USAAAO — 2023 First Round
Question 19
In 1995, researchers at the University of Geneva discovered an exoplanet in the main-sequence star 51 Pegasi. This was the first-ever discovery of an exoplanet orbiting a Sun-like star! When they observed the star, a periodic Doppler shifting of its stellar spectrum indicated that its radial velocity was varying sinusoidally; this wobbling could be explained if the star was being pulled in a circle by the gravity of an exoplanet. The radial velocity sinusoid of 51 Pegasi was measured to have a semi-amplitude of 56 m/s with a period of 4.2 days, and the mass of the star is known to be 1.1M☉. Assuming that the researchers at Geneva viewed the planet's orbit edge-on and that the orbit was circular, what is the mass of the exoplanet in Jupiter masses?
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USAAAO 2023 First Round. Question text, choices, and answer were transcribed from the official USAAAO exam and solutions PDFs.