The same pheonomenon that causes the siren of a passing ambulence to change pitch also causes a moving star to change color. It is called the Doppler effect and while it is a much smaller effect for light than for sound, modern instrumentation allows us to track the subtle motions of stars down to human walking speeds! The wobble, or “reflex motion” of stars due to orbiting planets produces a recognizeable signature, and we use this method to find planets and to confirm planets found by other methods such as transits. If the properties of the host star are known, the Doppler method gives us the mass of the planet, and the eccentricity and radius of the planetary orbit.
The tiny, periodic diminutions of star light due to an exoplanet moving across the face of a star (eclipse, or transit) can be detected allowing the existence of the exoplanet to be inferred. The transit method of planet detection is performed by monitoring the light of stars with very high precision in search of these characteristic signatures. Transits allow us to measure planetary radii and combined with Doppler measurements give planetary mass. These properties are combined to reveal the bulk density providing insight into planetary compositions and interior structures. High-sensitivity follow-up observations with spectroscopy and photometry can be used to measure planetary temperatures, weather patterns, spin-orbit alignments, and atmospheric properties.
Minerva will be an array of small-aperture robotic telescopes built atop Palomar Mountain outfitted for both photometry and high-resolution spectroscopy. It will be the first U.S. observatory dedicated to exoplanetary science capable of both precise radial velocimetry and transit studies. The multi-telescope concept will be implemented to either observe separate targets or a single target with a larger effective aperture. The flexibility of the observatory will maximize scientific potential and also provide ample opportunities for education and public outreach. The design and implementation of Minerva will be carried out by postdoctoral and student researchers at Caltech.
Website: Project Minerva
Lock-in Amplified Externally Dispersed Interferometry (LAEDI) is an experiment to develop novel instrumentation capable of measuring ultra-precise radial velocities of stars (< 10 cm/s of systematic uncertainty). The instrument combines new technologies recently developed in the medical industry (optical coherence tomography), large-format zero-read-noise detectors, and externally dispersed interferometry to achieve high Doppler precision. The goal of the experiment is to recover the 30 cm/s, 5-minute-duration seismological oscillations in the Sun, first observed using optical resonance spectroscopy of a single Solar potassium line. With LAEDI we hope to recover the oscillations with broad-band light, paving the way for future instruments on large telescopes to study asteroseismology of stars and measure the mass of Earth-like planets found to transit their host stars. LAEDI was recently awarded a JPL DRDF grant to develop a prototype (PIs: Phil Muirhead and Gautam Vasisht, Co-Is John Johnson and Kent Wallace).
Cutting-edge astronomical instrumentation now allows astonomers to detect objects very near a star that are a hundred thousand times fainter. This technology of high-contrast imaging has led us in the last 5 years to the direct detection of exoplanets. This planet finding technique is most sensitive to massive exoplanets with orbits larger than that of Jupiter in our own solar system, and is complementary to the results of the radial velocity and transit methods. These direct images of exoplanets give us information about their atmospheric chemistry, compositions and thermodyanamic properties.
The planets in our Solar System all orbit the Sun in the same direction, and in nearly the same plane (known as the ecliptic plane). In addition, the rotation axis of the sun is aligned with the ecliptic. To astronomers as early as the 18th century, this was evidence supporting the origin of our planets out of a spinning disk of circumsolar material, with little orbital disruption since their formation. Until very recently it was not known whether the architecture of exoplanet systems was similar to our Solar System in this way. Recently however, astronomers have devised a technique to measure the so-called "spin-orbit alignment" of transiting planet systems: this involves taking radial velocity measurements of the star while the planet is transiting to look for a characteristic "Rossiter-McLaughlin" Doppler signal. The shape of this signal can tell us whether the planet's orbit is aligned or misaligned with the rotation axis of its host star. Over the last few years the spin-orbit alignment of dozens of systems have been measured, many of which are turning out to be highly misaligned—in some cases, the planets even orbit their stars backwards (reverse to the stellar spin)—potentially indicative of violent dynamical histories. This ensemble of spin-orbit angle measurements is beginning to provide important clues to the formation and evolution of planetary systems, suggesting that many exoplanet systems may have histories quite different than that of our Solar System.
To understand exoplanets requires a good understanding their host stars; the derived mass and orbit of the planet depends on the mass of the star, the derived radius of the planet depends on the radius of the star, and the planet equilibrium temperature is a function of the temperature and luminosity of the star. We therefore strive to observationally determine stellar properites with accurate observations. The extremely precise and repetitive observations of the Kepler mission can detect small variations in stellar brightness due to sound waves within the star. In a similar way that seismology is used to determine the structure of the Earth's interior, these observations can be used to derive stellar mass, age, and composition. Also, the large, growing library of Keck HIRES spectra gathered for low-mass stars over the past 15 years is being used to identify subtle features in the starlight that can be calibrated to determine stellar mass and metallicity.
The centuries-old quest for other worlds like our Earth has been rejuvenated by the intense excitement and popular interest surrounding the discovery of hundreds of planets orbiting other stars. There is now clear evidence for substantial numbers of three types of exoplanets; gas giants, hot-super-Earths in short period orbits, and ice giants. The challenge now is to find terrestrial planets (i.e., those one half to twice the size of the Earth), especially those in the habitable zone of their stars where liquid water might exist on the surface of the planet. The Kepler Mission, NASA Discovery mission #10, is specifically designed to survey our region of the Milky Way galaxy to discover hundreds of Earth-size and smaller planets in or near the habitable zone and determine the fraction of the hundreds of billions of stars in our galaxy that might have such planets. more