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:
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|
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).