Was Our Solar System Born Tilted?

Every planet in the Solar System orbits in almost exactly the same flat plane. The one clear exception is the Sun itself, whose spin axis leans about six degrees away from that shared plane, a small but stubborn mismatch that astronomers have never been able to fully explain.

Planets form out of swirling disks of gas and dust, and in theory a young star and its own disk should spin in step. That relationship between a star’s rotation and the orbital plane of the material around it is called stellar obliquity, and it matters because hundreds of exoplanets have already been found on tilted, sometimes wildly tilted, orbits around their stars. The open question has always been whether that tilt shows up later, stirred up by gravitational fights with other planets or passing stars, or whether it was baked in from the very beginning.

Answering that question directly requires catching stars while they are still very young, before dynamics have had time to scramble anything. Earlier attempts came close but never had the numbers to be conclusive. The largest previous effort looked at only seven single Sun-like stars, and a handful of other studies used debris disks, the second-generation dust rings that appear once a system is a bit older, to check alignment in barely two dozen more.

A new assessment finally has enough infant systems to say something with confidence. Researchers combined resolved disk images from the Atacama Large Millimeter Array with rotation measurements from the TESS and K2 space telescopes to compare the tilt of 49 young, single, Sun-like stars against the outer regions of their own protoplanetary disks. The stars are between roughly one and three million years old, still deep in the process of building planets, and none of them has a known stellar companion that could complicate the picture.

About a third of these infant systems are already misaligned before their planets have even finished forming. Sixteen of the 49 systems, a rate of 33%, show a clear mismatch between the star’s spin and its disk, with an average offset of 17 degrees and a spread reaching as far as 60 degrees in a few cases. The team checked whether this tilt tracked with stellar mass, temperature, spectral type or rotation speed and found no meaningful pattern. The misalignment does not seem to single out any particular type of star.

Several forces could produce a tilt this early. Turbulent, chaotic collapse of the gas cloud that a star forms from can twist the disk by tens of degrees on its own. Late streams of gas falling in from the surrounding envelope can also torque the outer disk after the fact. And in some systems, a sufficiently massive planet can effectively cut the disk in two, letting the inner and outer regions tilt independently of each other by wide, sometimes extreme angles.

The Sun’s own six degree tilt fits neatly into that picture. The rest of the Solar System is remarkably flat by comparison. The mutual tilt among the gas and ice giants is only about 0.3 degrees, a flatness that holds all the way out to the Kuiper belt. Estimating what spectral type the young Sun would have had around five million years old places it comfortably inside the range of stars in this new sample, most of which prefer a modest tilt under about 10 to 25 degrees. In other words, the Sun’s lean looks entirely ordinary for a star of its age, not like the leftover scar of some later collision or planetary tug of war.

That does not settle the matter completely. The angle the team measured is technically a lower limit, since the full 3D geometry of each star and disk is not always known, so the true misalignment could run higher in some systems. The researchers themselves are careful to note that this primordial explanation and later dynamical disruption are not mutually exclusive, and the next generation of data, including a major release from the Gaia space observatory, should help pin down how obliquity evolves as systems age from a few million years old to the billions of years old planetary systems we can observe today.

The findings come from Lauren I. Biddle, Brendan P. Bowler, Marvin Morgan, Quang H. Tran and Ya-Lin Wu, “One-third of Sun-like stars are born with misaligned planet-forming disks,” published in Nature in 2025 (DOI: 10.1038/s41586-025-09324-0).