient at capturing photons than black paint. The reason is that at these high temperatures the outer atmosphere of a giant planet may have no reflective condensates – no clouds – and the warm swirling gas is exceptionally good at absorbing incoming photons of light. By contrast, frigid Jupiter in our system is rich in reflective ammonia cloud structures and bounces back about 50% of light when seen head-on. Not all hot Jupiters may be quite as dark as TrES2-b, but we have yet to fully examine their properties and this could well be a common characteristic. This kind of measurement provides fantastic insight to the atmospheric state of worlds unlike anything in our own solar system – great warm gaseous orbs, soaking up photons.

The notion of a murky black planet is very intriguing, and got this result into the news, but what would TrES2-b look like if we could see it with our own eyes? This is where a bit of a reality check and perspective is useful. The starlight hitting this planet is roughly 700 times brighter than the light we receive on Earth from the Sun. So if we were sitting in orbit around TrES2-b even 1% of that light reflected towards us would still look like an awful lot to our puny eyes. The fact is that this close to the star then the sheer flood of photons will make all reflecting objects look very bright to us, even if they are darker than charcoal. What if we were instead sitting out at a respectable 1 astronomical unit (the equivalent to Earth’s orbit)? We can use the planet Venus as a yardstick. Venus reflects an incredible 75% of visible light, but is about 20 times further from the Sun than TrES2-b is from its star, and is 15 times smaller in radius than TrES2-b. So how bright would TrES2-b appear to us compared to Venus in full phase?

I’ve brushed a few things under the carpet, but very roughly the reflected flux of photons we’d see from TrES2-b would still be about 3,000 times more than that from Venus. Since the human eye has an approximately logarithmic response to brightness this means that despite the very low reflectivity of TrES2-b it would still appear to shine as much as 3 times stronger than Venus in our sky – assuming of course you could ever separate it from the glare of the Sun given its tiny orbit. So the funny thing is that although this planet is so awfully good at absorbing light, because of its location it would not actually appear to be pitch black if we could look at it up close and personal.

Hot Jupiters can indeed be very dark, but what you end up seeing is all relative.

[bystander]

'Darkest' world enlightens astronomers about mysterious light-gobbling planet
Princeton University | Morgan Kelly | 2011 Sept 26