astronomy

Sub-Earth

Sub-Earth 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 Sub-Earth rather than just read about it. In short: A sub-Earth is a planet "substantially less massive" than Earth and Venus. In the Solar System, this category includes Mercury and Mars.

Key takeaways

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

Reference excerpt

A sub-Earth is a planet "substantially less massive" than Earth and Venus. In the Solar System, this category includes Mercury and Mars.

Characteristics Sub-Earth exoplanets are among the most difficult type to detect because their small sizes and masses produce the weakest signals. Despite the difficulty, one of the first exoplanets found was a sub-Earth around a millisecond pulsar PSR B1257+12. The smallest known is WD 1145+017 b with a size of 0.15 Earth radii, or somewhat smaller than Pluto. However, WD 1145+017 b is not massive enough to qualify as a sub-Earth classical planet and is instead defined as a minor, or dwarf, planet. It is orbiting within a thick cloud of dust and gas as chunks of itself continually break off to then spiral in towards the star, and within around 5,000 years it will have more-or-less disintegrated. The Kepler space telescope opened up a new realm of sub-Earth discoveries. On January 10, 2012, Kepler discovered the first three sub-Earths around an ordinary star, Kepler-42. As of June 2014, Kepler has 45 confirmed planets that are smaller than Earth, with 17 of them being smaller than 0.8 Rⴲ. In addition, there are over 310 planet candidates with an estimated radius of <1 Rⴲ, with 135 of them being smaller than 0.8 Rⴲ. There is a confirmed sub-Earth orbiting Proxima Centauri, the closest star to the Sun. The mass of Proxima d is believed to be between that of Mars and Venus, over 0.26 Earth masses.

Deficiency Sub-Earths commonly lack substantial atmospheres because of their low gravity and weak magnetic fields, allowing stellar radiation to wear away their atmospheres. Due to their small sizes, and unless there are significant tidal forces when orbiting close to the parent star, sub-Earths also have short periods of geologic activity.

References

Illustrations

Worked examples

Example 1 — a first encounter with Sub-Earth

Start with the simplest possible case. Write down what Sub-Earth 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 Sub-Earth 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 Sub-Earth 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 Sub-Earth

In research: Sub-Earth 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 Sub-Earth 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: Sub-Earth is common in secondary-school and first-year university syllabi. It links to neighbouring topics Sub-Earth exoplanets, Terrestrial planets, Types of planet, so understanding it makes those chapters shorter.
In everyday life: Look for Sub-Earth 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 Sub-Earth in 20 minutes

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

Frequently asked questions

What is Sub-Earth in simple terms?

A sub-Earth is a planet "substantially less massive" than Earth and Venus. In the Solar System, this category includes Mercury and Mars.

Why does Sub-Earth 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 Sub-Earth?

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 Sub-Earth.