Exoplanets

Exoplanets are planets orbiting other stars rather than the Sun.

In the early days, they all show that the exoplanetary populations were dominated by really massive companions. Most of them were more massive than Jupiter, and only 15% were less massive than Neptune.

The Kepler came along and showed that most planets are more common than we thought. The most common planets of all are the so-called Super-Earth or Mini-Neptune. These planets could be large rocky planets or water worlds with a deep ocean on the surface, we don’t know.

Transit method

Kepler looked for planets using the transit method.

When a planet passes in front of its star, it blocks out some of the light, and the telescope detects this brief decrease in brightness.

However, the planet’s orbital plane has to be aligned just right with the telescope’s line of sight, for this to work. Assuming the exoplanet’s orbital axes are randomly distributed in space, then the geometric probability of a transit is only about half a percent. So to find a lot of planets, you need to observe a lot of stars. Of the more than half a million stars in Kepler’s field of view, around 150,000 were chosen for observation.

And Kepler observed all of these stars simultaneously. When an exoplanet is found, the transit itself is used to characterize the planet. In particular, the transit depth tells you how large the planet is in relation to its host star. Either way, you’d observe the same original. You have to know something about your star to constrain the radius of the planet.

To solve this, Kepler has an input catalog, containing information on hundreds of thousands of stars in the telescope’s field of view. One of the catalog properties is the stellar radius, which is then used to calculate the planetary radius for each detected exoplanet. In this way, scientists are able to characterize the vast population of exoplanets being discovered.