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# What is the Weakest Surface Gravity for an Exoplanet So Far?

The surface gravity of a planet is the free-fall acceleration experienced by an object on the surface of the planet and is a measure of the object's weight as opposed to its mass. Both the planet's mass and a radius are required to calculate the surface gravity of the planet, but not all exoplanets have both mass and radius measurements or estimates. On 5 August 2012, only about 32% of confirmed exoplanets had sufficient data for the surface gravity to be calculated.

In order to calculate the surface gravities of the exoplanets listed in the Extrasolar Planets Encylopedia catalog, you have to download the data table, filter the rows that have both mass and radius entries filled, and then calculate the surface gravities as shown in the surface gravity distribution page. You can do this by reading the data into an analysis program that you are familiar with (for example, EXCEL).

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As an example, on 5 August 2012, the exoplanet with the weakest surface gravity was WASP-17 b. This planet has a radius of $1.991\pm 0.081$ Jupiter radii, a minimum mass estimate of about $0.486\pm 0.032$ times the mass of Jupiter, and a calculated surface gravity of 0.324 times that of the Earth. (For reference, the surface gravity of Earth is about 9.8 meters per second per second, or 32 feet per second per second). Thus, a person weighing 150 pounds on Earth would weigh $150 \times 0.324 = 48.6$ pounds near the surface of WASP-17 b. For comparison, the surface gravity of our Moon is about one-sixth that of Earth, so this is actually even weaker than the exoplanet with the weakest surface gravity (so far).

A word of caution: you should always try to find out what measurement uncertainties are associated with any quoted number (not just for exoplanet mass and radius values, but any measurement in general). However, the measurement uncertainties on various parameters are often not given, even in the Extrasolar Planets Encyclopedia: you have to dig a little deeper into the literature. Remember also that most exoplanet masses are minimum masses so the calculated surface gravity will also be a minimum value, or lower limit.

File under: What is the least value of the surface gravity of an exoplanet found so far? What exoplanet has the smallest surface gravity? How can I find the latest information on the exoplanet with the weakest surface gravity?