What does the “typical” exosolar system look like? We know it’s not likely to look like our own Solar System, given that our familiar planets don’t include entire classes of planets (Hot Jupiters! Mini-Neptunes!) that we’ve found elsewhere. And our discovery methods have been heavily biased toward planets that orbit close to their host star, so we don’t really have a strong sense of what might be lurking in more distant orbits.
A new study released on Thursday describes a search for what are called “microlensing” events, where a planet acts as a gravitational lens that magnifies the star it’s orbiting, causing it to brighten briefly. These events are difficult to capture, but can potentially indicate the presence of planets in more distant orbits. The researchers behind the new work find indications that there’s a significant population of rocky super-Earths that are traveling in orbits similar to that of Jupiter and Saturn.
Lenses go micro
The two primary methods we’ve used to discover exoplanets are called transit and radial velocity. In the transit method, we simply watch the star for dips in the light it sends to Earth, which can be an indication of a planet orbiting in a way that it eclipses a small fraction of the star. For radial velocity, we look for red- or blue-shifts in the light received from the star, caused by a planet tugging the star in different directions as it orbits.
Obviously, a planet’s gravitational influence is stronger when it’s closer to the host star. And stars can temporarily dim for all sorts of reasons, so we’ve generally set a standard for discovery that involves observing multiple transits. That, in turn, means a shorter orbital period, and so also biases us toward discovering planets that are close to their host star. As a result, most of what we know about exosolar systems comes from planets that are far closer to their host star than Earth is to the Sun. Even the most distant object discovered by the Kepler mission orbits is only about as distant as Mars.