astronomy

TRAPPIST-1c

TRAPPIST-1c 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-1c rather than just read about it. In short: TRAPPIST-1c is a mainly rocky 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 Aquarius. It is the third most massive and third largest planet of the system, with about 131% the mass and 110% the radius of Earth.

Key takeaways

TRAPPIST-1c 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-1c to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of TRAPPIST-1c from memory before moving on to harder problems.

Reference excerpt

TRAPPIST-1c is a mainly rocky 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 Aquarius. It is the third most massive and third largest planet of the system, with about 131% the mass and 110% the radius of Earth. Its density indicates a primarily rocky composition, and observations suggests against a thick CO2 atmosphere, which does not exclude a thick abiotic oxygen-dominated atmosphere.

Physical characteristics

Mass, radius, and temperature TRAPPIST-1c was observed with the transit method, which enabled scientists to calculate its radius. Transit-timing variations and computer simulations were able to determine the mass, density, and gravity of the planet. TRAPPIST-1c is the third-largest planet of the TRAPPIST-1 system, with a radius of 1.097 R🜨. It is also the third-most massive of the system, with a mass of 1.308 M🜨, slightly lower than that of the next most massive, TRAPPIST-1g. Initial estimates suggested that TRAPPIST-1c has a lower density (4.89 g/cm3) and gravity (0.966g) than Earth, consistent with a rock-based composition and a thick, Venus-like atmosphere. However, refined density estimates show that the planet's density is similar to Earth. TRAPPIST-1c's atmosphere was expected to be large enough to raise its surface temperature far above the calculated 334.8 K (61.7 °C; 143.0 °F) equilibrium temperature. However, an observation of the secondary eclipse of TRAPPIST-1c by the James Webb Space Telescope, announced in 2023, suggests against a thick CO2 atmosphere, however this does not exclude a thick abiotic oxygen dominated atmosphere as is hypothesized to be common around red dwarf stars, with a measured surface temperature of 380 K (107 °C; 224 °F). In addition, 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 The orbit of TRAPPIST-1c is very close to its host star. One year on the planet lasts a mere 2.42 days (58 hours), a fraction as long as that of the Solar System's innermost planet, Mercury, at 176 days. The planet orbits at a distance of 0.0158 AU, which is about 1.6% the distance between Earth and the Sun. At this proximity, TRAPPIST-1c is most likely tidally locked. However, due to the small size of its host star, the planet only receives about 2.1 times the sunlight as Earth (similar to Venus, at 1.9 times). Its orbital eccentricity is very low at 0.00654, similar to that of TRAPPIST-1b. It is in 8:5 resonance with TRAPPIST-1b and 5:3 resonance with TRAPPIST-1d.

Host star

TRAPPIST-1c orbits the ultracool dwarf star TRAPPIST-1. It is 0.12 R☉ and 0.09 M☉, with a temperature of 2566 K and an age of 7.6 billion years old. For comparison, the Sun has a temperature of 5778 K and is about 4.5 billion years old. TRAPPIST-1 is also very dim, with about 0.0005 times (0.05%) the luminosity 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 spectrum of TRAPPIST-1 b and c rules out a cloud-free hydrogen-dominated atmosphere for each planet, so they are unlikely to harbor an extended gas envelope. Prior to JWST observations, other atmospheres, from a cloud-free water-vapor atmosphere to a Venus-like atmosphere, remained consistent with the featureless spectrum. In 2018, the composition of TRAPPIST-1c was determined, and has been found to be rock-based. The presence of an atmosphere could not be confirmed. An observation of the secondary eclipse of TRAPPIST-1c by the James Webb Space Telescope, announced in 2023 rules out a thick carbon dioxide atmosphere like that of Venus. This is similar to JWST results on the inner planet TRAPPIST-1b announced earlier the same year, which suggest that it does not have a thick CO2 dominated atmosphere. In 2024, transmission spectra of the planet had ruled out a hydrogen-dominated atmosphere with >3σ confidence and an atmosphere rich in water, ammonia or carbon monoxide with a pressure of 1 bar when taking into account stellar contamination.

See also 55 Cancri e GJ 1132 b List of terrestrial exoplanet candidates for atmosphere detection

References

Illustrations

Worked examples

Example 1 — a first encounter with TRAPPIST-1c

Start with the simplest possible case. Write down what TRAPPIST-1c 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-1c 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-1c 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-1c

In research: TRAPPIST-1c 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-1c 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-1c 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-1c 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-1c in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what TRAPPIST-1c 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-1c out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is TRAPPIST-1c in simple terms?

Why does TRAPPIST-1c 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-1c?

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-1c.