Spotty stars and exoplanets

James Miller considers the role of spotty stars in our search for habitable planets

Have you ever wondered if one day humans could live on another planet? Until the discovery of exoplanets, some thought our Solar System was the only one with planets. The Kepler telescope quickly changed this view, discovering countless different families of systems. As it turns out, our meagre system is quite unique compared to the zoo of others out there. Some have huge ‘Hot Jupiter’ planets whizzing around close to their host stars at unimaginable speeds. Others have strange half-and-half worlds where one side of the planet is tortured by its star while the other is left to freeze, facing the emptiness of space. It becomes a real struggle, then, to find any potential habitable planets.

How can spots help us with this? It turns out that studying another star’s spots may give us insight into the potential habitability of its exoplanets. The Sun has a major effect on the Earth’s climate—by extension, so will other stars on their respective exoplanets.

But what are these ‘spots’? We have known about sunspots for a very long time. It’s not that the Sun is unwell, nor has it recently tried some bad skin cream. Unfortunately for the Sun, its spots are unavoidable and are caused by variations in its magnetic field. The important thing to know about these spots is that they are cooler than the rest of the Sun’s surface (despite still being some 3800 degrees Kelvin!). Many solar flares and storms originate from sunspots, so it’s crucial they’re monitored.

From observing these spots and various other changes, we know that the Sun has a roughly 11-year cycle of activity. The spots form at high latitudes, slowly moving inward over time until disappearing as they reach the Sun’s equator. These movements can be plotted with time to produce a ‘butterfly diagram; this gives a deeper insight into the Sun’s magnetic activity lifecycle. It is believed that the change in the Sun’s spot activity even contributed to the ‘Little Ice Age’ experienced in the 15th century.

So, do these spots exists on other stars? And if so, how can we track them? Despite these being perfectly reasonable questions, they are incredibly difficult to answer. The light we receive from other stars is represented by a few tiny pixels on a screen, rendering it impossible to distinguish any details.

There have been successful attempts, however, at indirectly observing other stars’ spots, so we know they exist. These techniques vary from detecting slight variations in temperatures over time, to measuring small differences in spectral observations of the star’s light. But these techniques cannot provide important information about where the spots are on the surface, or how they move—here come the exoplanets!

As a planet transits in front of its host star, it blocks out some of its light. This causes noticeable dips in brightness, which can tell us all sorts about the planet and its star. Now, imagine the star has spots. A planet that transits in front of a spot will give a slight rise in these brightness dips, as the spot is cooler and hence less brightness is blocked. That little bump in the spectrum is how a star spot can be tracked. There are,of course, many other complications, but this is the underlying principle. By plotting the butterfly diagram for the star, we can then begin to analyse its effects on surrounding exoplanets. One day, this may enable us to find the perfect host star for a new human colony!

From Issue 14