A Transiting Planet of a Sun-like Star

A planet transits an 11th magnitude, G1 V star in the constellation Corona Borealis. We designate the planet XO-1b and the star XO-1, also known as GSC 02041-01657. XO-1 lacks a trigonometric distance; we estimate it to be 200 ± 20 pc. Of the 10 stars currently known to host extrasolar transiting planets, the star XO-1 is the most similar to the Sun in its physical characteristics: its radius is 1.0 ± 0.08 R☉, its mass is 1.0 ± 0.03 M☉, its V sin i < 3 km s-1, and its metallicity [Fe/H] is 0.015 ± 0.04. The orbital period of the planet XO-1b is 3.941534 ± 0.000027 days, one of the longer ones known. The planetary mass is 0.90 ± 0.07MJ, which is marginally larger than that of other transiting planets with periods between 3 and 4 days. Both the planetary radius and the inclination are functions of the spectroscopically determined stellar radius. If the stellar radius is 1.0 ± 0.08 R☉, then the planetary radius is 1.30 ± 0.11RJ and the inclination of the orbit is 87.°7 ± 1.°2. We have demonstrated a productive international collaboration between professional and amateur astronomers that was important to distinguishing this planet from many other similar candidates.