Detecting Exoplanets

A rapidly shifting observational field in astronomy is the study of exoplanets, or planets outside of our solar system. Exoplanets can be detected through a number of methods, all of which provide a piece of information about that planet. Kepler and K2, for example, find exoplanets by taking a series of photometric images of the star and recording the brightness for a period of time, called the light curve. When a planet crosses over the host star, a portion of that star’s light is blocked, and we see a dip in the light curve. Analyzing the shape of the dip provides the radius and orbital period of the exoplanet. Precise astrometry takes images of a star system over time and searches for wobbles in the star’s position that can be attributed to a planet’s gravity tugging on the star. In this case, the star and planet’s orbit is in a plane orthogonal to Earth.

A transiting exoplanet. If the planet’s and star’s orbit are on the same plane relative to our line of sight from Earth, the planet will eclipse the star, blocking some of the star’s light for a period of time. Measuring changes in a star’s brightness as a function of time allows us to find exoplanet candidates. Then RV instruments like G-CLEF can follow-up and study the exoplanet candidate in detail, revealing information about the planet’s mass and atmosphere.

G-CLEF, like Kepler/K2, detects transiting exoplanets, or planets orbit fully or partially across the host star relative to Earth. Instead of measuring the change in brightness due to the transiting planet, G-CLEF measures the changes in a star’s radial velocity. A star with an orbiting planet will have a small orbit around the system’s center of mass in response to the planet’s gravity. This causes small variations in the star’s speed as seen from Earth, the radial velocity, as the star moves closer and farther away from us in its orbit. Radial velocity instruments like HARPS-N, ESPRESSO, and G-CLEF measure the Doppler shifts from this orbital wobble. The planet’s mass and orbit eccentricity can be determined from the radial velocities measured over time.

Precision radial velocity measurement
Measuring radial velocity. As the star moves around the center of mass of the system, the star at some points will move towards then away from Earth. Due to the Doppler effect, spectral features in the star’s spectrum will appear at shorter wavelengths when the star is moving towards Earth (blue line), and at longer wavelengths when it is moving away (red line). Measuring these shifts in spectral features over time, we can determine the projected mass of the orbiting planet. Image credit: ESO

G-CLEF Exoplanet Studies

The science objectives for G-CLEF in performing studies of exoplanets are:

  • Measure radial velocities to sub-meter precision, and in the best observing conditions down to 10 cm/s
  • Identify exoplanets with significant atmospheres
  • Discover biomarkers in the atmospheres of exoplanets
  • Understand energy transport in giant exoplanet atmospheres
  • Measure exoplanet atmosphere vertical temperature profiles
  • Determine atomic and molecular composition of exoplanet atmospheres