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Why Hot Jupiters Stay So Inflated

For more than two decades, astronomers have puzzled over a cosmic mystery:

Why are many “hot Jupiters” — giant exoplanets orbiting scorchingly close to their stars — so abnormally large?

These planets, similar in mass to Jupiter but roasting under their host stars’ intense heat, often have radii far bigger than scientists expected. Their interiors appear hotter, puffier, and more inflated than standard models predict. From the very first famous case, the planet HD 209458 b, researchers realized something wasn’t adding up. The planet was almost half Jupiter’s mass, yet significantly larger.

Since then, dozens of ground- and space-based surveys such as WASP, HATNet, KELT, NGTS, CoRoT, Kepler, K2, and TESS have discovered many similar worlds. A clear pattern emerged: the hotter the planet (specifically, planets with equilibrium temperatures above ~1000 K), the larger its radius tends to be.

This persistent trend became known as the hot Jupiter radius inflation problem. And despite years of work, the exact physical mechanism behind this inflation remained unclear.

Now, a new study by Schmidt, Thorngren, and Schlaufman offers a compelling solution — one that reshapes how scientists understand the heating and cooling of these exotic planets. Their findings point to a surprisingly powerful process occurring not deep in the planet’s core, but much closer to the surface.

A Long-Standing Puzzle in Exoplanet Science

Hot Jupiters orbit very close to their stars, often in just a few days. Because of this proximity, they receive enormous amounts of stellar radiation, which was originally expected to make them somewhat larger than Jupiter. Hot atmospheres expand, and their heat escapes more slowly.

But right from the start, scientists realized the observed radii were too big to be explained by atmospheric heating alone.

To give a sense of the scale:
Some hot Jupiters are 30–80% larger than theoretical models predicted, even billions of years after their formation. To maintain such large radii, the planets must remain unusually hot inside — far hotter than they should be if they were cooling normally over time.

Early clues and key discoveries

Over the years, several patterns helped scientists narrow down the suspects:

  • Incident flux, or the amount of stellar energy falling onto a planet, is strongly correlated with a planet’s size.

  • Mass also matters — lighter planets inflate more easily.

  • Stellar metallicity plays a small role.

  • Some planets around evolving (brightening) stars appear to reinflate, suggesting external heating affects their interiors.

  • Younger hot Jupiters seem to cool more slowly than expected, hinting at some process preventing heat from escaping.

The overall picture suggested that two things were happening:

  1. Deep heating — something was pumping energy into the deep interior, allowing the planet to reinflate as the star brightens.

  2. Delayed cooling — something else was slowing the ability of heat to escape, especially in younger systems.

These two requirements pointed toward two different kinds of heat deposition: deep and shallow.

But until now, it was unclear how both could operate together — or which one actually dominates.

Deep Heating vs. Shallow Heating: Two Competing Ideas

To solve the mystery, astronomers developed physical mechanisms to explain where and how heat might be deposited in hot Jupiters.

🟦 Deep Heating (interior heating)

This includes mechanisms that deliver energy deep into the planet, such as:

  • Tidal dissipation — internal friction caused by gravitational interactions

  • Thermal tides — atmospheric temperature differences causing deep distortions

  • Interior wave breaking

These mechanisms can heat a planet’s core and directly inflate its radius, explaining the reinflation seen in planets orbiting brightening stars.

But deep heating alone does not slow cooling enough to explain why young hot Jupiters stay large.

🟧 Shallow Heating (near the surface)

These mechanisms operate much closer to the radiative–convective boundary, such as:

  • Ohmic dissipation — atmospheric winds move charged particles through magnetic fields, generating heat

  • Longitudinal temperature advection — large-scale transport of heat by winds

  • Turbulent mixing or compositional gradients

Shallow heating naturally slows interior cooling by raising temperatures near the surface. But shallow heating cannot reinflate planets — it doesn’t deposit energy deep enough to expand the whole interior.

The tension

  • Deep heating explains reinflation, but not delayed cooling.

  • Shallow heating explains delayed cooling, but not reinflation.

Scientists needed a model that could handle both phenomena.

A New Modeling Approach: Solving Both Problems at Once

Schmidt, Thorngren, and Schlaufman developed an enhanced thermal evolution model that includes both deep heating and delayed cooling, with a new parameter that controls how efficiently heat escapes from the planet’s interior.

They applied this model to one of the most carefully assembled and homogeneous catalogs of hot Jupiter properties available. Homogeneity is essential, because variations in stellar age, mass, or radius can bias results.

Because exoplanet ages are often poorly constrained, the team used a hierarchical Bayesian framework. This approach does not rely on perfect measurements from any single system; instead, it infers population-level trends by combining information from many planets.

The key innovation was allowing interior cooling rates to vary — making it possible to test how much cooling must be slowed to match real observations.

What they found was astonishing.

A Stunning Result: Hot Jupiters Cool 95–98% More Slowly Than Models Predicted

The researchers discovered that, on average, hot Jupiters lose internal heat 95–98% more slowly than standard deep-heating models predicted.

This means:

  • The interiors are effectively insulated.

  • The outward flow of heat is drastically reduced.

  • Very little deep heating is needed to keep the planets inflated.

Simply put: something near the surface is trapping heat extremely efficiently.

This is strong evidence that substantial shallow heating is the missing ingredient in hot Jupiter models.

What about deep heating?

Interestingly, once shallow heating is included, the model requires far less deep heating to maintain the radius. That suggests that the classic deep-heating mechanisms may play only a secondary role.

Why Shallow Heating Makes Sense

The results line up well with the physics of known shallow-heating mechanisms:

✔ Ohmic dissipation

Hot Jupiters have strong winds — thousands of kilometers per hour — moving electrically charged particles across planetary magnetic fields. This generates heat just below the atmosphere.

✔ Advection and atmospheric circulation

The fierce day–night temperature contrasts drive powerful winds that move heat around the planet. This heat can get deposited just below the radiative–convective boundary.

✔ Slower cooling

Shallow heating warms the upper layers of the planet, flattening the temperature–pressure profile and reducing how efficiently heat can escape. The result is a planet that stays warm and inflated for billions of years.

✔ Phase curve predictions

Because these shallow heating processes depend on atmospheric winds and circulation, the authors predict a new observational signature:

  • Phase curve offsets (where the hottest region of the planet shifts away from the substellar point)

    • Increase for Teq < ~1500 K

    • Peak around 1500–1800 K

    • Decrease for Teq > ~1800 K

Future observations from JWST, Ariel, and other missions can test this prediction.

Why This Discovery Matters

The implications of this study are far-reaching:

1. It solves a long-standing exoplanet mystery

For years, scientists could not reconcile the need for both reinflation and delayed cooling. This model finally unifies both effects.

2. It identifies shallow heating as a dominant mechanism

Rather than relying on exotic or rare processes deep inside the planet, the model points to processes that occur naturally in hot Jupiter atmospheres.

3. It reduces the need for powerful deep interior heat sources

This simplifies the physical models and makes them more realistic given the diversity of observed systems.

4. It improves thermal evolution predictions

Accurate models are essential for understanding not just hot Jupiters, but also:

  • Warm Jupiters

  • Brown dwarfs

  • Young exoplanets

5. It provides new observational tests

Phase curve trends offer a direct way to confirm or falsify the shallow heating hypothesis.

Rethinking Hot Jupiter Models and Atmospheric Circulation

Because shallow heating appears to dominate:

  • Global circulation models (GCMs) of hot Jupiters need to be updated.

  • Atmospheric–interior coupling must be included.

  • Wind-driven heating must be modeled in detail.

  • Predictions of infrared emission, temperature maps, and atmospheric chemistry may need revision.

This will influence the interpretation of data from upcoming missions and may explain inconsistencies in earlier atmospheric measurements.

A New Era in Understanding Inflated Exoplanets

The research by Schmidt, Thorngren, and Schlaufman represents a major step forward in exoplanet science. By carefully combining precise planetary data with advanced statistical modeling, they revealed that the interiors of hot Jupiters cool far more slowly than previously thought.

The most natural explanation is that shallow heating — especially from atmospheric winds and magnetic interactions — traps heat extremely effectively, keeping these planets large long after they formed.

And importantly, this heating doesn’t need to be huge. Even modest shallow energy deposition can dramatically reduce cooling.

The final picture

  • Deep heating: supports reinflation (secondary role)

  • Shallow heating: slows cooling (dominant role)

  • Combined: fully explains radius inflation trends

This elegant solution resolves long-standing tensions between different models and sets the stage for the next generation of atmospheric studies.

Conclusion

Hot Jupiters continue to capture our imagination with their extreme temperatures, blistering winds, and inflated sizes. Thanks to the innovative work of Schmidt, Thorngren, and Schlaufman, we now understand much more about what keeps these worlds so puffy.

Their study shows that:

  • Hot Jupiters cool 95–98% more slowly than expected.

  • This slowdown is best explained by shallow heating near the radiative–convective boundary.

  • Deep heating plays a smaller role once shallow heating is accounted for.

  • Phase curve measurements can provide a strong observational test.

  • Future atmospheric models must include shallow heating to remain accurate.

As new telescopes probe these exotic worlds in greater detail, this refined understanding will help scientists decode the climates, winds, and internal structures of planets unlike anything in our own solar system.

Reference: Stephen P. Schmidt, Daniel P. Thorngren, Kevin C. Schlaufman, "Hot Jupiters are Inflated Primarily by Shallow Heating", AAS, 2025. https://arxiv.org/abs/2512.08932

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