Apart from a few special exceptions, directly observing planets hidden in the glare of their stars is beyond our current technology. So we have to rely on indirect methods to detect and characterize them. By far, the easiest methods involve measuring the small effects that the planets have on their star. The most fruitful of these techniques has been Transit Photometry - measuring the dimming of a star’s light when its planets pass between us and the star. Radial Velocity and Stellar Astrometry are other methods that detect the subtle motions of stars caused by orbiting planets tugging on them gravitationally. The TRAPPIST-1 discovery used the transit method.

Transit methods only work for the small percentage of star systems that are oriented edge-on towards Earth. Fortunately, that’s still a lot of stars! On a regular interval each planet transits the disk of its star, temporarily causing a slight reduction of the star’s light (on the order of 1 part in 10,000) as viewed from Earth. The amount of the reduction is proportional to the size of the planet. The dip re-occurs every time the planet completes an orbit. By recording the brightness of the starlight continuously over a long period of time, we can determine the length of a planet’s year and the radius of its orbit.

The graphs of starlight versus time, known as light curves, can be complex if multiple planets are transiting – each with a different dip amount, duration, and interval – all summed together. Many stars exhibit inherent variability in their brightness, too, but the exoplanet transit dips are usually brief by comparison.

Small, Earth-sized planets produce tiny dips in light intensity. If a star is being examined using a ground-based telescope, daytime prevents continuous measurements, and atmospheric interference (i.e., twinkling) adds noise to the light curve, limiting the sensitivity and preventing the detection of small planets. To overcome this, observatories such as the Kepler Space Telescope are launched into orbit. They detect and characterize exoplanets, especially Earth-like ones, by using Transit Photometry continuously on a single small patch of sky. Kepler has discovered thousands of exoplanets, including many hot Jupiters and Neptune-sized planets, and a good number of Earth-sized planets. The Spitzer space telescope is another space-based observatory that was originally launched as an infrared observatory. With its primary mission long completed, and some of its instruments no longer operational, it is still functional as an exoplanet observer.

The time taken for the transit yields the speed of the planet, and the time interval between transits provides its orbital period (its year). These parameters allow the orbit to be calculated using laws of orbital dynamics, including the planet’s mass and its distance from the star. The amount that the host star’s light diminishes while an exoplanet transits gives astronomers the diameter of the exoplanet. Knowing its diameter and mass provides its density.

Combined with the star’s luminosity and spectral type, astronomers estimate the surface temperature of the planet. Moreover, if an exoplanet has a translucent atmosphere, or is a gas world, the reduction in light drops gradually, while one that is rocky and airless diminishes more abruptly.