Exoplanet Magnetopause Standoff Distance

Star & Orbit

Orbital distance 1.00 AU

Stellar wind speed 400 km/s

Stellar activity (density ×) 1.0×

Planet

Magnetic moment 1.00 M⊕

Readouts

Magnetosphere shields the atmosphere.

Standoff distance

10.5 R_p

Dynamic pressure

1.8 nPa

Wind density

5.0 cm⁻³

Surface field B_p

31.2 µT

How it works

A magnetized planet carves a cavity — the magnetosphere — out of the oncoming stellar wind. The boundary (magnetopause) sits where the wind's ram pressure exactly balances the planet's dipole magnetic pressure. This is the Chapman-Ferraro pressure balance:

P_dyn = n m_p v²                (stellar wind ram pressure)
P_B   = B(r)² / (2 μ₀),  B(r) = B_p (R_p/r)³   (dipole field, falls as r⁻³)

Setting f·P_B = P_dyn and solving for r gives the standoff distance:
R_mp / R_p = [ f B_p² / (2 μ₀ n m_p v²) ]^(1/6)

with f ≈ 2 (the compression/form factor for a confined dipole). Because the exponent is 1/6, the standoff distance is very insensitive to the wind — but a close-in orbit (higher n ∝ 1/d²) or a weak intrinsic field can still collapse it onto the planet.

Orbital distance — wind density scales as 1/d² from the star, so hot Jupiters and close-in rocky worlds face a far denser wind than Earth does.
Magnetic moment — sets the dipole field strength at the surface; a planet with a weak or absent dynamo (e.g. tidally locked, or slowly rotating) has little to balance the wind with.
Stellar activity — young and active stars drive winds tens to hundreds of times denser and faster than the present-day Sun.

When R_mp collapses to within a couple of planetary radii, the magnetopause sits inside the upper atmosphere (the exobase), exposing it directly to stellar-wind sputtering and ion pickup — a leading candidate for why close-in exoplanets with weak fields lose their atmospheres over geological time.

Dipole field lines Stellar wind Magnetopause

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