When Planets Go Their Own Way: A Stellar Ejection Explains a Misaligned Planetary System

The young star system IRAS04125+2902 (IRAS04125) presents astronomers with a curious puzzle: it contains a baby planet that appears to orbit at a very different angle compared to the dusty disc surrounding its host star. Even more confusingly, this star is part of a wide binary system, where two stars orbit each other, and both the planet and the companion star share an orbital orientation that’s tilted by around 60° from the disc. In a new paper, Rebecca Nealon and colleagues explore whether this surprising misalignment might have resulted from a dramatic stellar encounter: the chaotic breakup of a triple-star system.

Understanding the Mystery

The IRAS04125 system resides in the Taurus star-forming region and is around 3 million years old. Recent observations detected a planet just slightly larger than Neptune, orbiting the primary star every 8.83 days. This makes it the youngest transiting planet ever found. However, the biggest surprise is geometric: the planet and binary orbit edge-on, while the disc tilts about 60° away. How could such an arrangement arise when young stars and their planets typically form in the same disc and thus share the same orientation?

The authors consider whether IRAS04125 might have once been a triple system. In star-forming regions like Taurus, triple stars often interact and eject one member. If this ejection was forceful and happened close enough to the disc, it might have twisted the disc and shifted the orbits of the remaining stars and planet, leaving behind the system we observe today.

A Simplified Model of Chaos

Because it's nearly impossible to model the full complexity of three stars interacting with a disc, the team used a simpler setup. They simulated a “flyby” star — representing the ejected third member — passing by a star-disc-binary system. By changing the flyby star’s mass, path, and timing, they tested whether this kind of encounter could recreate the observed tilts. The simulations used a 3D code called Phantom to model how the gas in the disc would react to the encounter.

They found that if the flyby star was about 0.35 times the mass of the Sun and passed closely in a retrograde (backward) direction, it could indeed torque the disc enough to make it tilt significantly — over 60°, in some cases. Meanwhile, the orbits of the binary star and the planet also tilted, but differently. Interestingly, the binary orbit became more stretched (eccentric), which is something future observations could look for as a test of the model.

Reproducing the Disc-Planet Tilt

To further test their scenario, the authors added a planet to some of their simulations, placing it at either its observed location (0.074 au) or at 20 au (the disc’s inner edge). In both cases, the planet survived the flyby and ended up orbiting at a very different angle compared to the disc — up to 124° misaligned. Even when starting in the same plane as the disc, the planet’s orbit shifted because of the overall disturbance from the flyby. This outcome supports the idea that a stellar ejection event could explain the planet-disc misalignment.

Was the Flyby Likely?

The authors then asked how likely such an event might be. Using results from a separate simulation of the Taurus region’s star dynamics, they found that systems like IRAS04125 could plausibly experience this kind of triple-star breakup. In fact, the flyby star’s mass and timing in their best-fit model matched well with what those larger simulations predicted for real ejections. This adds weight to the idea that IRAS04125 is a fossil record of a violent stellar past.

Could Other Forces Be at Work?

One alternative explanation involves von Zeipel–Kozai–Lidov (vZKL) oscillations. In triple systems, these gravitational effects can swap a disc's tilt and shape over time, possibly leading to misalignments like the one observed. However, for this to work, the gas disc in IRAS04125 would need to stretch out to over 230 au — much larger than the 60 au seen in dust. Since gas often spreads farther than dust, this remains a possibility but depends on future observations.

Which Scenario Wins?

In the end, Nealon and collaborators suggest that the most likely explanation is the ejection of a third star early in the system’s history. This chaotic event would have reshaped the disc and disturbed the orbits of the remaining stars and planet. If the planet is later found to orbit in the same plane as the binary, however, this would challenge the flyby explanation and suggest a different, still-unknown mechanism at work. For now, IRAS04125 serves as a reminder that planetary systems can form in messy, violent environments — and sometimes, planets end up going their own way.

Source: Nealon