Tidal Constraints on Planetary Habitability

Rory BarnesU. of Washington

Virtual Planetary Lab

Sean N. Raymond, Brian Jackson, Richard Greenberg

Distance from Star (AU)

Mas

s of S

tar (

M)

GJ 581

Sun

Likely habitablePossibly habitable

From Selsis et al. (2007)

Distance from Star (AU)

Mas

s of S

tar (

M)

GJ 581

SunTides

Important

Distance from Star (AU)

Mas

s of S

tar (

M)

GJ 581

Sun

Tidal Locking

Prot ∞ Porb

1 + e2

≠Synchronous

Rotation!

obliquity 0

Distance from Star (AU)

Mas

s of S

tar (

M)

GJ 581

Sun

Porb = 66 de = 0.38Prot = 28 – 39 d

High e => more habitable?

No CloudsTotal cloud

coverage

Williams & Pollard (2002) + Selsis et al. (2007)

Barnes et al. (2008)

GJ 581

But tides circularize orbits…

HZ

0.2 M

semi-majoraxis

Orbit

But tides circularize orbits…

Orbital Decay -> Tidal Heating

Io has ~2 W/m2

heat flux

Such heating could be bad for life

But Some Heating is Good…

Plate tectonicsrequires

~40 mW/m2

(Williams et al. 1997)

Preventsrunaway

greenhouse

If heating > 2 W/m2

=> uninhabitable

If 0.04 < heating < 2 W/m2

=> habitable

If heating < 0.04 W/m2

=> uninhabitable(need radiogenic sources)

“Tidal Habitable Zone”

d

ce

“Super-Ios”

Super-Earth?

“Super-Ios”

Super-Mars?

or

10 M

e =

0.0

1

e =

0.5

SummaryTidal locking ≠ Synchronous rotation Rotation rates are fasterObliquities go to zero Tides may remove planets from HZ Tidal heating may be severe (super-Io) But tidal heating may drive plate tec. “Tidal Habitable Zone” First-detected rocky planets in HZ will be affected by tides

ec = 0 ec = 0.01