Exoplanets

Description

The detection of planets orbiting nearby stars has been one of astronomy’s great success stories. As of 2010, more than 460 exoplanet discoveries have been made using the Doppler technique (http://exoplanet.eu/catalog.php), more than 100 transit detections and roughly a dozen planets each discovered by microlensing, imaging and measurement of timing variation methods. The NASA Kepler mission is poised to announce hundreds of new candidates will learn how often small rocky planets form. Exoplanet detections have launched a subfield of astronomy that includes host star characterization (e.g., planet-metallicity and host star mass correlations), measurements of planet radii and density, studies of exoplanet atmospheres, interior structure, theory of exoplanet formation and orbital evolution.

The priority for the next decade is clearly the search for planets that are one or two times the mass of Earth, orbiting in the habitable zones of their host stars.

· The NRC report (2006) notes, “The discovery of truly Earth-like planets around nearby stars would be transformational.”

· The Exoplanet Task Force Report (Lunine et al. 2008), with extensive community input, outlined a 15-year plan that highlights the importance of searches for Earth-mass planets at habitable zone distances.

· And one of the three recommendations of the ASTRO 2010 decadal survey is: “locating the closest habitable Earth-like planets beyond the solar system for detailed study.”

Based on Doppler measurements of 166 Sun-like stars, Howard et al. (2010) predict that 25% of stars will host Earth-mass planets with orbital periods less than 50 days. However, low mass, terrestrial planets are extremely difficult to detect using the Doppler method; the only induces a reflex stellar velocities of ~10 cm s^-1 in the Sun. From the perspective of high-resolution spectrometers, this corresponds to stellar line shifts across a few dozen silicon atoms in the CCD detectors of modern spectrographs.

Since the search for planets is motivated by a search for life, it is important to detect many potentially habitable worlds around nearby stars in order to carry out a statistical survey for life (e.g., by obtaining spectra of exoplanet atmospheres with imaging missions from space). If Doppler precision can be improved (to better than 0.5 m s^-1), then we can usher in an era where potentially habitable worlds could be detected around nearby stars as we steadily plug away from the ground over the next couple of decades. In addition to the highest possible Doppler precision, a strategy of “high cadence” (many observations per night, many nights per year) may help to average over astrophysical noise sources.

Measuring the tiny stellar line shifts induced by a true Earth analog requires a significant improvement over current Doppler precision, driving the need for instrumentation and software that outperform those in our current programs. Exoplanet research at Yale aims to address some these challenges, making use of Keck and CTIO telescopes.