Free Astronomy Magazine July-August 2020

7 JULY-AUGUST 2020 EXOPLANETS disk. In this case, the calculation of the mass of the transiting planet is straightforward and we can obtain a “true mass” for the planet. When the planes are not aligned and we do not observe passage of the planet across the stellar disk, we are left to predict a range of possible masses for a planet based on how that planet causes its host star to oscillate towards and away from us. From this range of possible masses, we are able to determine a planet’s “minimum mass” for the case when that planet’s or- bital plane places it just above or below the stellar disk when it passes between its star and Earth (the largest gravitational influ- ence it can have on the star with respect to our observing position without being able to observe a transit). For the observed Prox- O pposite page, the position in the sky of Alpha Centauri’s triple star system. [ESO/B. Tafreshi (twanight.org) / DSS 2. Ack: Da- vide De Martin/ Mahdi Zamani] A n artistic view of the dust belt dis- covered in 2017 around Proxima Centauri. The band extends from 1 to 4 AU away from the star. The planet Proxima b is represented here a little larger and farther from the star than it actually is. [ESO/M. Kornmesser]