Earth-like moons could (okay, do) exist around large planets in other solar systems

Sunday, December 14, 2008

Given the number of stars in the universe there's no way that there aren't a huge number of Earth-like moons orbiting large planets here and there in the universe; we just haven't found them yet. There are so many factors in habitability that it's almost impossible to come up with a clear standard for which places are habitable and which aren't. For example:

  • With the presence of extremophiles, habitability is a pretty hard thing to judge. There are creatures on Earth that live near volcanic vents deep in the ocean, and if there was to be an entire planet with that sort of environment we would probably judge it to be unsuitable for life. The same goes with extremely cold environments, extremely acidic ones, etc. However, since planets change their environments over time, it's probably safe to assume that a planet hot enough to be on the edge of habitability even for extremophiles on Earth would probably surpass even that level from time to time due to long-term climate change, and the opposite would be true of extremely cold planets. Earth is in a nice middle range where changes in climate have no effect on the possibility of life itself. So the general rule would probably be that we shouldn't cancel out planets with somewhat extreme climates as possible sources for life, but the chance will probably be lower.
  • Planets even outside the so-called "habitable zone" could still be warm enough to support life depending on their atmosphere, and even rogue planets that have been thrown out into the reaches of space could retain a warm climate with a thick enough atmosphere thanks to internal heating (for a few billion years).
  • Moons orbiting planets outside a star's habitability zone could also have a fair amount of internal heat thanks to the gravity of the planet it orbits.
The reason I'm writing this is because there was an article on on the same subject, that of Earth-like moons orbiting large planets in other solar systems. A moon here doesn't really mean anything except "a body orbiting a planet" so these bodies could (and will be) easily be larger than the Earth. In addition to observing the wobble given off by stars as their planets orbit them, we can also detect these moons orbiting their planets by their wobbles too.
David Kipping, whose work is funded by the UK's Science and Technology Facilities Council (STFC), has found that such moons can be revealed by looking at wobbles in the velocity of the planets they orbit. His calculations, which appear in the Monthly Notices of the Royal Astronomical Society today (11th December), not only allow us to confirm if a planet has a satellite but to calculate its mass and distance from its host planet – factors that determine the likely habitability of a moon.

Out of the 300+ exoplanets (planets outside our Solar System) currently known, almost 30 are in the habitable zone of their host star but all of these planets are uninhabitable gas giants. The search for moons in orbit around these planets is important in our search for alien life as they too will be in the habitable zone but are more likely to be rocky and Earth-like, with the potential to harbour life.
So there it is, already 30 good targets to take a good look at to see if they have any Earth-sized moons. What the article doesn't mention though is that there is another plus to observing these planets to find Earth-like moons: their much smaller orbits. Wikipedia gives the following on astrometry:
One potential advantage of the astrometric method is that it is most sensitive to planets with large orbits. This makes it complementary to other methods that are most sensitive to planets with small orbits. However, very long observation times will be required — years, and possibly decades, as planets far enough from their star to allow detection via astrometry also take a long time to complete an orbit.
Europa, for example, orbits Jupiter once every 3.5 days. Io orbits every 1.7 days, and even Callisto only takes 16 days for a complete orbit. Saturn's moon Titan also takes 15 days.


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