“Some planets are darker than coal.” Astronomers have discovered worlds that barely reflect light

Some planets beyond our solar system are so dark that they reflect almost none of the light falling on them. Astronomers have found worlds that appear darker than coal or fresh asphalt, not because they sit in permanent darkness, but because their atmospheres absorb most incoming starlight instead of sending it back into space.

One of the most extreme examples is TrES-2 b, a giant planet orbiting very close to its star. NASA describes it as the darkest planet yet discovered around another star, with less than 1% of the light reaching it reflected back into space.

That raises a strange question: how can a planet sitting next to an intensely bright star appear almost black?

TrES-2 b reflects less than 1% of the light it receives

TrES-2 b is a gas giant somewhat larger than Jupiter, but its most famous characteristic is its extraordinary darkness. NASA says the planet reflects less than 1% of incoming starlight, making it darker than coal.

Astronomers first discovered the planet in 2006 by watching it pass in front of its host star. Its unusual darkness was identified later using observations from NASA’s Kepler spacecraft.

NASA’s current catalog still describes TrES-2 b as the darkest planet known to orbit a star.

The planet circles its star in only about 2.5 days, meaning it sits extremely close to an intense source of heat and radiation.

Takeaway: TrES-2 b is not dark because little light reaches it. It is dark because almost all of that light appears to be absorbed rather than reflected.

Coal is actually brighter than this planet

Calling a planet “darker than coal” sounds like exaggeration, but the comparison comes from reflectivity. Astronomers describe this using a measurement called albedo, which represents how much incoming light a surface or atmosphere reflects.

Fresh snow has a high albedo because it reflects a large fraction of sunlight. Dark materials such as charcoal absorb much more.

TrES-2 b’s measured reflectivity is extraordinarily low. NASA’s description says less than 1% of the light striking the planet is reflected.

That makes it darker in visible light than many materials people naturally think of as black.

Takeaway: “Darker than coal” refers to measurable reflectivity, not simply an artist deciding to color the planet black.

Scientists are still not completely sure why it is so dark

The planet’s temperature and atmospheric chemistry probably play major roles, but astronomers have not reduced its darkness to one simple explanation.

Hot Jupiters such as TrES-2 b orbit extremely close to their stars. Their atmospheres can become too hot for the bright reflective clouds that appear on cooler worlds.

Instead, gases and chemicals capable of absorbing visible light may dominate the atmosphere.

NASA notes that the precise reason for TrES-2 b’s extreme darkness remains an active area of research, even though its low reflectivity has been clearly measured.

Takeaway: Astronomers can measure how little light TrES-2 b reflects more confidently than they can explain exactly which atmospheric processes make it so dark.

WASP-12b is another almost-black world

TrES-2 b is not alone. Hubble observations of WASP-12b revealed another extremely dark hot Jupiter that absorbs at least 94% of the visible light falling onto its daytime side.

NASA describes WASP-12b as appearing almost as black as fresh asphalt. Its atmosphere reaches about 4,600 degrees Fahrenheit on the side permanently facing its star.

At those temperatures, many molecules cannot survive in the same forms they would on cooler planets.

Reflective clouds are also difficult to maintain, allowing incoming starlight to penetrate deeper into the atmosphere and be absorbed.

Hubble researchers reported that this unusually dark atmosphere was unexpected when they measured the planet’s reflected light.

Takeaway: More than one exoplanet absorbs nearly all visible starlight, suggesting extreme darkness can emerge naturally under the right atmospheric conditions.

These worlds are dark but definitely not cold

On Earth, darkness is often associated with cold because shaded places receive less sunlight. That intuition fails completely for these planets.

WASP-12b is nearly black while simultaneously being heated to temperatures comparable with some stars on its day side.

The same incoming starlight that fails to bounce back into space is absorbed and converted into heat.

A planet can therefore be extremely dark in visible light while radiating enormous amounts of energy at infrared wavelengths.

Astronomers can study that thermal radiation even when very little reflected visible light reaches us.

Takeaway: A black-looking planet can be extremely hot because absorbing light heats the atmosphere instead of making the planet bright.

A planet’s color depends heavily on its atmosphere

Planets are not simply colored by the solid material beneath them. Thick atmospheres and clouds can dominate what an outside observer sees.

Venus provides a familiar example. Its clouds reflect a large fraction of sunlight, making the planet extremely bright even though conditions underneath are hostile.

Hot Jupiters can behave very differently. Without bright reflective clouds, their atmospheres can absorb much more light.

NASA notes that many hot Jupiters reflect considerably more light than WASP-12b, showing that similar-looking giant planets can have very different atmospheric chemistry.

Takeaway: Two planets with similar sizes and temperatures can have dramatically different brightness depending on what gases and clouds exist in their atmospheres.

Astronomers can measure darkness without directly seeing the planet clearly

Most exoplanets are far too distant and too close to their bright host stars for astronomers to photograph them like planets in our own solar system. Scientists instead measure tiny changes in the total light coming from the star-and-planet system.

One useful moment occurs when a planet passes behind its star.

If the planet normally reflects some starlight toward Earth, the total brightness of the system should decrease slightly when the planet disappears from view.

Hubble used this method with WASP-12b. Researchers detected almost no reflected visible light, allowing them to calculate just how dark its daytime side must be.

NASA explains why these measurements are difficult in its overview of exoplanet observations.

Takeaway: Astronomers do not need a detailed photograph to determine how dark a distant planet is. Tiny brightness changes can reveal its reflectivity.

Some exoplanets sit at the opposite extreme

The diversity becomes even clearer when extremely dark planets are compared with highly reflective ones. ESA’s Cheops mission found that the exoplanet LTT9779 b reflects about 80% of the light reaching it.

That makes it brighter than Earth, which reflects roughly 30% of incoming sunlight, and comparable with highly reflective Venus.

ESA researchers think reflective clouds containing metals such as silicates and titanium may help explain its remarkable brightness.

The contrast is remarkable: some intensely heated exoplanets absorb almost everything, while another can behave like a giant mirror.

Takeaway: Exoplanets range from almost pitch-black worlds to planets that reflect most of the light reaching them, showing how varied planetary atmospheres can be.

A dark planet would not necessarily look completely black up close

TrES-2 b absorbs nearly all visible light, but scientists think it might still produce a faint glow because it is extremely hot. NASA has suggested the planet could appear almost black with a dim reddish appearance from its heated atmosphere.

This would be more like glowing charcoal than an invisible black hole.

The planet still emits thermal radiation, especially at wavelengths human eyes cannot see.

An observer equipped with infrared instruments would therefore detect an enormous amount of energy even though the world looked remarkably dark in visible light.

Takeaway: “Almost black” refers mainly to reflected visible light. These planets can still glow strongly in infrared because of their extreme temperatures.

Darkness can tell astronomers what an atmosphere contains

Measuring reflectivity is not just about identifying unusual planets. It provides clues about clouds, gases, temperature and atmospheric circulation.

A bright atmosphere may contain reflective clouds or hazes, while a very dark atmosphere may contain substances that absorb visible light efficiently.

Researchers can combine these measurements with spectroscopy, which separates light into wavelengths and helps identify atmospheric chemicals.

The planet’s darkness therefore becomes another piece of evidence for reconstructing an environment that astronomers may never photograph directly in detail.

Takeaway: Knowing how much light a planet reflects helps scientists infer what may be happening inside its atmosphere.

Could a rocky planet be this dark too?

In principle, a rocky planet could have a very dark surface or atmosphere, but the famous extreme examples discovered so far include giant planets orbiting very close to their stars.

A rocky world covered in certain dark minerals could absorb substantial visible light.

Thick atmospheric haze could also alter its reflectivity.

Astronomers are increasingly able to study smaller exoplanets, so future observations may reveal rocky worlds with similarly unexpected optical properties.

The wider lesson is that planets do not have to resemble anything in our solar system.

Takeaway: Extremely dark rocky worlds are physically possible, but the best-known nearly black exoplanets today are intensely heated gas giants.

Related: Steven Spielberg Warns There’s “Overwhelming” Proof Aliens Exist and It Changes Everything

So how can a planet be darker than coal?

Because brightness depends on how much incoming light is reflected, not how much light reaches the planet. TrES-2 b sits close to a bright star but sends less than 1% of that starlight back into space.

WASP-12b absorbs at least 94% of incoming visible light and becomes incredibly hot in the process.

Scientists think extreme temperatures, unusual atmospheric chemistry and the absence of highly reflective clouds contribute to these conditions, although important details remain under investigation.

The discoveries reveal something broader about planets beyond our solar system.

Some can be brighter than expected, while others absorb so much starlight that even coal would appear reflective beside them.

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