Kepler-51

Key takeaways

Kepler-51 belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Kepler-51 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Kepler-51 from memory before moving on to harder problems.

Reference excerpt

Kepler-51 is a Sun-like star that is about 500 million years old. It is orbited by four planets—Kepler-51b, c, d and e—first three of which are super-puffs and have the lowest known densities of any known exoplanet. The transiting planets in the system (b, c and d) are similar in radius to gas giants like Jupiter, but have unusually small masses for their size, only a few times greater than Earth's.

Properties Kepler-51 is a small G-type star, with a slightly lower radius, mass and effective temperature than the Sun. It is a young star, less than one billion years old, and hence is highly active compared to the Sun. Around 4 to 6% of the star's surface is covered by starspots. Its EUV and X-ray fluxes are likely influencing the chemistry, dynamics and atmospheric mass loss of its planets.

Planetary system

Kepler-51 has four planets, discovered between 2013 and 2024. The first planets discovered in the system were Kepler-51 b, c and d, detected by the transit method. The radius of these planets were initially measured using transit data, yielding values of 7.1, 9.0 and 9.7 R🜨 respectively, while masses were measured using transit-timing variations, giving masses of 2.1, 4.0 and 7.6 M🜨 respectively. These estimates imply very low densities, less than 0.05 g/cm3, one of the lowest of any exoplanets, or 14 times less than Saturn. The low masses were later confirmed in 2020 and 2024, and the densities have been improved to less than 0.14 g/cm3. Kepler-51 b, c and d are called super-puffs, planets with masses a bit larger than that of Earth, but radii larger than Neptune. The reason for the low density of these planets remains elusive, and many hypotheses have been proposed to explain the nature of these planets, all of which have flaws. In 2024, the Kepler-51 system was revealed to have a new planet, detected using transit timing variations by the James Webb Space Telescope and named Kepler-51e.

Kepler-51b The innermost planet, Kepler-51b, has an orbital period of 45 days. It is 6.8 times larger than Earth and 319 times more voluminous, but its mass is only 3.5 times that of Earth. This translates to a very low density of 0.06 g/cm3, much lower than that of any planet in the Solar System. Given the planet's proximity to its host star, its equilibrium temperature is of 543 K. Transmission spectroscopy with the Hubble Space Telescope revealed that Kepler-51b has a featureless spectrum, implying that its extended atmosphere has a high photochemical haze layer. Over time, the planet will contract, lose part of its atmosphere and become a sub-Neptune.

Kepler-51c Kepler-51c takes 85 days to complete an orbit around its host star, about the same as the planet Mercury. It has 6.4 times Earth's radius (40,770 km) and is 262 times more voluminous, while its mass is only around 5.65 Earth masses. This implies a low density of 0.14 g/cm3.

Kepler-51d Kepler-51d is the puffiest planet in the system, with an density of just 0.0381 g/cm3. It is also the largest planet orbiting Kepler-51, with 9.32 times Earth's radius (59,400 km), almost the same size as Saturn. Its mass, however, is only 3.8 times that of Earth. Transmission spectroscopy with the Hubble Space Telescope revealed that Kepler-51d has a featureless spectrum, implying that its extended atmosphere has a high photochemical haze layer. Over time, the planet will contract and lose part of its atmosphere, but will still have a low density. The rotation of the planet has been measured to be larger or equal than 40 hours.

Kepler-51e Kepler-51e is the outermost planet in the system. It was discovered via transit-timing variations of Kepler-51d: The planet's transit time, measured with the James Webb Space Telescope, was found to be discrepant with the predictions made by a three-planet model, implying the presence of an yet unseen fourth planet. The best-fit planet model, which implies a 2:1 orbital resonance with Kepler-51d, give a mass between 1.6 and 6.1 Earth masses and an orbital period of 260 days. However, it is possible that it is a more massive planet with a longer and more eccentric orbit. Since it was not observed with the transit method like the inner planets, its radius and hence density cannot be measured.

See also V1298 Tauri - a young system with four super-puff planets

References

Further reading Kepler-51 is Home to Three Super-Puff Exoplanets, Sci-News, Dec 20, 2019 by Natali Anderson

Worked examples

Example 1 — a first encounter with Kepler-51

Start with the simplest possible case. Write down what Kepler-51 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 Kepler-51 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 Kepler-51 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 Kepler-51

In research: Kepler-51 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 Kepler-51 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: Kepler-51 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Cygnus (constellation), G-type main-sequence stars, Planetary systems with four confirmed planets, so understanding it makes those chapters shorter.
In everyday life: Look for Kepler-51 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.

How to study Kepler-51 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Kepler-51 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 Kepler-51 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Kepler-51 in simple terms?

Kepler-51 is a Sun-like star that is about 500 million years old. It is orbited by four planets—Kepler-51b, c, d and e—first three of which are super-puffs and have the lowest known densities of any known exoplanet.

Why does Kepler-51 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 Kepler-51?

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 Kepler-51.