Free Astronomy Magazine March-April 2022

23 MARCH-APRIL 2022 ASTRO PUBLISHING T his composition com- bines a view of the southern sky above the dome of ESO’s 3.6-meter telescope at La Silla Ob- servatory in Chile with Hubble Space Telescope images of the stars Prox- ima Centauri (bottom right) and the Alpha Cen- tauri AB binary (bottom left). [Y. Beletsky (LCO)/ ESO/ ESA/NASA/ M. Zamani] size of the Earth or not, much less whether it is potentially habitable. To obtain this information, it is in- dispensable to also know the diam- eter of the planet. There are essen- tially two ways to do this: getting a direct image with an angular reso- lution sufficient to measure the ap- parent size of the planet (with the distance known, you can calculate the actual size) or record its transit in front of the stellar disk and de- duce the diameter from the per- centage of the eclipsed photo- sphere (with the diameter and the