What Even ARE Exoplanets?
At their core, exoplanets are any planets outside the solar system (Figure 1). They are of special interest to many astronomers because if life is harbored anywhere in the universe, it is likely to be found at an exoplanet (Apai et al. 2016). It then becomes imperative to be able to find such exoplanets that may harbor life.
There are, of course, several techniques scientists use to find these exoplanets. You can find out more about them here. The most common of these methods are the transit and radial velocity methods. The radial velocity method, although the oldest, is also more likely to be able to detect an exoplanet than the rest (Lovis & Fischer 2015). The radial velocity method works by taking into account the fact that when two objects are rotating, they both exert a gravitational influence on each other and rotate about their shared center of mass. For example, the Sun and the Earth are both rotating around a point: their shared center of mass. This causes the Sun to appear to “wobble” about its axis, which can be detected through the Sun’s spectra as the Sun doppler shifts the light moving back and forth with respect to an observatory. This doppler shift is incredibly small — the Earth causes the Sun to move at a mere 13 cm/s on its axis, causing a Doppler shift of just about five atoms. Tiny, right?
As of right now, technology is just emerging so that we can detect such small exoplanets, but there are still hurdles we have to face. The biggest of these is stellar activity. Stellar activity, like starspots, stellar flares, or plages, is any inhomogeneity on a star’s surface. Stellar activity can cause spurious Doppler shifts in a star’s spectra, thereby masking, or even posing as an exoplanetary signal. These signals often show up at or at harmonics of the stellar rotation period the star takes to rotate about its own axis (just like the rotation period of the Earth is 24 hours), and usually produces quasi-periodic signals. Many experts say that the main hindrance to finding Earth-sized exoplanets as of right now isn’t instrumentation limits, but instead the way we handle stellar activity. This is what I decided to focus my science fair project on. You can find out more of my interactions with exoplanet science here!
