astronomy

TRAPPIST-1b

TRAPPIST-1b is a astronomy topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand TRAPPIST-1b rather than just read about it. In short: TRAPPIST-1b is a terrestrial, Earth-sized exoplanet orbiting around the ultra-cool red dwarf star TRAPPIST-1, located 40.7 light-years (12.5 parsecs) away from Earth in the constellation of Aquarius. The planet was detected using the transit method, where a planet dims the host star's light as it passes in front of it.

Key takeaways

TRAPPIST-1b belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect TRAPPIST-1b to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of TRAPPIST-1b from memory before moving on to harder problems.

Reference excerpt

TRAPPIST-1b is a terrestrial, Earth-sized exoplanet orbiting around the ultra-cool red dwarf star TRAPPIST-1, located 40.7 light-years (12.5 parsecs) away from Earth in the constellation of Aquarius. The planet was detected using the transit method, where a planet dims the host star's light as it passes in front of it. It was first announced on May 2, 2016, and later studies were able to refine its physical parameters. The planet is about 37% more massive than Earth and about 39% larger in volume; thus its density is very similar. It is the innermost of seven planets orbiting TRAPPIST-1, all of which are terrestrial, but is too close to its star to be in the habitable zone. Observations by the James Webb Space Telescope announced between 2023 and 2024 suggest that it is either airless or has a hazy CO2-rich atmosphere. Its albedo is very low, making it dark in color.

Physical characteristics

Mass, radius, and temperature TRAPPIST-1b is very similar in both mass, radius, and gravity to Earth. It has a radius of 1.116 R🜨, a mass of 1.374 M🜨, and about 110% Earth's surface gravity. Initial estimates of the planet's density suggested that it is not entirely rocky; with a density of 3.98 g/cm3, about ≤5% of its mass must be volatiles, likely in the form of a thick Venus-like atmosphere due to it receiving nearly four times more energy than Earth does. However, refined density estimates show that the planet is only slightly less dense than Earth. Assuming the presence of an atmosphere, the planet's surface temperature was initially estimated to be between 750 K (477 °C; 890 °F) and 1,500 K (1,230 °C; 2,240 °F), potentially as high as 2,000 K (1,730 °C; 3,140 °F). This is much hotter than the surface of Venus and may be hot enough that the surface is molten lava. An observation of the secondary eclipse of TRAPPIST-1b by the James Webb Space Telescope, announced in 2023, suggests that the planet does not have any significant atmosphere, with a measured surface temperature of about 503 K (230 °C; 446 °F), and a low albedo. The planet may be very geologically active due to tidal squeezing similar to Jupiter's moon Io, which happens to have a similar orbital period and eccentricity (see TRAPPIST-1#Resonance and tides for references).

Orbit TRAPPIST-1b orbits very close to its parent star. One orbit requires only, about 1.51 days (36 hours). It orbits about 0.0115 AU (1.72 million km; 1.07 million mi) from its star, just 1.2% the distance between Earth and the Sun. The close proximity to its host star means that TRAPPIST-1b is likely tidally locked. It also has a very circular orbit, with an eccentricity of 0.00622, significantly more circular than Earth's orbit, which has an eccentricity of 0.0167086. It is in 8:5 resonance with TRAPPIST-1c.

Host star

TRAPPIST-1b orbits the ultracool dwarf star TRAPPIST-1. It has a mass of 0.09 M☉ and is only 0.12 R☉, with a surface temperature of 2,566 K (2,293 °C; 4,159 °F) and an age of 7.6 billion years old. The Sun, in comparison, has a surface temperature of 5,778 K (5,505 °C; 9,941 °F) and is about 4.5 billion years old. TRAPPIST-1 is also very dim, with a luminosity about 0.0005 times that of the Sun. It is too faint to be seen with the naked eye, having an apparent magnitude of 18.80.

Atmosphere

The combined transmission spectra of TRAPPIST-1 b and c rule out cloud-free hydrogen-dominated atmospheres for both planets, so they are unlikely to harbor extended gas envelopes. Also, no helium emission from TRAPPIST-1b has been detected. Prior to JWST observations, other atmospheres, from a cloud-free water-vapor atmosphere to a Venus-like atmosphere, remained consistent with the featureless spectra. In 2018, the planet's atmosphere was better examined by the Spitzer Space Telescope and suggested to be quite large and hot, although the presence of an atmosphere could not be confirmed. The planet's transmission spectrum and refined density estimate suggested two main possibilities for the atmosphere: one rich in carbon dioxide, or one rich in water vapor. The more likely CO2 atmosphere would have a scale height of approximately 52 kilometers (32 miles) (Earth's being 8 km [5 mi], and Venus' at 15.9 km [9.9 mi]) and an average temperature in excess of 1,400 K (1,130 °C; 2,060 °F), far greater than the planet's equilibrium temperature of 397.6 K (124.5 °C; 256.0 °F). A water vapor atmosphere would need to have a scale height of >100 km (62 mi) and a temperature >1,800 K (1,530 °C; 2,780 °F) to produce the variations seen in the planet's transit depths and its transmission spectrum, and would be vulnerable to photodissociation where CO2 would not be. Other sources for the effects seen, such as hazes and thick clouds, would require an even larger atmosphere. TRAPPIST-1b will have to be studied further to confirm its potential large atmosphere. An observation of the secondary eclipse of TRAPPIST-1b by the James Webb Space Telescope, announced in March 2023, suggested that the planet does not have any significant atmosphere. Atmospheres containing carbon dioxide with surface pressures greater than 0.1 bar can be ruled out at 3-sigma, and pressures greater than 0.01 bar at 1-sigma. Further studies of the exoplanet by transmission spectroscopy (primary eclipse), reported in September 2023, also confirmed the absence of a hydrogen-rich atmosphere, but due to stellar contamination were unable to determine the presence or absence of other types of atmospheres based on the transmission spectroscopy data alone. This does not affect the previous results based on emission spectroscopy. Analysis of ten secondary eclipses observed by JWST published in December 2024, five in 12.8 μm and five in 15 μm, showed a shallower eclipse depth in the shorter wavelength that may be caused by photochemical hazes in a thick, CO2-rich atmosphere. The presence of photochemical hazes in the atmosphere of TRAPPIST-1 b could create a thermal inversion that warms the upper atmosphere relative to the lower atmosphere, decreasing the infrared brightness temperature at 12.8 μm. The high UV flux of TRAPPIST-1 b would make for efficient photodissociation and photochemistry. However, it is unclear if such hazes can form in a hot, oxidized, CO2-rich atmosphere. These results are also compatible with a young ultramafic surface and no atmosphere, which the authors of the 2024 paper slightly favor.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with TRAPPIST-1b

Start with the simplest possible case. Write down what TRAPPIST-1b claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In astronomy, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to TRAPPIST-1b before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about TRAPPIST-1b ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of TRAPPIST-1b

In research: TRAPPIST-1b appears in astronomy research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses TRAPPIST-1b in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: TRAPPIST-1b is common in secondary-school and first-year university syllabi. It links to neighbouring topics Aquarius (constellation), Exoplanets discovered in 2016, Near-Earth-sized exoplanets, so understanding it makes those chapters shorter.
In everyday life: Look for TRAPPIST-1b outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study TRAPPIST-1b in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what TRAPPIST-1b means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain TRAPPIST-1b out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is TRAPPIST-1b in simple terms?

Why does TRAPPIST-1b matter?

Because it connects several astronomy ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study TRAPPIST-1b?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on TRAPPIST-1b.