Mar 282012

At first…

New ‘life in space’ hope after billions of ‘habitable planets’ found in Milky Way

But then…

A total of nine super-Earths – planets with masses between one and 10 times that of Earth – were found. Two were located within the habitable zones of the stars Gliese 581 and Gliese 667 C.

First off, a “super Earth” is not necessarily a “habitable planet.” In fact… almost certainly *not.* Second, that’s kind of a small sample size…

Still, it’s looking more and more like this might be a crowded galaxy.

 Posted by at 7:21 pm
  • Peter Hanely

    As I understand the methods used to find extra-solar planets, they have a strong bias towards big planets in orbits close to their star. We might estimate a greater number of smaller planets exist, and distributed farther out on average.

  • Any large moons around such super-Earths might be good targets for terraforming. If no moons are present, or the moons are too small, we can always arrange to construct some via geo-braking of any sufficiently massive asteroidal material in the neighborhood:-). Two planets for the price of one!

  • Seems to me that any intelligent race that evolved on a “super-Earth” would be pretty much stuck there. The gravity well would be too deep for chemical-propulsion rockets to escape.

    Could this be part of the reason for the Fermi Paradox? Planets more massive than Earth are gravitational roach motels, while planets less massive that Earth lose their atmospheres?

    Or perhaps we’ve been surveyed but not visited by extraterrestrials from less massive planets who consider our world undesirable real estate because we’re not worth the energy expense.

    • Mass is only a part of it. If the radius of the planet were also larger than Earth then the gravitational pull could be equal to that of Earth.

      You can have a planet with a larger mass than Earth and a larger radius and have a surface gravity equal to Earth.

      I remember doing the calculations several semesters ago but don’t have the paperwork handy. But you can find what Radius and Mass are required using the formula:


      where G equals 6.673×10^-11 (the Gravitational Constant) and m is mass and r is radius

      Just find Earth’s g then adjust the variables till you get the same number for a different planet.

      • Anonymous

        > Mass is only a part of it. If the radius of the planet were also larger than Earth then the gravitational pull could be equal to that of Earth.

        To a certain definition of “gravity.” Yes, you can have a much larger planet with that same 9.81 m/sec^2 surface gravity. But the gravity well would be much deeper, and much more difficult to escape from.

        Take Saturn for example. According to Wikipedia, the “surface” (cloudtop) gravity is a paltry 1.065 g’s. You’d hardly notice the difference. But if you tried to get *off* Saturn, its escape velocity of 35.5 km/sec – three times greater than Earths – would suddenly seem to be rather oppressive.

        A rocky Super Earth would almost certainly have a higher surface gravity *and* a higher orbital/escape velocity than Earth. To tinker the numbers so that you get a larger diameter but not a larger surface gravity means that you have a planet with lower bulk density than Earth. More rocks, less nickel-iron core. And this would *probably* mean less accessible heavy metals on the surface…copper or iron as rare on Super Earth as gold is on Earth. Which would make any culture that evolves on such a world one that never goes through so much as a Bronze Age. Aluminum might well be just as plentiful… but without metals such as iron or copper, electricity might never be something that they can master, and thus would never be able to use electrical means to turn aluminum oxide into metallic aluminum.

        An intelligent species with little to no access to electricity and metals, stuck in a deep gravity well, seems like a poor prospect to ever develop starflight.

        • Fair enough, but a deep gravity well also means a deep atmosphere. Aircraft/baloon launched spacecraft might have more of a chance of being useful in such a situation.

          The atmospheric losses otherwise would be frightful.

          • Anonymous

            Think of getting off a Super Earth kinda like getting off Venus (though hopefully without the heat and acid). You’d *have* to get above the bulk of the atmosphere, and that could mean starting from a LTA platform perhaps 50 or a 100 km above the surface.

            Another potential concern: with a thick enough atmosphere, the local sun might be little more than a vaguely brighter smudge in the sky; stars, moons and planets could be unknown until high-altitude aeronautics was developed. Cultures evolving under such conditions might (or might not, who the hell knows) not even really develop much of a concept of “out there.” Whole areas not only of engineering but of *physics* might entirely escape their notice.

        • Anonymous

          Aluminum conducts electricity rather well per unit of weight, not much worse than copper. The Hall Heroult process is further not the only process nor might not even be the most economical process to turn aluminum oxide into aluminum, but it is the most developed today. I even disagree on the gravity well part. It could just mean a civilisation might have to advance technologies a few centuries further beyond our present technology level before being able to do so, which is negligible as compared to how long earth has been around or in the larger development scheme of evolution.