astronomy

K2-21

K2-21 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 K2-21 rather than just read about it. In short: K2-21, also known as EPIC 206011691, is a red dwarf star located 273 light-years (84 parsecs) away in the constellation Aquarius. It hosts two known exoplanets, discovered in 2015 by the transit method as part of Kepler's K2 mission.

Key takeaways

K2-21 belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect K2-21 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of K2-21 from memory before moving on to harder problems.

Reference excerpt

K2-21, also known as EPIC 206011691, is a red dwarf star located 273 light-years (84 parsecs) away in the constellation Aquarius. It hosts two known exoplanets, discovered in 2015 by the transit method as part of Kepler's K2 mission. Both planets have significantly lower densities than Earth, indicating that they are not rocky planets and are better described as mini-Neptunes. The inner planet, K2-21b, is less dense than the outer planet, K2-21c.

References

Worked examples

Example 1 — a first encounter with K2-21

Start with the simplest possible case. Write down what K2-21 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 K2-21 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 K2-21 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 K2-21

In research: K2-21 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 K2-21 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: K2-21 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Aquarius (constellation), M-type main-sequence stars, Planetary systems with two confirmed planets, so understanding it makes those chapters shorter.
In everyday life: Look for K2-21 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.

Ask Teacher Smith questions about this articleOpens your AI tutor with a question about “K2-21” →

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study K2-21 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what K2-21 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 K2-21 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is K2-21 in simple terms?

Why does K2-21 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 K2-21?

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 K2-21.