Lesser-Used Methods For Detecting Earth-Sized Planets

Astrometry
Astrometry is the oldest search method for extra solar planets. It consists of measuring a star's position in the sky and observing how that position changes over time. Astronomers use astrometric techniques to try and detect extra solar planets by measuring the gravitational influence that a planet will have over its parent star, causing the star to wobble in a tiny circular or elliptical orbit. Unfortunately for astronomers searching for Earth-sized planets, the astrometric method is most sensitive to planets with large orbits. Only planets with 6.6 Earth masses or greater have been detected, and the ability to find smaller, Earth-sized objects hardly exists. For these reasons, astrometry continues to be a lesser-used technique for finding them.

Optical Interferometry
Optical Interferometry combines the light of multiple telescopes to act as one larger telescope. Interferometry once used radio wavelengths to observe the structure of distant galaxies, but for the last 15 years, astronomers have used optical wavelengths, which are leading to new advances in the search for extra solar planets. Optical interferometry has the potential to provide information on the orbits and masses of extra solar planets of any age or mass. It can make very precise astrometric measurements and can detect the motion of the planet and parent star around their common center of gravity. The Keck Interferometer is a telescope in Hawaii, which is made of two twin telescopes, 85 meters apart from each other. This telescope will study dust clouds around stars in the hope to find the formation of Earth-sized planets.

Radial Velocity
Radial Velocity, also known as Doppler spectroscopy, detects extra solar planets through observations of Doppler Shifts in the spectrum of the star around which the planet orbits. Unfortunately for seekers of Earth-sized planets, this method is best suited for detecting very massive objects close to the parent star. These massive planets cause the largest changes in their radial velocity and are therefore easiest to detect. The chance of observing smaller and more distant planets is increasing, but Earth-sized planets remain undetectable with current techniques. Even so, the hunt for these Earth-sized planets through this technique continues. The High Accuracy Radial Velocity Planet Searcher (HARPS) is currently the best radial velocity instrument available for detecting extra solar planets. HARPS has already detected some Uranus- and Neptune- sized planets with his technique, which represents an important step in the future detection of smaller, potentially Earth-sized extra solar planets.