1. Tidal Constraints on Planetary Habitability
Rory Barnes
U. of Washington
Virtual Planetary Lab
Sean N. Raymond, Brian Jackson, Richard Greenberg

2. Distance from Star (AU)
Sun
From Selsis
et al. (2007)
Mass of Star (M)
GJ 581
Likely habitable
Possibly habitable
Distance from Star (AU)

3. Distance from Star (AU)
Tides
Important
Sun
Mass of Star (M)
GJ 581
Distance from Star (AU)

4. Distance from Star (AU)
Tidal Locking ≠
Synchronous
Rotation!
Sun
Porb
Mass of Star (M)
Prot ∞
1 + e2
GJ 581
obliquity 0
Distance from Star (AU)

5. Distance from Star (AU)
Sun
Porb = 66 d
e = 0.38
Prot = 28 – 39 d
Mass of Star (M)
GJ 581 ?
High e => more habitable
Distance from Star (AU)

6. GJ 581 No Clouds Total cloud coverage
Williams & Pollard (2002) + Selsis et al. (2007)
GJ 581
No Clouds
Total cloud
coverage
Barnes et al. (2008)

7. But tides circularize orbits…
semi-
major
axis HZ
Orbit
0.2 M

8. But tides circularize orbits…

9. Orbital Decay -> Tidal Heating
Io has ~2 W/m2
heat flux
Such heating could
be bad for life

10. But Some Heating is Good…
Plate tectonics
requires
~40 mW/m2
(Williams et al. 1997)
Prevents
runaway
greenhouse

11. If heating > 2 W/m2
=> uninhabitable
If 0.04 < heating < 2 W/m2
=> habitable
If heating < 0.04 W/m2
=> uninhabitable
(need radiogenic sources)

12. “Tidal Habitable Zone”

13. d c e

14. Super-
Earth?
“Super-
Ios”

15. or
Super-
Mars?
“Super-
Ios”

17. e = 0.01
e = 0.5
10 M

20. Summary Tidal locking ≠ Synchronous rotation Rotation rates are faster
Obliquities 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

22. ec = 0
ec = 0.01