Exoplanet Reflects Practically No Light—and Scientists Have No Idea Why

By Joseph Castro | August 12, 2011 3:16 pm

spacing is important

What’s the News: Using data from NASA’s Kepler spacecraft, astronomers from Princeton University and the Harvard-Smithsonian Center for Astrophysics have discovered the darkest known planet. The exoplanet, called TrES-2b, is located about 750 light-years away from Earth and reflects less than 1 percent of the incident light from its parent star, making it blacker than the blackest piece of coal. The discovery was published recently in the Monthly Notices of the Royal Astronomical Society (pdf).

What’s the Context:

  • For astronomers, albedo—the percentage of light that is reflected off an object’s surface—is a very useful concept that allows them to infer much about an object’s properties. For example, by comparing the albedo of an asteroid with the albedos of known materials here on Earth, astronomers can figure out how much of the body’s surface is covered with ice, dust, or metallic compounds. Albedo can also help pinpoint the temperature of a solar body.
  • For comparison, the Earth has an albedo of about 0.37, meaning that it reflects 37 percent of incoming light. Jupiter’s albedo, on the other hand, is 0.52 because of the large amount of reflecting clouds of ammonia and water ice in the planet’s upper atmosphere. The albedo of TrES-2b, as you could probably guess, is less than 0.01, while coal has an albedo of 0.05–0.1.

How Do They Know:

  • The Kepler spacecraft cannot see extrasolar planets directly—it detects them via the transit method. As a planet passes in front of its star, it causes a dip in the star’s brightness. Astronomers know that the star’s dimming is caused by a planet if it happens periodically (i.e. every time the planet revolves around the star).
  • Astronomers can also tease out a planet’s albedo with this method. Just as the Moon goes through phases as it revolves around the Earth, a planet goes through phases as it orbits its star, reflecting more or less light to an observer as it moves.
  • When the planet’s behind the star, we see only light from the star. As the planet crosses the front of its star—its transit—it blocks some of the starlight. When the planet’s to the side of the star (from our perspective), it reflects some starlight, allowing researchers to find its albedo. “By combining the impressive precision from Kepler with observations of over 50 orbits, we detected the smallest-ever change in brightness from an exoplanet,” lead author David Kipping said in a prepared statement.

Why Is the Planet So Dark:

  • The researchers can’t quite explain TrES-2b’s unusually low albedo. They have proposed that the planet’s atmosphere is composed of certain light-absorbing chemicals, such as vaporized sodium and potassium or gaseous titanium oxide, but even these molecules can’t account for the planet’s extreme blackness. “There’s a good chance it’s a chemical we haven’t even thought of yet,” Kipping told Space.com.

[via Harvard-Smithsonian Center for Astrophysics]

Image courtesy of David Aguilar, Center for Astrophysics

  • eyesoars

    It sounds like there may be an interaction between planet and star…

    given the large number of orbits, there may be a tidal (or other) effect between the planet and star, disturbing the independence of the star’s brightness from the transiting planet.

    The ’50 orbits’ suggests the planet must be quite close to the star, and that also strongly suggests that the planet is hot. The albedo is much lower than reasonable: it’s *hard* to make something with an albedo that low.

  • Xodus

    Maybe it’s just pure evil. 😉

  • Chris Gilgallon

    Death Star

  • Tim

    Perhaps it’s an uninhabited planet, converted into a giant battery – a solar energy collector and storage system. Advanced civilizations (maybe living on a nearby, yet undetected planet) surely need advanced power sources, right?

  • mrzeigler

    When this news reaches beings on that planet, they’re going to have some harsh words for the cloaking device manufacturer.

  • Komeess

    Not everything dark is evil, but from my understanding this dark planet and probably there are more dark planets like this that are not yet to be found are actually helping us. It looks like they absorb most UV light from the sun act like filter so that these rays would not be a problem to the life on earth.

  • Pippa

    Mrzeigler – thanks for the laugh. Brilliant comment???????

  • Durant S.

    Maybe it’s composed entirely of dark matter.

  • quarkman

    We have found Giedi Prime!

  • Ken

    Is there anything that can repeat the observations, but in the infrared? If it’s absorbing all that light it must be heating up.

  • Georg

    That is some “soot” or carbon black very likely. Temperature
    in the upper atmosphere and UV from the nearby star will crack the methane
    (or other hydrocarbons) to form sooty material and hydrogen.
    That hydrogen can diffuse into space.
    In case that soot floats for some time in the atmosphere, it
    wiil make the gas behaving as a black body thus radiating
    like a candle flame.
    Saying that it is “black” is not correct. It radiates a light red
    light, (1000 °C, a candle is anout 1200 °C, thus yellow).

  • Jovan

    It’s Wesley Snipes.

  • Erik

    Somebody tell Flavor Flav!

    Seriously, any chance this is actually a sunspot? A sunspot would produce a cyclical dimming and of course no albedo at the end of the transit. Is 1% reflectivity close to the noise level when looking close to the star right beside it to measure the albedo of the candidate planet?

  • scott

    perhaps it’s completely covered in super-efficient photosynthetic life…?

  • Jacob
  • m

    Why don’t they look at Mercury and see what it’s reflectivity is? Too bad we do not know over what time period those 50 orbits occured. Also, could this planet have an oversized moon that would give a false reading?

    It’s fun to conjecture. And yes, maybe it is composed of a material we don’t know about…..yet.

    750 ly isn’t so far. We should send a probe. Our ancestors 2 million years from now will thank us. :)

  • Devin

    There’s something about this planet that’s so black, it’s like how much more black could this be? And the answer is none. None more black.

  • Tetzauh

    I remember reading something similar about pluto. Maybe by 2015, when New Horizons flies by we may have an answer to the puzzle.

  • david

    How about a small black star in orbit around the star? No reflection, right?

  • david

    How about a small black hole in orbit around the star? No reflection, right?

  • http://fiske.colorado.edu Chris

    This is clearly a Xindi artifact intended to frighten us. We must destroy it, without mercy. Or mayonnaise.

  • Chris

    @ david 19&20
    They can determine how big the planet is by the amount of light that is dimmed when it passes in front. If our Sun were to become a black hole it’s Schwartzchild radius would be only 3 km. So if a black hole were to have a size even approaching a small moon, much less a planet, the wobble on the host star would be easily noticeable and the star would have probably be eaten up long ago.

  • http://www.bowlofknowledge.blogspot.com Moonman

    Man this planet is Black!

    How black is this planet? it’s blacker than the blackest person on earth painted black


  • Kees

    The link: “lead author David Kipping said in a prepared statement” is broken. (should be .html; not .htmll)


  • Karlin

    It is possible that this dark planet was at one time populated by humanoids, and as on earth, they would need energy, so maybe they burned up 100 million years worth of stored fossil fuels in a short time, in about 100 years or so.

    Inefficient combustion led to hydrocarbon pollution which blackened the atmosphere.

    Just a thought…

  • http://discovermagazine.com Iain

    Oh! I know! I know! Because the planet absorbs light.
    Next problem please.

  • floodmouse

    Tim #4 states: “Perhaps it’s an uninhabited planet, converted into a giant battery – a solar energy collector and storage system. Advanced civilizations (maybe living on a nearby, yet undetected planet) surely need advanced power sources, right?”

    I agree with Tim, except for the location of the advanced civilization. How about this sci-fi scenario: The whole planet has been terrascaped & textured to absorb light like a giant solar panel, creating immense amounts of electricity to power a civilization located UNDERNEATH the planet’s surface . . . possibly in response to some environmental catastrophe. Maybe the lights are still on, but nobody is home due to a massive population die-off. I saw something similar in a movie. I think this was the movie in question:


    The technology they used to blacken the planet is undoubtedly derived from the special optical properties of giant space-bug wings. You will want to check out this link to see a Terran analogy:

    http://discovermagazine.com/2011/jun/03-bugs-can-teach-us-a-lot-about-solar-power/?searchterm=insect solar black

  • gw

    Any chance it could be mostly carbon? Too simple for the techies and the conspiracy theorists, I guess.

  • lacey redd

    like a bottle of oil and vinagar…our earth is settling, and seperating into like elements…maybe this planet connsists of only one element that will not seperate until it colides with another planet…. maybe it is a part of our of our indrocrine system and our planet and solar system is an artry….different laws obide…does this mean that the big picture fell down and skinned it’s knee?

  • Odin


    LOL, how can a planet from outside the solar system protect life on earth from the sun’s UV rays?

  • pj

    I like floodmouse’s hypothesis the best! Write us a story, floodmouse!

  • Stephen Fretwell

    If it were dark matter, it would not dim it’s sun when it passes in front. Dark matter planets have to be discoverd through solar wobble, or periodically distorted light bending around the star.

  • http://bossy-girls.net Lila Sovietskaya

    Many humorous comments do not solve this puzzle. Either the astrophysicist came to the wrong conclusion. An atmosphere of carbon gas would be read not black. Very likely the atmosphere has low albedo because itemits non-visible light, either infrared or ultraviolet

  • Ray Fowler

    @Devin, surely this planet is the stuff of album covers

  • tangojuli

    Mystery solved: It is Z’ha’Dhoum–homeworld of the “shadows” (Babylon 5 ref)
    @Lila-humor will keep this in the front of people’s minds.

  • BohdanUke1

    Blind inhabitants painted it black by mistake. End of story. WAIT! just one more idea……. DOH!

  • Ray Fowler

    To me, it seems like an error in the observation measurements. That seems a lot more likely than the planet being covered in some heretofore unknown substance that would be the darkest substance ever discovered.

    Measurement error. HAPPENS ALL OF THE TIME.

  • praedor

    Sheesh. CLEARLY it is merely an alien high school science project. The students converted a planet into a true black body to test black body radiation – for real.

    The project took 3rd place in their local science fair.

  • Ashran

    I have a low libido also. Does that make me a black hole?

  • Grimace

    Maybe our instruments are wrong!!!

  • Grimace

    Maybe it’s Maybelline!

  • charlie

    #4 ‘Tim’ states an advanced civilisation would need energy so built a giant solar collector. Shame they didn’t just use the massive energy source right next to them known as a sun.

  • Duane

    Have we taken into consideration that there could be planets that revolve around distant stars orbiting in a plane where the planet does not cross the stars light from our perspective? I doubt that every planetary system in the galaxy has an orbital path of planets on the same plane as our system is on. If this is true then there could be far more planetary systems out there than we can detect with current methods.

  • Jonny

    It’s probably a primitive form of cloaking device around an alien mega-cruiser that doesn’t actually bend light around it properly, just absorbs it. Actually by absorbing 100% light the mega-cruiser might actually be using that as a power source perhaps temporarily to charge a super-powerful weapon or transgalactic FTL drive.

    To respond to #43, direct use of the sun as an energy source is almost impossible and only the Ancients managed to figure that out (you fly one of their crafts into a star to recharge it) without being ripped apart by radiation, heat, electromagnetic flux, gravity, or all of the above .

    @#39. Hah, maybe somebody wants to create a binary system eh?

  • Big_Ed

    I agree with Erik. Maybe it’s a giant sunspot like the big red spot found on Jupiter, and it just never goes away. If there is a wobble, there may be another undetected planet, possibly with the same period as the rotation of the star / sunspot.

  • http://drgustavosantos.com Gustavo Santos

    Didn’t you get it? Remember the black monolith on Arthur Clark’s book “2001 – space odissey”? In that book, coincidentally, it was a mistery to scientists why the black slab was so black. In fact, if reflected no light at all.
    So that’s it: the planet is in fact the black monolyth

  • Jeremy P

    if you read the pdf, the transit time is roughly 4624 seconds (roughly 77 minutes, give or take 42 seconds) and the orbital period is only 2.470619 days. (section 3.2).

  • http://www.quantumzk.com Zoltan

    Sphere symmetrical accelerating collapse at speed i=lim(adt)=c will not return in practice any light impact, rather “swollows” it as red shift. We are probably experiencing a neutron process of this magnitude. Can we call it as neutron star?

  • Jonny

    @Gustavo Santos – Oh indeed! But then the black monoliths turned Jupiter into a glowing star of its own! I think I got the reference, if I didn’t or even missed part of it I am deeply sorry.

    Pray I don’t let Larry Niven down now. “Larry Niven” should have a familiar “ring” to it for most of you guys. I know I know that was really bad, roasting an orange cat would have been funnier.

  • Tony Milton

    The inhabitants probably burned evrything.


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