astronomy

Ultra-short period planet

Ultra-short period planet 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 Ultra-short period planet rather than just read about it. In short: An ultra-short period (USP) planet is a type of exoplanet with an orbital period of less than one Earth day (24 hours). At this short distance, tidal interactions lead to relatively rapid orbital and spin evolution.

Key takeaways

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

Reference excerpt

An ultra-short period (USP) planet is a type of exoplanet with an orbital period of less than one Earth day (24 hours). At this short distance, tidal interactions lead to relatively rapid orbital and spin evolution. Therefore when there is a USP planet around a mature main-sequence star, it is most likely that the planet has a circular orbit and is tidally locked. There are not many USP planets with sizes exceeding 2 Earth radii. There is a strong negative correlation between their occurrence rate and the mass of the host star. About one out of 200 Sun-like stars (G dwarfs) has an ultra-short-period planet, and their occurrence rate falls from 1.1%±0.4% for M dwarfs to 0.15%±0.05% for F dwarfs. Ultra-short-period planets also tend to orbit older stars. USP planets seem mostly consistent with an Earth-like composition of 70% rock and 30% iron, but K2-229b has a higher density, suggesting a more massive iron core. WASP-47e and 55 Cnc e, however, have a lower density and are compatible with pure rock, or a rocky-iron body surrounded by a layer of water (or other volatiles). A difference between hot Jupiters and terrestrial USP planets is the proximity of planetary companions. Hot Jupiters are rarely found with other planets within a factor of 2–3 in orbital period or distance. In contrast, terrestrial USP planets almost always have longer-period planetary companions. The period ratio between adjacent planets tends to be larger if one of them is a USP planet suggesting the USP planet has undergone tidal orbital decay which may still be ongoing. USP planets also tend to have higher mutual inclinations with adjacent planets than for pairs of planets in wider orbits, suggesting that USP planets have experienced inclination excitation in addition to orbital decay. There are several known giant planets with an orbital period shorter than one day. Their occurrence must be lower by at least an order of magnitude than that of terrestrial USP planets. It had been proposed that USP planets were the rocky cores of evaporated hot Jupiters, however, the metallicity of the host stars of USP planets is lower than that of hot Jupiters' stars, so it seems more likely that USP planets are the cores of evaporated gas dwarfs. Other theories suggest that USP planets form as terrestrial planets and migrate inward over billions of years due to dynamical interactions with nearby planetary companions. A study by the TESS-Keck Survey using 17 USP planets found that USP planets predominantly have an Earth-like compositions with iron core mass of about 32% and have masses below runaway accretion. USP planets are also almost always found in multiple-planet systems around stars with solar metallicity. As of 2025, 12 known Jovian USP planets have been found.

Examples

Notes

References

Illustrations

Worked examples

Example 1 — a first encounter with Ultra-short period planet

Start with the simplest possible case. Write down what Ultra-short period planet 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 Ultra-short period planet 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 Ultra-short period planet 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 Ultra-short period planet

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

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

Frequently asked questions

What is Ultra-short period planet in simple terms?

Why does Ultra-short period planet 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 Ultra-short period planet?

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 Ultra-short period planet.