More On “Super Earths”

My last rant on “Super-Earths” got a number of comments. One of them, from Murgatroyd, pointed out the importance of understanding that even larger planets can have low surface gravities, using Uranus (a low density gas giant) as a reference. In addition, this article about “Super-Earths” that also deals with gravity was printed the same night as mine: http://goo.gl/Ipb7a . In the article, Dr. Micheal Chorost shows how the surface gravities on various known “Super-Earths” aren’t proportionally greater relative to their mass because, obviously, the larger radius of the planets gets you farther away from the center of that mass’s gravity. Here’s his table showing the Mass, Radius and Surface Gravities for those “Super Earths”, plus a fictional one with a surface gravity equal to Earth’s:

Mass radius Surface Gravity
earth 1 1 1
Gliese 581g: 2.6 1.4 1.33
Gliese 581d: 6.9 2.2 1.43
Gliese 667Cc: 4.9 1.9 1.36
Kepler 22b: 6.4 2.1 1.45
HD40307g: 8.2 2.4 1.42
HD85512b: 4 1.7 1.38
Gliese 163c: 8 2.4 1.39
Fictional 8 2.83 1

But I pointed out to Dr. Micheal Chorost that ever on his fictional world, escape velocity would be higher (he did talk about landing/launching in the article). He said it was an interesting point and would ask some of his rocket science friends about it. Well I took the liberty of figuring it out myself and here is a table showing the escape velocities for all those planets, as well as the rocky planets in our solar system:

Mass radius Surface Gravity Escape Velocity
Mercury 0.055 0.383 0.380 0.381
Venus 0.815 0.950 0.900 0.928
earth 1.000 1.000 1.000 1.000
Luna 0.012 0.273 0.165 0.213
Mars 0.107 0.533 0.380 0.450
Gliese 581g: 2.600 1.400 1.330 1.360
Gliese 581d: 6.900 2.200 1.430 1.770
Gliese 667Cc: 4.900 1.900 1.360 1.610
Kepler 22b: 6.400 2.100 1.450 1.740
HD40307g: 8.200 2.400 1.420 1.850
HD85512b: 4.000 1.700 1.380 1.530
Gliese 163c: 8.000 2.400 1.390 1.830
Fictional 8.000 2.830 1.000 1.680

As you can see, on the fictional “Super-Earth” with a surface gravity equal to Earth’s, the escape velocity is still x1.68 that of Earth. HD40307g is the recent “Super-Earth” discovery that prompted these rants, and it’s surface gravity is x1.42 Earth’s, with an escape velocity x1.85 Earth’s. So clearly the surface gravities of a planet is not the attribute you want to look at when determining the difficulty of launching from/landing on that planet. I wish it was as simply as saying that the density of the planet is the best indicator… but it just doesn’t appear to work that way (I checked).

-Space Colonizer

2 comments on “More On “Super Earths”

  1. Murgatroyd on said:

    Yes, you’re quite right about the importance of escape velocity. I wonder if that’s an answer to the Fermi Paradox: planets more massive than Earth could be “roach motels” for civilizations. We can barely climb out of Earth’s gravitational well using chemical fuels in our rockets, so to escape from a more massive planet you probably would have to use nuclear propulsion … which the people back at the launch site might object to, if the spacecraft is anything like our Project Orion designs.

    If our solar system had HD40307g instead of Mars in the next orbit out from the Sun, any explorers from Earth probably would be on a one-way trip. They could land, but it would be very hard to leave. (Unless they used an Orion craft, of course.)

    Hmm. And it may be that planets less massive than ours can’t hold onto enough of a Earth-type N2/O2atmosphere for a long enough time for intelligent life to evolve. Mars didn’t do very well, did it? Venus is less massive than Earth, but the molecular weight of its CO2 atmosphere is much heavier. (And who knows how much of that it’s lost over the past four billion years?)

    • SpaceColonizer on said:

      LOL. “Roach Motels”… I love that. I once heard it said that if the orbit of Venus and Mars had been reversed, we would have three habitable planets in our solar system. In Venus’s case I find that easy to believe, but for Mars the problem is the lack of a strong magnetic field. I’m wondering if the people who said that thought the increased tidal forces or heat from the sun would make Mars’s core more active and produce a stronger magnetic field.

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