The tilt of a planet ’ s spin axis off its orbital axis ( “ obliquity ” ) is a basic physical characteristic that plays a central role in determining the planet ’ s global circulation and energy redistribution .
Moreover , recent studies have also highlighted the importance of obliquities in sculpting not only the physical features of exoplanets but also their orbital architectures .
It is therefore of key importance to identify and characterize the dominant processes of excitation of non-zero axial tilts .
Here we highlight a simple mechanism that operates early on and is likely fundamental for many extrasolar planets and perhaps even Solar System planets .
While planets are still forming in the protoplanetary disk , the gravitational potential of the disk induces nodal recession of the orbits .
The frequency of this recession decreases as the disk dissipates , and when it crosses the frequency of a planet ’ s spin axis precession , large planetary obliquities may be excited through capture into a secular spin-orbit resonance .
We study the conditions for encountering this resonance and calculate the resulting obliquity excitation over a wide range of parameter space .
Planets with semi-major axes in the range 0.3 \mathrm { AU } \lesssim a \lesssim 2 \mathrm { AU } are the most readily affected , but large- a planets can also be impacted .
We present a case study of Uranus and Neptune and show that this mechanism likely can not help explain their high obliquities .
While it could have played a role if finely tuned and envisioned to operate in isolation , large-scale obliquity excitation was likely inhibited by gravitational planet-planet perturbations .