5.5 Mass measurements for Kepler planets
Because Kepler stared at all the stars in a particular small region of sky, most of the stars it studied are faint. The radial velocity method needs the starlight to be spread out so the tiny changes in the wavelengths of the chemical fingerprints can be measured. This is quite difficult to do when the star is so far away and consequently so faint: there is simply not enough light to spread out in this way. Radial velocity measurements require large telescopes and specialised equipment. The telescope must be dedicated to making a radial velocity measurement of only one star at a time. This is in stark contrast to a transit search like Kepler, which simultaneously measured the brightness of about 150 000 stars.
So, while Kepler has discovered a lot of planets, most of them have not had radial velocity measurements of their masses.
One of the most interesting new things Kepler revealed was that many stars host several transiting planets. When there are several planets, these have gravitational pulls on each other. This can cause the planets to slightly speed up or slow down in their orbit. When this happens, the planets’ transits arrive slightly early or slightly late. These transit timing variations can be used to work out how strong the gravitational attraction between the planets must be. From this, astronomers can then work out how massive the planets must be.
A few of the Kepler planets do have known masses, if they are bright enough for the radial velocity method to be applied or if the method using transit timing variations can be applied precisely, but the majority have unknown masses, or masses that have considerable uncertainty. This makes it more difficult to know what the planets are really like.