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How Fast Do Exoplanets Travel Around Their Parent Stars?

-- | January 25, 2015

Question: Looking at data on exoplanets, I’ve noticed a few with what seem like incredibly short projected orbital periods; single digit numbers of days per orbit.

This would seem to put these planets travelling through space at incredible velocity relative to the speed of light, at least compared to anything else in our experience.

What effects would this have, if any, on the planet – from a general relativity point of view. Would time dilation be measurable to an outside observer surveying one of these planets from up close (say, 3 or 4 AU)? — Timothy

Answer: In fact, exoplanets travel just somewhat faster than a rocket. An average velocity for a rocket is something like 30000 feet per second, or about 10,000 meters per second. The velocity of an exoplanet with an orbital semimajor axis of about 0.1 AU and an orbital period of about 10 days is about 100,000 meters per second, or about 10 times that of the rocket. Finally, the speed of light is about 300,000,000 meters per second, or about 1000 times the speed of the exoplanet. Therefore, the space velocities of exoplanets do not appear to be fast enough to experience general relativistic effects.

Jeff Mangum

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