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I'm curious how many Earth-sized planets have a twin planet like our moon. Without the moon we'd be tidally locked, and despite all our water we wouldn't be nea

I'm curious how many Earth-sized planets have a twin planet like our moon. Without the moon we'd be tidally locked, and despite all our water we wouldn't be nearly as life-friendly.

ceejayoz 14y ago
Mercury and Venus aren't tidally locked.
- CapitalistCartr 14y ago
  They're quite close to it. Mercury takes two of its own years to have one day, and Venus's year is less than two of its days.
  - nfg 14y ago
    This is generally suspected to be because of large primordial impacts, much like the one which it is proposed created our moon. I don't think there's anything intrinsic about Goldilocks zone orbiting rocky planets which would lend to tidal-locking with the star.
    - mullingitover 14y ago
      It's not their distance from the sun, it's the presence of liquid being affected by gravity and creating a tidal bulge. The bulge slows down the planet's rotation via friction, and eventually the planet stops rotating.
- JoeAltmaier 14y ago
  Mercury is in a more elliptical orbit; its 'tidally locked' at the perihelion but when its further away it gets a half-turn in. So each side spends half a year baked, half a year frozen.
SoftwareMaven 14y ago
Having Jupiter nearby to suck up a lot of stray flotsam and jetsam is quite helpful, too. Much harder to make life while under a constant rain of interplanetary debris.
BerislavLopac 14y ago
And also how many of these are in the Goldilocks zone.
andrewfelix 14y ago
I'd be curious too. For me the Fermi paradox is best satisfied by the 'Rare Earth' hypothesis. There are just too many infinitely rare combinations of variables required to support the kind of life we see on our planet. I don't hold out any hope of finding much beyond the odd microbe.
outworlder 14y ago
Is having a large moon a requirement for not being tidally locked? My understanding was that planets further from the star would take more time to get tidally locked. Since we are quite far from the sun, it would take some time even without a moon.
- Udo 14y ago
  You're correct, it isn't. Planets in very close orbits tend to be tidally locked. Instead the significance of the moon is thought to be as a stabilizer for the Earth's rotational axis. However, our axis is not exactly as ultra stable as some people think - it's still shifting up to a few degrees every few million years. It's also worth noting that six out of nine planets in this system are oriented in the same way as Earth and none of them has the same moon configuration as us. For example, Mars has pretty much the same tilt as us and its moons are pitiful. I personally don't believe it's a requirement for life. We simply lack the empirical data needed to make any assumptions about habitability or even the basic configuration of habitable star systems. Until then, looking towards twin Earths is probably a good start to look for life but it's certainly not the only kind of environment stable enough to support it.

I'm curious how many Earth-sized planets have a twin planet like our moon. Without the moon we'd be tidally locked, and despite all our water we wouldn't be nearly as life-friendly.

ceejayoz 14y ago

Mercury and Venus aren't tidally locked.

CapitalistCartr 14y ago
They're quite close to it. Mercury takes two of its own years to have one day, and Venus's year is less than two of its days.
- nfg 14y ago
  This is generally suspected to be because of large primordial impacts, much like the one which it is proposed created our moon. I don't think there's anything intrinsic about Goldilocks zone orbiting rocky planets which would lend to tidal-locking with the star.
  - mullingitover 14y ago
    It's not their distance from the sun, it's the presence of liquid being affected by gravity and creating a tidal bulge. The bulge slows down the planet's rotation via friction, and eventually the planet stops rotating.
JoeAltmaier 14y ago
Mercury is in a more elliptical orbit; its 'tidally locked' at the perihelion but when its further away it gets a half-turn in. So each side spends half a year baked, half a year frozen.

SoftwareMaven 14y ago

Having Jupiter nearby to suck up a lot of stray flotsam and jetsam is quite helpful, too. Much harder to make life while under a constant rain of interplanetary debris.

BerislavLopac 14y ago

And also how many of these are in the Goldilocks zone.

andrewfelix 14y ago

I'd be curious too. For me the Fermi paradox is best satisfied by the 'Rare Earth' hypothesis. There are just too many infinitely rare combinations of variables required to support the kind of life we see on our planet. I don't hold out any hope of finding much beyond the odd microbe.

outworlder 14y ago

Is having a large moon a requirement for not being tidally locked? My understanding was that planets further from the star would take more time to get tidally locked. Since we are quite far from the sun, it would take some time even without a moon.

Udo 14y ago
You're correct, it isn't. Planets in very close orbits tend to be tidally locked. Instead the significance of the moon is thought to be as a stabilizer for the Earth's rotational axis. However, our axis is not exactly as ultra stable as some people think - it's still shifting up to a few degrees every few million years. It's also worth noting that six out of nine planets in this system are oriented in the same way as Earth and none of them has the same moon configuration as us. For example, Mars has pretty much the same tilt as us and its moons are pitiful. I personally don't believe it's a requirement for life. We simply lack the empirical data needed to make any assumptions about habitability or even the basic configuration of habitable star systems. Until then, looking towards twin Earths is probably a good start to look for life but it's certainly not the only kind of environment stable enough to support it.