astronomy

KELT-9b

KELT-9b 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 KELT-9b rather than just read about it. In short: KELT-9b, also designated HD 195689 b, is an exoplanet and ultra-hot Jupiter that orbits the late B-type/early A-type star KELT-9, located about 670 light-years from Earth. Detected using the Kilodegree Extremely Little Telescope, the discovery of KELT-9b was announced in 2016.

Key takeaways

KELT-9b belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect KELT-9b to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of KELT-9b from memory before moving on to harder problems.

Reference excerpt

KELT-9b, also designated HD 195689 b, is an exoplanet and ultra-hot Jupiter that orbits the late B-type/early A-type star KELT-9, located about 670 light-years from Earth. Detected using the Kilodegree Extremely Little Telescope, the discovery of KELT-9b was announced in 2016. As of June 2017, it is the hottest known exoplanet.

Host star The host star, KELT-9, also designated HD 195689, is 2.3 times larger and 2.4 times more massive than the sun. The surface temperature is 10,170 K (9,897 °C; 17,846 °F), unusually hot for a star with a transiting planet. Prior to the discovery of KELT-9b, only six A-type stars were known to have planets, of which the warmest, WASP-33, is significantly cooler at 7,430 K (7,157 °C; 12,914 °F); no B-type stars were previously known to host planets. KELT-9, classified as B9.5-A0 could be the first B-type star known to have a planet. KELT-9b occupies a circular but strongly inclined orbit a mere 0.03462 AU from KELT-9 with an orbital period of less than 1.5 days.

Physical properties

KELT-9b is a relatively large giant planet at about 2.2 times the mass of Jupiter; however given that its radius is nearly twice that of Jupiter, its density is less than half that of it. Like many hot Jupiters, KELT-9b is tidally locked with its host star. The outer boundary of its atmosphere nearly reaches its Roche lobe, implying that the planet is experiencing rapid atmospheric escape driven by the extreme amount of radiation it receives from its host star. In 2020, atmospheric loss rate was measured to be equal to 18 - 68 Earth masses per billion years. The planet's elemental abundances remain largely unknown as of 2022, but a low carbon-to-oxygen ratio is strongly suspected.

As of 2022, KELT-9b is the hottest known exoplanet, with dayside temperatures approaching 4,600 K (4,327 °C; 7,820 °F) — warmer than some K-type stars. Molecules on the day side are broken into their component atoms, so that normally sequestered refractory elements can exist as atomic species, including neutral oxygen, neutral and singly ionized atomic iron (Fe and Fe+) and singly ionized titanium (Ti+), only to temporarily reform once they reach the cooler night side, which is indirectly confirmed by measured enhanced heat transfer efficiency of 0.3 between dayside and nightside, likely driven by the latent heat of dissociation and recombination of the molecular hydrogen. Surprisingly, spectra taken in 2021 have unambiguously indicated a presence of metal oxides and hydrides in the planetary atmosphere, although higher resolution spectra taken in 2021 have not found any molecular emissions from the planetary dayside. The thermosphere layer of KELT-9b is expected to heat up to 10,000–11,000 K (9,727–10,727 °C; 17,540–19,340 °F), driven by ionization of heavy metals atoms like iron. In June 2026 scientists found a self-absorption phenomena of Hα in the upper atmosphere KELT-9b. Due to this, it is suspected that the planet is losing 1013 grams per second and has a vertical outflow wind speed of ~5 km/s.

See also 51 Pegasi b, the first-discovered Hot Jupiter List of hottest exoplanets

References

External links

HD 195689 SIMBAD entry for HD 195689

Illustrations

Worked examples

Example 1 — a first encounter with KELT-9b

Start with the simplest possible case. Write down what KELT-9b 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 KELT-9b 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 KELT-9b 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 KELT-9b

In research: KELT-9b 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 KELT-9b 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: KELT-9b is common in secondary-school and first-year university syllabi. It links to neighbouring topics Cygnus (constellation), Exoplanets discovered by KELT, Exoplanets discovered in 2016, so understanding it makes those chapters shorter.
In everyday life: Look for KELT-9b 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 KELT-9b in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what KELT-9b 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 KELT-9b out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is KELT-9b in simple terms?

Why does KELT-9b 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 KELT-9b?

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 KELT-9b.