View Full Version : I have questions on exoplanets? need clarity.

2012-Jul-17, 06:24 AM
What is the minimum mass for an exoplanets or exomoons to hold and atmosphere and have plate tectonics. I read a few articals where it said it was 0.3 earth masses and was wondering if that sounded resonable. Is there any chance these small exoplanets could hold complex life?

Van Rijn
2012-Jul-17, 08:32 AM
Well, Titan has about .02 of the Earth's mass, but has a thicker atmosphere than Earth. Of course, it's cold. Temperature and time are both issues - Titan wouldn't have held onto its atmosphere this long if it was warmer. Mars (about .10 Earth mass) probably had a thick atmosphere for a billion years or so.

I'm not as familiar with the issues involved in plate tectonics, so I won't comment on that.

As for life, nobody knows. There's not enough established science to do more than guess on that.

2012-Jul-17, 09:07 AM
Beyond what we currently can tell from our own Solar System, right now no one knows for sure to a certain degree. We just haven't seen very small planets yet around other stars (or life anywhere beyond Earth) but a lot of curious details about exoplanets has called things about planetary formation into question.

Plate tectonics is a curious one, for example, because you need a molten core in the center for it to happen which is why Earth has it but Mars (whose core cooled down long ago due to its smaller mass) hasn't had tectonic activity after its first few hundred million years of formation it's believed- that's why Olympus Mons got so big, the volcanoes there just plain never moved. Thing though is Venus doesn't have a magnetic field which is similar to Earth's size but tiny Mercury and Ganymede (Jupiter's moon) do and even then you still have moons like Io with constant volcanism because of its proximity to Jupiter, so yeah knowing whether plate tectonics can and will happen just isn't that simple!

2012-Jul-17, 10:03 PM
No simple answer to this, as has already been said. A low-mass body can hold on to a very significant atmosphere if it's cold, and if the atmospheric gases themselves are massive (carbon dioxide or nitrogen being easier to hold on to than hydrogen or helium). Venus's atmosphere is much denser than Earth's, but if you moved it out to Neptune's orbit, the remaining gaseous envelope wouldn't be very dense at all. Plate tectonics (or let's just broaden it to say convective tectonic activity regardless of the existence of plates, or even silicates), as mentioned, can be influenced by factors other than mass under special conditions, such as by composition or tidal forces. Note than an Earth-mass planet with say, a third as much metals might not have any plate tectonics at all; tectonics on Earth are also heavily influenced by water, both on the surface and in the minerals themselves.