How warm Jupiters broke the rules of planet formation

Warm Jupiters – gas giants parked closer to their stars than Jupiter is to the Sun – keep turning up with orbits that are stretched but strangely well-behaved. Many of these planets move in long ovals yet still trace paths neatly aligned with their stars.

A growing set of measurements shows this pattern is real, not a fluke. The signal, seen across systems hundreds of light-years away, is forcing theorists to rethink how giant planets form and move. That’s where Diego Muñoz comes in.

Muñoz, an assistant professor of astronomy at Northern Arizona University (NAU), is leading a three-year project to test new explanations with advanced computer models.

The goal of the research project is to find out why these eccentric giants behave with such surprising order.

Consistent geometry of warm Jupiters

A 2024 analysis found that single-star systems hosting warm Jupiters usually stay aligned, even when their host stars are hot.

One striking example is TOI-2005 b, a warm Jupiter on a roughly 17-day orbit with a high eccentricity – a measure of how stretched an orbit is. The planet’s path still sits nearly on its star’s equatorial plane.

Another recent case is TOI-4127 b, which travels on an orbit with eccentricity near 0.75 yet remains closely aligned with its host star’s spin. That pairing – wild shape plus tidy alignment – is hard to square with standard migration stories.

How astronomers measure alignment

The angle between a star’s spin and a planet’s orbital plane is called obliquity – the tilt between the stellar equator and the planet’s path. Astronomers often use subtle line shifts during a transit to read this angle.

For hot Jupiters, the record looks different. Observations a decade ago found that hot stars partnered with hot Jupiters often show large tilts.

Cooler stars, by contrast, tend to be aligned – a pattern that helped launch today’s debates.

Three theories test planetary order

One idea is a hidden neighbor. A second planet can pump up eccentricity through long-term gravitational nudges without necessarily twisting the orbital plane. That route can keep alignment intact while making the orbit oval shaped.

A second idea points to the birth environment. Protoplanetary disk, the gas and dust ring where planets coalesce, can interact with young planets and steer them in ways that reduce tilts while reshaping their orbits.

A third idea moves inside the star. Some hot stars support internal gravity waves – ripples that carry angular momentum and can sap orbital energy over time.

These waves may also reorient stellar spins, a proposed mechanism that could explain close alignment in certain systems.

Muñoz is especially interested in whether stellar physics can realign systems without erasing their orbital stretch. “The data tells us that warm Jupiters are not just the tail end of hot Jupiters,” he said.

What counts as a warm Jupiter

A warm Jupiter – a gas giant with an orbital period of about 10 to 200 days – sits closer to its star than Jupiter does but not as close as a typical hot Jupiter.

That middle distance weakens tidal effects that can otherwise force alignment.

Because tides do less work at these distances, many warm Jupiters should preserve their primordial obliquity – the original tilt set early in a system’s life. That is why their consistent alignment is so surprising.

Hot Jupiters look different

Hot Jupiters sit tight to their stars where tides can be ferocious. Over time, those tides can scramble or realign spins and orbits.

The result is a mix of aligned, misaligned, and even retrograde paths – a trend first mapped by earlier evidence and later studies.

Warm Jupiters avoid the strongest tidal torques, so their tidy alignment hints at gentler migration or a stabilizing influence that preserves the original geometry. That contrast is a key clue.

Companion planets drives eccentric paths

NASA’s Transiting Exoplanet Survey Satellite (TESS) scans nearly the entire sky for tiny dips in starlight when planets pass in front of their stars, steadily adding new warm-Jupiter candidates.

These targets give follow-up teams the chance to measure obliquities with precision spectrographs. They also allow scientists to track orbital changes over time and search for companion planets that might be shaping each system’s dynamics.

The goal is to find what drives these stretched orbits. If most eccentric warm Jupiters also host unseen companions orbiting in the same plane, that would favor a companion-driven scenario.

If many stars instead show strong internal gravity waves alongside aligned planets, stellar physics may be the dominant force.

And if neither pattern fits, interactions between planets and the leftover gas and dust of their birth disks could hold the key. Each pathway makes distinct predictions that future models and measurements can test.

Tilts reveal planet origins

Determining whether alignment stays intact or gets rebuilt reveals when a planetary system underwent its major sculpting.

When alignment stays intact, the protoplanetary phase shapes the system’s structure, and later events play only a minor role.

If the system rebuilds, late interactions, tidal forces, or stellar physics reshape it long after the gas dissipates. That distinction matters when comparing our own solar system to the broader population.

Researchers are now expanding the sample of warm Jupiters with measured tilts, and new observations continue to reinforce the alignment trend. Recent studies suggest this pattern may be common.

One analysis covered eight systems with aligned warm gas giants, while a broader survey co-authored by Muñoz reached similar conclusions.

As the catalog grows, the focus will shift from early hints to rigorous tests, and the leading explanation will be the one that accounts for both the stretched orbits and the tight alignment across many stars.

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