Surface gravity on many exoplanets similar to that of Earth, study finds

The acceleration that a body experiences on the surface of a planet, the gravity on surface, depends on the mass M and radius R according to a simple formula of Newton's: a=GM/R², in which G denotes the universal gravitational constant. Therefore we expect that planets with masses and sizes different from Earth would exhibit very different gravity values on their surfaces.

In fact, this occurs, for instance, on our moon, whose surface gravity is about g/6; this explains the particular way of walking of astronauts on the moon that we see in documentaries. If movie scripts are faithful to the laws of physics, should we not see similar effects in many fictional planets, like Tatooine or Alderaan of Star Wars?

Surprisingly, it does not appear to be so. A recent study published in the Astrobiology Journal has found that, despite showing ostensible differences in mass and size, a considerable number of the extrasolar planets discovered so far have a surface gravity very similar to that of Earth.

The article, titled "Walking on exoplanets: Is Star Wars right?" and authored by Fernando J. Ballesteros, from Observatorio Astronómico at Universidad de Valencia and Bartolo Luque, from Universidad Politécnica de Madrid, reveals this interesting property of gravity that the models of planet formation do not explain.

In order to carry out this study, researchers used the exoplanets.org database and estimated the surface gravity of those exoplanets whose mass and radius are already known (about 1,200 exoplanets out of the 3,500 detected exoplanets). If the planets' masses (units of mass Earth) were represented facing their surface gravity (in units of terrestrial gravity) on double log-log plot, we would observe three scaling regions clearly distinguished.

Firstly, the surface gravity of the small bodies in the Solar System and rocky planets smaller than Venus grows with the square root of the mass. Secondly, in the case of gaseous giant exoplanets, the surface gravity linearly grows with the mass. And surprisingly, in the transition zone (between 1 and 100 land masses), we find some sort of plateau that shows a constant surface gravity roughly similar to that of Earth.

The results revealed by Ballesteros and Luque are confirmed in our own planetary system: although Uranus, Neptune and Saturn are, respectively, 14, 17 and 95 times more massive than Earth, their surface gravities barely vary between 0.9g and 1.1g. According to the authors, the current models of planet formation fail to predict this constant law, but they do predict the power laws whose exponent changes smoothly when passing from the completely rocky planets to the gas giants.