A Radially Resolved Magnetic Field Threading the Disk of TW Hya
In one sentence
For the first time, we measured how the magnetic field changes with distance across a real planet-forming disk — TW Hya, the closest one to us — and found a field of roughly 10 milligauss that flips from vertical to in-plane right where the disk has a known gap.
What’s the question?
Stars and planets form inside flat, rotating disks of gas and dust. Theory has long insisted that magnetic fields are central to this story: they help disk material drift inward, drive winds, and influence where planets can grow. But measuring the magnetic field inside a real disk has been almost out of reach. The signals are faint, and we need a tracer whose response to magnetism we actually understand.
What did we do?
Working with TW Hya — the closest, best-studied protoplanetary disk to us — we used very sensitive ALMA observations of CN emission lines and exploited a more subtle effect than polarization: the Zeeman broadening of the unpolarized line itself. A magnetic field splits the molecular energy levels into sub-levels at slightly different frequencies; we don’t see the splitting directly, but we do see the line getting a bit broader than it otherwise should. Modelling that excess width, while accounting for the disk’s velocity field and the non-LTE excitation of CN, gives us the field strength as a function of distance.
The result: a magnetic field of roughly 10 mG between 60 and 120 au, predominantly poloidal (i.e. vertical) inside a known gap at 82 au, and reorienting into the disk plane outside it. The implied gas densities and plasma-β values are exactly in the regime where magnetic forces matter for disk evolution.
Why does it matter?
A field that varies smoothly with radius — and changes orientation at a real structural feature — is not just a number. It is a direct fingerprint of magnetism shaping where gas goes, where it accumulates, and where planets can grow. We are moving from “magnetic fields exist in disks” to “we can measure them and read off their geometry.”
My role
I developed this novel Zeeman-broadening method together with Richard Teague, and contributed the modelling that connects observed CN line widths to magnetic-field strengths — including the radiative-transfer and quantum-mechanical Zeeman framework needed to interpret the data quantitatively.
In the press
- NRAO press release — Astronomers Reveal Planet Building's Secret Ingredient: Magnetism