astronomy

TRAPPIST-1d

TRAPPIST-1d 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-1d rather than just read about it. In short: TRAPPIST-1d is a small exoplanet (about 40 percent the mass of the Earth), which orbits on the inner edge of the habitable zone of 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 exoplanet was found by using the transit method.

Key takeaways

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

Reference excerpt

TRAPPIST-1d is a small exoplanet (about 40 percent the mass of the Earth), which orbits on the inner edge of the habitable zone of 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 exoplanet was found by using the transit method. The first signs of the planet were announced in 2016, but it was not until the following years that more information concerning the probable nature of the planet was obtained. TRAPPIST-1d is the second-least massive planet of the system. It receives just 4.3% more sunlight than Earth, placing it on the inner edge of the habitable zone. A 2025 study, based on a James Webb Space Telescope observation, found that the data was "satisfactorily" consistent with TRAPPIST-1d having no atmosphere at all. Nevertheless, there is still a "marginal possibility" the planet has an atmosphere, but determining this with greater confidence would require a different detection technique.

Physical characteristics

Radius, mass and temperature TRAPPIST-1d was detected with the transit method, allowing scientists to accurately determine its radius. The planet is about 0.788 R🜨 with a small error margin of about 70 km. Transit timing variations and complex computer simulations helped accurately determine the mass of the planet, which led to scientists being able to calculate its density, surface gravity and composition. TRAPPIST-1d is a mere 0.388 M🜨, making it one of the least massive exoplanets yet found. Initial estimates suggested that it has 61.6% the density of Earth (3.39 g/cm3) and just under half the gravity. Compared to Mars, it has nearly three times that planet's mass but was thought to still be significantly less dense, which would indicate the presence of a significant atmosphere; models of the low density of TRAPPIST-1d indicated a mainly rocky composition, but with about ≤5% of its mass in the form of a volatile layer. The volatile layer of TRAPPIST-1d may consist of atmosphere, ocean and/or ice layers. However, refined estimates show that the planet is more dense, closer to 79.2% of Earth's bulk density (4.35 g/cm3). TRAPPIST-1d has an equilibrium temperature of 282.1 K (9.0 °C; 48.1 °F), assuming an albedo of 0. For an Earth-like albedo of 0.3, the planet's equilibrium temperature is around 258 K (−15 °C; 5 °F), very similar to Earth's at 255 K (−18 °C; −1 °F).

Orbit TRAPPIST-1d is a closely orbiting planet, with one full orbit taking just 4.05 days (97.2 hours) to complete. It orbits at a distance of just 0.0223 AU (3.34 million km; 2.07 million mi) from the host star, or about 2.2% the distance between Earth and the Sun. For comparison, Mercury, the Solar System's innermost planet, takes 88 days to orbit at a distance of about 0.38 AU (57 million km; 35 million mi). It is in 5:3 resonance with TRAPPIST-1c and 3:2 resonance with TRAPPIST-1e. The size of TRAPPIST-1 and the close orbit of TRAPPIST-1d around it means that the star, as seen from the planet, appears 5.5 times as large as the Sun from the Earth. While a planet at TRAPPIST-1d's distance from the Sun would be a scorched world, the low luminosity of TRAPPIST-1 means that the planet gets only 1.043 times the starlight that Earth receives, placing it within the inner part of the conservative habitable zone.

Host star

The planet orbits an ultracool dwarf star named TRAPPIST-1. The star has a mass of 0.09 M☉ (close to the boundary between brown dwarfs and hydrogen-fusing stars) and a radius of 0.12 R☉. It has a temperature of 2,566 K (2,293 °C; 4,159 °F), and is 7.6 billion years old. For comparison, the Sun is 4.6 billion years old and has a temperature of 5,778 K (5,505 °C; 9,941 °F). The star is metal-rich, with a metallicity ([Fe/H]) of 0.04, or 109% the solar amount. This is particularly odd, as such low-mass stars near the boundary between brown dwarfs and hydrogen-fusing stars should be expected to have considerably less metals than the Sun. Finally, its luminosity is 0.05 L☉. Stars like TRAPPIST-1 have the ability to live up to 4–5 trillion years, 400–500 times longer than the Sun will live (the Sun only has about 5 billion years of lifespan left, slightly more than half of its lifetime). Because of this ability to live for long periods of time, it is likely TRAPPIST-1 will be one of the last remaining stars when the universe is much older than it is now, when the gas needed to form new stars will be exhausted and the remaining ones begin to die off. The star's apparent magnitude, or how bright it appears from Earth's perspective, is 18.8. Therefore, it is too dim to be seen with the naked eye (the limit for that is 6.5). The star is not just very small and far away, it also emits comparatively little visible light, mainly shining in the invisible infrared. Even from the close-in proximity of TRAPPIST-1d (about 50 times closer than Earth is from the Sun), the planet receives less than 1% the visible light Earth sees from the Sun. This would probably make the days on TRAPPIST-1d never brighter than twilight is on Earth. However, that still means that TRAPPIST-1 could easily shine at least 3000 times brighter in the sky of TRAPPIST-1d than the full moon does in Earth's night sky.

Atmosphere

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with TRAPPIST-1d

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

In research: TRAPPIST-1d 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-1d 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-1d 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 in the habitable zone, so understanding it makes those chapters shorter.
In everyday life: Look for TRAPPIST-1d 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-1d in 20 minutes

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

Frequently asked questions

What is TRAPPIST-1d in simple terms?

Why does TRAPPIST-1d 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-1d?

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-1d.