TrES-2b

TrES-2b is an extrasolar planet (exoplanet) orbiting the star GSC 03549-02811, located in the constellation Draco, approximately 750 light-years from Earth. It gained scientific prominence for being the darkest known exoplanet ever discovered—reflecting less than 1% of the light that falls on it, making it darker than coal or black acrylic paint. TrES-2b is a hot Jupiter, a class of gas giant planets that orbit very close to their parent stars, resulting in extreme surface temperatures and unique atmospheric conditions.

Discovery and Observation

TrES-2b was discovered in 2006 by the Trans-Atlantic Exoplanet Survey (TrES), a network of small, ground-based telescopes located in the United States, Spain, and the Canary Islands. The planet was detected using the transit method, which measures the slight dimming of a star’s light when a planet passes in front of it.
The discovery was announced by astronomers led by Georgi Mandushev and colleagues, and subsequent observations confirmed its planetary nature. Later, the Kepler Space Telescope observed TrES-2b extensively, allowing astronomers to study its light curve and albedo (reflectivity) in greater detail.

Orbital and Physical Characteristics

TrES-2b orbits its parent star, a G0V-type star similar to the Sun, but slightly hotter and more massive.
Key parameters include:

• Host star: GSC 03549-02811 (similar to the Sun)
• Distance from Earth: ~750 light-years (230 parsecs)
• Constellation: Draco
• Orbital period: 2.47 Earth days
• Orbital distance (semi-major axis): ~0.035 AU (about 5.2 million kilometres)
• Inclination: Nearly edge-on, allowing transits to be visible from Earth.

Because of its close orbit, TrES-2b is tidally locked, meaning one side always faces its star, much like the Moon does with Earth.
Physical characteristics:

• Planet type: Gas giant (Hot Jupiter)
• Mass: Approximately 1.2 times the mass of Jupiter
• Radius: About 1.3 times Jupiter’s radius
• Mean density: Around 0.83 g/cm³ (less dense than Jupiter)
• Surface temperature: Estimated at around 1,000–1,100°C (1,300–1,400 K)

The intense stellar radiation causes the planet’s atmosphere to glow faintly in infrared, even though it reflects almost no visible light.

Darkness and Reflectivity

TrES-2b’s most extraordinary feature is its extremely low albedo, meaning it reflects only a tiny fraction of incident light. Observations from NASA’s Kepler telescope showed that its geometric albedo is less than 1%, making it the darkest exoplanet known.
For comparison:

• Earth reflects about 37% of sunlight.
• Jupiter reflects about 52%.
• Charcoal reflects about 5%.
• TrES-2b reflects less than 1%—possibly as little as 0.3%.

Astronomers attribute this darkness to its atmospheric composition, which likely contains:

• Vapours of sodium and potassium, which strongly absorb visible light.
• Possible titanium oxide (TiO) and vanadium oxide (VO) compounds, acting as optical absorbers.
• A lack of reflective clouds, unlike many other gas giants that have bright, ammonia-based clouds.

Despite its darkness, TrES-2b emits a faint red glow, due to thermal radiation from its intensely hot atmosphere.

Atmospheric Composition and Structure

TrES-2b’s atmosphere is dominated by hydrogen and helium, similar to Jupiter, but its proximity to its star leads to exotic chemical and thermal conditions.

• Temperature inversion: Likely absent, meaning that atmospheric temperature decreases with altitude rather than increasing, as seen on some other hot Jupiters.
• Photochemical reactions: High-energy radiation from the parent star drives complex photochemistry, possibly creating transient compounds that further reduce reflectivity.
• Thermal emission: The planet radiates strongly in the infrared spectrum, helping astronomers measure its temperature distribution.

The darkness also suggests that reflective condensates—such as silicate or ammonia clouds—cannot form under such extreme temperatures, leaving the atmosphere optically thin and absorptive.

Significance in Exoplanet Research

TrES-2b has become a benchmark case in the study of planetary atmospheres and light reflectivity. Its discovery challenged earlier assumptions about the optical properties of gas giants and demonstrated the diversity of exoplanetary atmospheres.
Scientific importance includes:

• Providing insight into energy absorption and heat redistribution in planetary atmospheres.
• Helping refine models of albedo variations and atmospheric chemistry.
• Offering a test case for studying phase curves—the change in brightness of a planet as it orbits its star.
• Demonstrating the capabilities of precision photometry missions such as Kepler for characterising exoplanet properties beyond detection.

TrES-2b also illustrates how planetary reflectivity can vary drastically depending on composition, temperature, and proximity to the host star, even among planets of similar size.

Comparative Context

TrES-2b belongs to the class of Hot Jupiters, similar to other well-known exoplanets such as:

• HD 209458b (“Osiris”) – the first exoplanet observed to have an atmosphere.
• WASP-12b – one of the hottest known exoplanets.
• Kepler-7b – a bright Hot Jupiter with reflective cloud layers.

Unlike Kepler-7b, however, TrES-2b lacks reflective clouds, explaining its extraordinary darkness. This diversity among Hot Jupiters highlights the complexity of atmospheric processes in exoplanets.

Future Research and Observations

TrES-2b continues to be a subject of interest for space-based observatories such as the James Webb Space Telescope (JWST), which can study its thermal emission spectrum to identify molecular signatures like water vapour, carbon monoxide, and methane.
Future studies aim to:

• Model its heat distribution between day and night sides.
• Detect potential chemical absorbers responsible for its darkness.
• Understand how stellar radiation influences atmospheric evolution in close-in gas giants.